A close binary system (A0 V, M1 III) with a shell-like spectrum indicating that mass transfer may be occurring from the late-type companion onto the A0 primary.
A nuclear reaction (3 4He → 12C + γ + 7 MeV) by which helium is transformed into carbon. The process is dominant in red giants. At a temperature of about 2 × 108 K and a density of 105 g cm-3, after core hydrogen is exhausted, three α-particles can fuse to form an excited nucleus of carbon 12, which occasionally decays into a stable carbon 12 nucleus. The overall process can be looked upon as an equilibrium between three helium nuclei and the excited 12C*, with occasional irreversible leakage out of the equilibrium into the ground state of carbon 12. Further capture of α-particles by carbon 12 nuclei produces oxygen 16 and neon 20. (also called the triple-α process)
A component of the Sagittarius arm with noncircular gas motions. It is seen in absorption against Sgr A with a velocity of -53 km s-1, implying that at least part of the arm is expanding away from the galactic center. The nearest "edge" is presently at a radius of 4 kpc from the Galactic center.
A giant H II region, at least 300 pc across - one of the largest known. It is larger and more luminous than any known in the Galaxy. It is the brightest object in the Large Magellanic Cloud at both optical and radio wavelengths, and contains the densest concentration of W-R stars. (The brightest object near the center is a O+ WN star of Mv = - 10.2.) It is characterized by very rapid, disordered, and complex motions.
A metal-rich globular cluster. It has roughly one-quarter the solar metal abundance. It has a high galactic latitude and low reddening. It is a member of the thick-disk population.
Nuclei possessing equal and even numbers of neutrons and protons. 4N nuclei are formed in supernova envelopes at temperatures of at least 2 × 109 K and are very stable.
One of about a dozen of the strongest Fraunhofer lines seen in the Solar spectrum, the A band at 7600 angstoms is due to telluric lines of molecular oxygen in the Earth's atmosphere. (originally thought to originate in the Sun by Fraunhofer)
Star with spectral type A in which the spectrum of the Balmer lines of hydrogen attain their greatest strength. Helium lines can no longer be seen. Some metallic lines are present; in late A stars the H and K lines of ionized calcium appear. A0 stars have a color index of zero.
A measure of the intrinsic brightness of a star or galaxy. Absolute magnitude is defined as the apparent magnitude the star or galaxy would have if it were 32.6 light-years (10 parsecs) from Earth. The lower an object's absolute magnitude, the greater its intrinsic brightness. For example, the Sun has an absolute magnitude of +4.83, while Sirius, whose intrinsic brightness is greater, has an absolute magnitude of +1.43. A star that is one absolute magnitude brighter than another (e.g., +4 versus +5) is 2.5 times intrinsically brighter; a star that is 5 absolute magnitudes brighter is 100 times intrinsically brighter; and a star that is 10 absolute magnitudes brighter is 10000 times intrinsically brighter.
The absolute magnitude (g) of a solar-system body such as an asteroid is defined as the brightness at zero phase angle when the object is 1 AU from the Sun and 1 AU from the observer.
Temperature measured on the Kelvin scale: 0 Kelvin = -273.15 ° Celsius. Absolute temperature is directly related to (kinetic) energy via the equation E = kBT, where kB is Boltzmann's constant. So, a temperature of 0 K corresponds to zero energy, and room temperature, 300 K = 27 °C, corresponds to an energy of 0.025 eV.
The lowest possible temperature, attained when a system is at its minimum possible energy. The Kelvin temperature scale sets its zero point at absolute zero (-273.15° on the Celsius scale, and -434.07° on the Fahrenheit scale).
Decrease in the intensity of radiation, representing energy converted into excitation or ionization of electrons in the region through which the radiation travels. As contrasted with monochromatic scattering (in which reemission occurs in all directions at the same frequency), the inverse process of emission refers to radiation that is reemitted in general in all directions and at all frequencies.
Sudden rises superposed on the smooth decrease of the curve of the attenuation coefficient, which cause the curve to have a typical sawtooth aspect. They generally occur at the limit of spectral lines.
Dark line in a spectrum, produced when light or other electromagnetic radiation coming from a distant source passes through a gas cloud or similar object closer to the observer. Like emission lines, absorption lines betray the chemical composition and velocity of the material that produces them.
Dark line superposed on a continuous spectrum, caused by the absorption of light passing through a gas of lower temperature than the continuum light source.
A process by which a star accumulates matter as it moves through a dense cloud of interstellar gas; or, more generally, whereby matter surrounding a star flows toward it (as in close binaries).
Lens (or combination of lenses) that brings different wavelenghts within a ray of light to a single focus, thus overcoming chromatic aberration. It was first successfully made by Joseph von Fraunhofer.
The slow rotation of the major axis of a planet's orbit in the same direction as the revolution of the planet itself, due to gravitational interactions with other planets and/or other effects (such as those due to general relativity).
Light in the nighttime sky caused by the collision of atoms and molecules (primarily oxygen, OH, and Ne) in Earth's geocorona with charged particles and X-rays from the Sun or outer space. The airglow varies with time of night, latitude, and season. It is a minimum at zenith and maximum about 10° above the horizon.
Art of bringing parts of the universe to the perfect state toward which they were thought to aspire - e.g., gold for metals, immortality for human beings.
An eclipsing system of at least three components (B8 V, K0, Am). Period of components A and B is about 68.8 hours; period of components A, B, and C is about 1.9 years. Long term observations also indicate a massive, unseen fourth component with a period of about 190 years. Algol is also an erratic radio source of about 0.5 AU diameter.
The most famous eclipsing binary, Algol was probably the first variable star discovered. It lies in the constellation Perseus and consists of two stars that orbit each other every 2.87 days. When one star passes in front of the other, the light of the system dims.
Spontaneous emission by a heavier element (such as uranium) of positively charged helium nuclei - alpha particles - comprising 2 protons and 2 neutrons. The result of this radioactive decay is that the original element is very gradually converted into another element, with a decreased atomic number and mass. Alpha particle emission may be simultaneous with beta particle decay.
The disintegration of an atomic nucleus, in which the final products are an alpha particle and a nucleus with two fewer protons and two fewer neutrons than the original.
A hypothetical process of nucleosynthesis, which consisted of redistributing α-particles in the region from neon 20 to iron 56 (and perhaps slightly higher). The α-process has been replaced by explosive and nonexplosive C, O, and Si burning occurring in rapidly evolving or even explosive stages of stellar evolution which at higher temperatures and densities becomes the e-process.
A form of mounting similar to that of a radar which allows the telescope tube to be moved horizontally (by rotation in azimuth or compass direction) and vertically (by rotation in altitude or elevation). To follow a star the telescope must be adjusted simultaneously in both axes. (also called alt-az)
A-type or F-type object to which no unique spectral type can be assigned. Usually the classifier provides a classification according to the hydrogen, metallic and calcium lines. Also call metallic-line stars.
Peculiar star whose metallic lines are as strong as those of the F stars but whose hydrogen lines are so strong as to require that they be classed with the A stars. They are generally short-period (<300d) spectroscopic binaries with high atmospheric turbulence and variable spectra, and are slower rotators than normal A stars.
constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 × 10-7 newton per meter of length
Unit of electric current. "The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 × 10-7 newton per meter of length" CIPM 1946, Resolution 2, approved by the 9th CGPM 1948). A current of 1 A is equivalent to the passage along the filament of a light bulb of about 6 × 1018 electronic charges per second.
A spiral galaxy (kS5 in Morgan's classification) in the Local Group, about 650-700 kpc distant (MV = -21), visible to the naked eye as a fuzzy patch in the constellation of Andromeda. Total mass about 3.1 × 1011 Msun ; i = 77°, heliocentric velocity - 180 km s-1. Its nucleus exhibits noncircular gas motions. It is similar to but slightly larger than our Galaxy.
Major spiral galaxy, 2.2 million light-years from Earth. Gravitationally bound to the Milky Way with which it shares membership in the Local Group, it is currently approaching us, rather than receding as is the case for most galaxies.
The largest galaxy in the Local Group. Also known as the Great Spiral and M31. It is about one and a half times the size of our own galaxy, and contains at least 300 globular clusters. Two smaller, elliptical galaxies (M32 and NGC 205) lie close to it.
The angular momentum of a system about a specified origin is the sum over all the particles in the system (or an integral over the different elements of the system if it is continuous) of the vector products of the radius vector joining each particle to the origin and the momentum of the particle. For a closed system it is conserved by virtue of the isotropy of space.
The product of mass and angular velocity for an object in rotation; similar to linear momentum. In quantum mechanics, angular momentum is quantized, i.e., is measured in indivisible units equivalent to Planck's constant divided by 2 pi. This corresponds classically to only certain frequencies of rotation being allowed.
The characteristic of being dependent upon direction. (Light coming with equal intensity from all directions is isotropic; a spotlight's beam is anisotropic.) The cosmic background radiation is generally isotropic - i.e., its intensity is the same in all parts of the sky - but small anisotropies have been detected which are thought to reflect the earth's proper motion relative to the framework of the universe as a whole.
A solar eclipse in which the solar disk is never completely covered but is seen as an annulus or ring at maximum eclipse. An annular eclipse occurs when the apparent disk of the Moon is smaller than that of the Sun.
A period of time based on the revolution of the Earth around the Sun, where a year is defined as the mean interval between successive passages of the Earth through perihelion.
Zeeman effet in which spectral lines are split into several components, in contrast to the normal Zeeman effect which results in only two distinct components.
The doctrine that the value of certain fundamental constants of nature can be explained by demonstrating that, were they otherwise, the universe could not support life and therefore would contain nobody capable of worrying about why they are as they are. Were the strong nuclear force slightly different in strength, for instance, the stars could not shine and life as we know it would be impossible.
The weak form of the anthropic principle states that life can exist only during a brief period of the history of our universe. The strong form of the principle states that out of all possible values for the fundamental constants of nature and the initial conditions of the universe, only a small fraction could allow life to form at all, at anytime. (See boundary conditions; fundamental constants of nature.)
Also AR coating. A layer of material of lower refractive index of just the right thickness (1/4 wave) is deposited on the optical surface to be coated. More complex coatings are possible which cover a large wavelength range.
The point in the galactic plane that lies directly opposite the galactic center. Here we gaze toward the edge of the Galactic disk. The nearest bright star to the anticenter is El Nath, in the constellation Taurus.
antimony (Sb, stibium from latin, 4S3/2 in ground state)
metalloid element with three forms. The metallic form is the more stable and is bright, silvery, hard and brittle
The antiparticle of a neutron. A neutron and antineutron both have the same mass and zero electric charge, but can be differentiated by their interactions: a neutron and an antineutron can annihilate into gamma rays, while two neutrons cannot.
An elementary particle of opposite charge but otherwise identical to its partner. Most of the observable universe consists of particles and matter, as opposed to antiparticles and antimatter.
Atomic particles that have the same mass as, but opposite charge and orbital direction to, an ordinary particle. Thus, instead of negatively charged electrons, atoms of antimatter have positrons. A quantity of antimatter coming into contact with matter would "cancel out" - annihilate, with total conversion of mass to energy - an exact proportion of matter corresponding to the original quantity of antimatter, provided that the elements in the matter also corresponded with the "elements" in the antimatter, i.e., that the atoms were equivalent but opposite.
For every known type of particle, there exists an antiparticle with exactly the same mass, but with the opposite electric charge. When a particle and its antiparticle come together, they can always annihilate to form gamma rays. The antiparticle of an electrically neutral particle is sometimes the same as the original particle (e.g., photons) and sometimes it is distinct (e.g., neutrons).
Particles predicted by combining the theories of special relativity and quantum mechanics. For each particle, there must exist an antiparticle with the opposite charge, magnetic moment and other internal quantum numbers (e.g., lepton number, baryon number, strangeness, charm, etc.), but with the same mass, spin and lifetime. Note that certain neutral particles (such as the photon and π0) are their own antiparticles.
Particles with identical mass and spin as those of ordinary matter, but with opposite charge. Antimatter has been produced experimentally, but little of it is found in nature. Why this should be so is one of the questions that must be answered by any adequate theory of the early universe.
Peculiar A-type stars ("magnetic" A stars) that show abnormally strong lines, sometimes of varying intensity, of certain ionized metals. Recent evidence indicates that all Ap stars are slow rotators compared with normal A stars.
Star with spectral type A in which the lines of one or several elements are abnormally enhanced. Traditionally the most important subgroups are Si λ4200, Hg-Mn and Cr-Eu-Sr stars. The latest objects of the latter group correspond to early F-type.
Stars can be classified according to their surface temperatures, which determine, in large part, the spectrum of radiation they emit. A stars have surface temperatures between about 7,500 and 11,000 degrees centigrade. Peculiar A stars are A stars whose emitted radiation spectra have many of the characteristics of A stars but are peculiar in certain ways.
Successful US lunar exploration program in which the Apollo spacecraft 1 to 6 were unmanned; 7 to 10 were manned but did not land; and 11, 12 and 14 to 17 landed and returned safely. (Apollo 13 was an aborted mission.) The first men to land on the Moon were Neil Armstrong and Edwin Aldrin, from Apollo 11, on 20 July 1969. The final Apollo flight (17) lasted from 7 to 19 December 1972, and left a considerable quantity of exploratory devices on the lunar surface.
A measure of how bright a star looks in the sky. The brighter the star, the smaller the apparent magnitude. A star that is one magnitude brighter than another (e.g., +1 versus +2) looks 2.5 times brighter. The brightest star of all, of course, is the Sun, whose apparent magnitude is -26.74, followed by Sirius, whose apparent magnitude is -1.46, Canopus (-0.72), Alpha Centauri (-0.27), Arcturus (-0.04), and Vega (+0.03). Stars of the Big Dipper are fainter, most of them around magnitude +2. On a clear, dark night, the unaided eye can see stars as faint as apparent magnitude +6, and the largest telescopes penetrate to apparent magnitude +30.
Measure of the observed brightness of a celestial object as seen from the Earth. It is a function of the star's intrinsic brightness, its distance from the observer, and the amount of absorption by interstellar matter between the star and the observer. The mv, of Sun, -26.5 mag. A sixth-magnitude star is just barely visible to the naked eye.
A beautiful orange star that is the brightest in the constellation Bootes and the fourth brightest in the night sky. It lies 34 light-years away and is a member of the thick-disk population. Historically, Arcturus is famous because it was one of the first stars to have its proper motion measured.
Physics as promulgated by Aristotle; includes the hypothesis that our world is comprised of four elements, and that the universe beyond the moon is made of a fifth element and so is fundamentally different from the mundane realm.
In the orbit of a solar-system body, the point where the body crosses the ecliptic from south to north: for a star, out of the plane of the sky toward the observer.
A sparsely populated grouping (mass range 102-103 Msun) of very young, massive stars lying along a spiral arm of the Milky Way, whose spectral types or motions in the sky indicate a common origin. The star density is insufficient for gravitation to hold the group together against shear by differential galactic rotation, but the stars have not yet had time to disperse completely. OB associations are composed of stars of spectral types O-B2; T associations have many young T Tauri stars. The internationally approved designation for associations is the name of the constellation followed by an arabic numeral - e.g., Perseus OB2.
A small planet-like body of the solar system, <e> ≃ 0.15, <i> ≃ 9°.7 . More than 1800 have been catalogued, and probably millions of smaller ones exist, but their total mass would probably be less than 3 percent that of the Moon. Their densities are poorly known (about 2.6 g cm-3), but they suggest a composition similar to carbonaceous chondrite. The bright asteroids are presumably original condensations and those fainter than about 14-15 mag are collision fragments. Asteroids and short-period comets have some orbital similarities. Also called minor planet
A small rocky body that orbits a star. In the solar system, most asteroids lie between the orbits of Mars and Jupiter. The largest asteroid is Ceres, about 900 kilometers in diameter.
Also called planetoids or minor planets, the asteroids are tiny planets most of which orbit the Sun between Mars and Jupiter. The largest - and the first discovered - is Ceres, with a diameter of 1,003 km. It is estimated that there may altogether be no fewer than 40000. A few have very elliptical orbits and cross the orbits of several other (major) planets. One or two even have their own satellites (moons).
A region of space lying between Mars (1.5 AU) and Jupiter (5.2 AU), where the great majority of the asteroids are found. None of the belt asteroids have retrograde motion.
Divination using the positions of the planets, the Sun and the Moon as seen against the stars in the constellations of the zodiac - a "science" almost as old as homo sapiens. Although at one stage in history astrology and astronomy were almost synonymous- the latter has advanced so far during the last three centuries that the two now bear little relation to each other.
The branch of astronomy that deals with measuring the positions of celestial objects, especially stars. Astrometrists measure parallaxes and proper motions, which allow astronomers to determine the distances and velocities of the stars.
The change in direction of travel (bending) of a light ray as it passes obliquely through the atmosphere. As a result of refraction, the observed altitude of a celestial object is greater than its geometric altitude. The amount of refraction depends on the altitude of the object and on atmospheric conditions.
The AU is the preferred unit for distances within the solar system. Mercury, the innermost planet, lies on average 0.39 AU from the Sun; Pluto, normally the farthest planet, lies on average 39.5 AU from the Sun.
The mean distance between the Earth and the Sun. The astronomical unit is defined as the length of the radius of the unperturbed circular orbit of a body of negligible mass moving around the Sun with a sidereal angular velocity of 0.017202098950 radian per day of 86400 ephemeris seconds. AU = 1.496 × 1013 cm ≈ 500 lt-sec.
The radius of a circular orbit in which a body of negligible mass, and free of perturbations, would revolve around the Sun in 2π / k days, where k is the Gaussian gravitational constant. This is slightly less than the semi-major axis of the Earth's orbit.
Is responsible for designing, developing, and operating data systems that support the processing, management, archiving and distribution of NASA mission data. The ADF serves three broadly-defined astrophysics disciplines: high-energy astrophysics, UV/optical astrophysics, and infrared/submillimeter/radio astrophysics. The ADF collaborates with the GSFC Laboratory for High Energy Astrophysics (LHEA) and the Laboratory for Astronomy and Solar Physics (LASP) in managing data for specific missions. The ADF staff also support the astrophysics community's access to multi-mission and multi-spectral data archives in the National Space Science Data Center (NSSDC).
The science that studies the physics and chemistry of extraterrestrial objects. The alliance of physics and astronomy, which began with the advent of spectroscopy, made it possible to investigate what celestial objects are and not just where they are.
Mantle of gases round a star planet or moon, sometimes even forming the apparent surface of the body. For a body to retain an atmosphere depends on the body's gravity, and the temperature and composition of the gases. Mean atmospheric pressure at the surface is 10330 kg/m2, and is also referred to as atmosphere.
An optical device usually comprising two thin prisms which can rotate to compensate for the elongation of a star image caused by the wavelength dependence of the refractive index of air.
Decrease in the intensity of light from a celestial body due to absorption and scattering by the Earth's atmosphere. The extinction increases from the zenith to the horizon and affects short wavelengths more than long wavelengths, so that objects near the horizon appear redder than they are at the zenith.
The total number of protons and neutrons in an atom's nucleus. For example, oxygen-16 has a mass number of sixteen, because it has eight protons and eight neutrons.
The masses of atoms and molecules are generally given in atomic mass units. These units are based on a scale in which the mass of carbon 12 is taken to be 12. Atomic masses were originally given as atomic weights on a scale where the mass of the hydrogen atom was unity, later they were based on oxygen or oxygen 16; these scales have all been replaced by the carbon 12 scale.
A radiationless quantum jump that occurs in the X-ray region. When a K-electron is removed from an atom and an L-electron drops into the vacancy in the K-shell, the energy released in the latter transition goes not into radiation, but into the liberation of one of the remaining L-electrons.
Light radiated by ions in the Earth's atmosphere, mainly near the geomagnetic poles, stimulated by bombardment by energetic particles ejected from the Sun (see solar wind). Aurorae appear about 2 days after a solar flare and reach their peak about 2 years after sunspot maximum.
Spectacular array of light in the night sky, caused by charged particles from the Sun hitting the Earth's upper atmosphere. The aurora borealis is seen in the north of the Northern hemisphere; the aurora australis in the south of the Southern.
A phenomenon occurring when a discrete double-excitation state of an atom lies in the ground-state continuum. In the autoionization process one of the excited electrons is ejected, leaving the ion in an excited state (see dielectronic recombination; see also Auger effect). (also called pre-ionization)
The number of atoms in 12 grams of carbon 12. The number of atoms in a gram-atom (mass in grams numerically equal to the atomic weight) or the number of molecules in a gram-molecule (mass in grams numerically equal to the molecular weight).
A hypothetical spin-0 particle with a very small mass of 10-5-10-3 eV. It was postulated in order to provide a natural solution to the "strong CP problem".
Angular distance from the north point eastward to the intersection of the celestial horizon with the vertical circle passing through the object and the zenith.
Blue-white star of spectral type B whose spectra are characterized by absorption lines of neutral helium which reach their maximum intensity at B2. The Balmer lines of hydrogen are strong, and lines of singly ionized oxygen and other gases are also present.
A clearing in the dust clouds of the constellation Sagittarius where astronomers can view stars in the Galactic bulge. Baade's window lies four degrees south of the Galactic center, so an observer's line of sight passes within 1800 light-years of the Milky Way's center.
A model for the Galaxy first published by John Bahcall and Raymond Soneira in 1980. In its original form, it sought to reproduce star counts in different parts of the sky by employing only a (thin) disk and a halo; it had no thick disk.
The sudden decrease in the intensity of the continuous spectrum at the limit of the Balmer series of hydrogen at 3646 Å, representing the energy absorbed when electrons originally in the second energy level are ionized.
The conspicuous sharp boundary which usually occurs at the head of a molecular band and which fades gradually toward either longer or shorter wavelengths, depending on the quadratic relation between frequency and rotational quantum number.
Peculiar low-velocity, strong lined red-giant stars of spectral types G, K, and M, with abnormally large abundances of heavy s-process (but not r-process) elements. They are usually regarded as old disk stars of ~ 1-2 Msun.
A faint binary star with the second largest proper motion known. Long-term observations of its light curve suggest a possible third component with a mass about 1.2 that of Jupiter, although this observation has been challenged.
(in Hubble's (1936) classification, SB: in Morgan's classification, B) A spiral galaxy whose nucleus is in the shape of a bar, at the ends of which the spiral arms start.
Irregular variables of spectral type B (or occasionally O or A) with hydrogen emission lines in their spectra. The Be phenomenon involves rapid stellar rotation, circumstellar shells, and mass loss.
Dwarf Cepheid in which two or more almost identical periods exist which cause periodic amplitude fluctuations in their light curves. The "beat" period averages about 2 to 21/2 hours.
Silvery-white, lustrous, relatively soft metal which is unaffected by air or water, even at red heat. Rare and fragile element. Nuclear reactions in stars destroy it. Most and possibly all beryllium originated when cosmic rays smashed into heavier atoms in space and split them into lighter ones, such as beryllium.
One of several quantities tabulated for the calculation of accurate predictions of an eclipse or occultation for any point on or above the surface of the Earth.
The period of one complete revolution in right ascension of the fictitious mean sun, as defined by Newcomb. The beginning of a Besselian year, traditionally used as as standard epoch, is denoted by the suffix ".0". Since 1984 standard epochs have been defined by the Julian year rather that the Besselian year. For distinction, the beginning of the Besselian year is now identified by the prefix B (e.g., B1950.0).
A small group of short-period pulsating variables lying slightly above the upper main sequence. They have a doubly periodic light curve, and are confined within a narrow band of the H-R diagram which lies near the end of core hydrogen-burning stars of roughly 10-20 Msun. beta Cephei itself has at least three components.
A class of eclipsing binary whose secondary minima are intermediate between those of Algol and those of W UMa. The prototype beta Lyr (B8.5 II, F V) is a complex eclipsing system and is presently in a state of rapid mass transfer. The spectrum of one companion is invisible; it may be a black hole. Beta Lyrae is also a weak radio source.
Emission of an electron and an antineutrino (or a positron and a neutrino) by a radioactive nucleus by any one of several processes. e.g., the spontaneous β-decay of a free neutron (n → p + e- + ν bar). The A-number is unchanged, but the Z-number is increased (or decreased) by 1. Beta-decay is a so-called weak interaction. Since electrons of all energies (up to a certain maximum) are emitted in β-decay, this process exhibits a continuous spectrum (unlike α-particle emission, which exhibits a line spectrum).
Spontaneous emission by a heavier element (such as uranium) of negatively charged electrons - beta particles. The result of this radioactive decay is that the original element is very gradually converted into another element. Beta particle emission may be simultaneous with alpha particle decay.
The disintegration of an atomic nucleus, in which an electron (which was historically called a beta particle) and an antineutrino are emitted. Since the electron carries away one unit of negative charge, the final nucleus has a charge one greater than the initial nucleus.
The process in which a neutron disintegrates into a proton, an electron, and an antineutrino. The escaping electron is sometimes called a beta ray. (See neutrino; neutron; proton.)
A general class of cosmological models that assume the universe is homogeneous and isotropic on large scales and that allow the universe to evolve in time. Most calculation in the standard big bang model assume a Friedmann cosmology. (See Friedmann equation; homogeneity; isotropy.) A cosmological model that has the same properties as a Friedmann model under some conditions is said to have a Friedmann limit.
A homogeneous, isotropic model of the Universe involving nonstatic (i.e., expanding or contracting) solutions to Einstein's field equations (with zero cosmological constant) calculated by the Russian mathematician A. Friedmann in 1922.
A model of the Universe which started with an initial singularity. The Friedmann model of a homogeneous, isotropic universe (composed of adiabatically expanding matter and radiation, as a result of a primeval explosion) is the standard example.
An evolutionary model of cosmology in which the universe began about 10 billion years ago, in a state of extremely high density and temperature. According to this model, the universe has been expanding, thinning out, and cooling since its beginning. It is an observational fact that distant galaxies are all moving away from our own galaxy, as predicted by the big bang model. (See closed universe; flat universe; Friedmann models; open universe.)
Model of cosmic history in which the universe begins in a state of high density and temperature, both of which decrease as the universe expands. Less a theory than a school of theories that attempt to trace how the universe evolved.
One of three standard Big Bang models that were formulated by Friedmann and Lemaitre of an isotropic and homogeneous universe composed of expanding matter and radiation. In these models space is unbounded.
The most widely accepted theory of the origin of the Universe. It asserts that the Universe began some 1010 years ago from a space-time point of infinite energy density (a singularity). The expansion of the Universe since that time is akin to the expansion of the surface of an inflating balloon: every point on the balloon's surface is moving away from every other point. So, microbes living on the surface see their two-dimensional world expanding, yet there is no center to the expansion which is everywhere uniform.
Theory originally proposed by Georges Lemaitre but elaborated by George Gamow and the α-β-hypothesis-γ that the Universe began with the Big Bang, the superexplosion of all the matter now dispersing in the Universe. Since the nuclear physics involved has been explained, and various supporting evidence - notably helium abundance and the sources of radio emission - has been discovered, the theory is almost universally accepted (although at one time the steady state theory rivaled it in popularity).
If the universe has a mass density exceeding the critical mass density, then gravity will eventually reverse the expansion, causing the universe to recollapse into what is often called the big crunch. See also closed universe.
One hypothesized future for the universe in which the current expansion stops, reverses, and results in all space and all matter collapsing together; a reversal of the big bang.
A system of two stars orbiting around a common center of gravity. Visual binaries are those whose components can be resolved telescopically (i.e., angular separation > 0'.5) and which have detectable orbital motion. Astrometric binaries are those whose dual nature can be deduced from their variable proper motion; spectroscopic binaries, those whose dual nature can be deduced from their variable radial velocity. At least half of the stars in the solar neighborhood are members of binary (or multiple) systems. (See photometric binaries; optical pairs.)
The energy required to break up a system. In particular, the binding energy of an atomic nucleus is the energy released in the formation of the nucleus. The most strongly bound nuclei are those with atomic weights between about 50 and 65 (the iron group). Lighter nuclei are less strongly bound because of their larger surface-to-volume ratios; heavier nuclei, because the effects of Coulomb repulsion increase with the nuclear charge.
A highly variable object (the most rapid radio variable known, also an optically violent variable - mv = 12 to 15 mag - and an infrared source). Probably an exceedingly compact nonthermal object, and undoubtedly extragalactic. Its optical spectrum is characterized by an absence of lines, so its redshift cannot be measured. (In 1974 Oke and Gunn infer z = 0.07 from an Hβ absorption line in the surrounding halo and conclude that it lies at the center of a bright (Mv = - 23) elliptical. If true, this would make BL Lac the nearest known quasar.)
A member of a class of astronomical objects with the following characteristics: (1) rapid variations in intensity at radio, infrared, and optical wavelengths; (2) energy distributions such that most of the energy is emitted at infrared wavelengths; (3) absence of discrete features in low-dispersion spectra; and (4) strong and rapidly varying polarization at visual and radio wavelengths.
The final stage in the evolution of a star of roughly 1 Msun. It is a mass of cold, electron-degenerate gas, and can no longer radiate energy, because the whole star is in its lowest energy state. No black dwarfs have ever been observed. Also, an object (M < 0.085 Msun) that is not massive enough to achieve nuclear chain reactions.
A gravitationally collapsed mass inside the Schwarzschild radius (q.v.), from which no light, matter, or signal of any kind can escape. A black hole occurs when the escape velocity of a body becomes the velocity of light (2GM / R = c2). If an object with the mass of the Sun had a radius of 2.5 km, it would be a black hole. Black holes represent one of the possible endpoints of stellar evolution for stars very much more massive than the Chandrasekhar limit.
A mass that is sufficiently compact that not even light can escape its intense gravity. Thus it appears black from the outside. If the sun were compressed to a sphere about four miles in diameter, it would become a black hole. It is believed that some massive stars, after exhausting their nuclear fuel, collapse under their own weight to form black holes.
A singularity in space, surrounded by an event horizon, caused by the collapse of a small but massively dense star through the effects of its own increasing gravity. By the time the state of singularity is reached, the remnants of the star may be minimal, but the gravitational force is so strong it prevents even light from escaping. Black holes may form the "power centers" of galaxies, thus explaining infrared radiation detected in several galactic centres. The properties of matter entering a black hole are the theme of John Wheeler's no hair theorem.
An object with such a strong gravitational field that even light cannot escape. Matter can fall into a black hole, but according to classical physics no matter or energy can leave it. (Hawking has used quantum theory to show that black holes emit blackbody radiation, but the effect is significant only for black holes much smaller than those that are expected to form by the collapse of stars, which have masses of several solar masses or more.)
Population II star (B3-A0) in the galactic halo, characterized by strong, sharp hydrogen lines and large Balmer jump, and very weak lines of all other elements.
A peculiar eclipsing binary (B2-B3) in the Trapezium, with a flat-bottomed light curve suggesting a total eclipse. The spectrum of the secondary has never been seen.
A compact, spherical dark nebula that absorbs radiation. Estimates of their mass suggest that their density is too low for gravitational collapse. They tend to lie in regions of much dust but less gas than would be expected for star-forming regions.
A measure of the total amount of energy radiated by a star at all wavelengths. Mbol of Sun = 4.72 mag. The fraction of total energy emitted by a very blue or very red star that lies in the visible range may differ from the total energy by 4 or 5 mag - i.e., only a few percent of the energy lies in the visible.
Non-metal with several forms - the most common form is a dark amorphous powder, unreactive to water, acids and alkalis. Rare and fragile element. Nuclear reactions in stars destroy it. Most boron is created in space, by cosmic rays that smash into heavier atoms and split them.
Nucleus of even A-number (i.e., those with integral spin) (cf. Fermi-Dirac nuclei). Bose-Einstein nuclei do not obey the exclusion principle, and their ground state has zero angular momentum.
A class of elementary particles whose spin is an integer multiple of a fundamental quantized value. The major function of bosons is to mediate the fundamental forces. The best-known boson is the photon.
A subatomic particle whose spin is an integral multiple of h bar (cf. fermion). Bosons include the photons, the pions, the gravitons, and all Bose-Einstein nuclei. Boson number is not conserved.
Elementary particles that have integral spins. Force particles such as the photon, gluon, and vector bosons are all bosons. But note that there can also exist composite particles formed out of collections of fermions such as a helium atom-which act collectively as bosons.
Elementary particles with integer spin that do not obey the Pauli exclusion principle. They include the photons and the W and Z particles, carriers of the electromagnetic and the electroweak forces respectively.
A mechanism first discovered by Bowen which explains the anomalously strong lines of O III in the spectra of some planetary nebulae as fluorescence involving the radiative excitation of the 2p3d3Po2 level of O2+ (54.71 eV) from the 2p2 3P2 state in the ground term by He II Lyman-α photons (54.17 eV).
Peculiar B stars whose spectra show a deficiency in helium and in which the lines of one or several elements are abnormally enhanced. Traditionally the most important subgroups are Si λ4200, Hg-Mn and Cr-Eu-Sr stars. The latest objects of the latter group correspond to early F-type.
Glancing angle between an incident X-ray beam and a given set of crystal planes for which the secondary X-radiation from the planes combines to give a single reflected beam.
Any of the extended objects that arise in string theory. A one-brane is a string, a two-brane is a membrane, a three-brane has three extended dimensions, etc. More generally, a p-brane has p spatial dimensions.
Radiation emitted or absorbed when a free electron is accelerated in the field of an atomic nucleus but remains in a hyperbolic orbit without being captured. Since bremsstrahlung is not quantized, photons of any wavelength can be emitted or absorbed. (Also called a free-free transition because the electron is free both before and after the transition.)
Slight changes in the frequency of radiation, caused by reflection or scattering from the high-frequency sound waves that arise from thermal vibrations of atoms in the medium.
A self-gravitating, self-luminous gaseous object which is not sufficiently massive to result in thermonuclear hydrogen fusion reactions in its core and cannot therefore be considered a star. Such objects are expected to have a mass less than 7% of the Sun's mass and represent a "missing link" between low-mass stars and gas giant planets like Jupiter (at 0.1% of the Sun's mass).
Star with too little mass to ignite its hydrogen 1 fuel. If brown dwarfs exist, they shine faint red for a time, as they convert gravitational energy into heat, and then fade and cool.
B star with weak helium lines - i.e., B stars which, if classified according to their colors, would have helium lines too weak for the classification, and which, if classified according to their helium lines, would have colors too blue for their spectral type.
Late type giant with strong bands of carbonated molecules (C2, CN, CH) and no metallic oxide bands. Formerly they were called R or N types, the R types being the hotter and the N types the cooler C stars.
The results of five intensive radio-astronomical surveys (1C, 2C, 3C, 4C and 5C) under the direction of Sir Martin Ryle and Anthony Hewish, during the l950s, 1960s and 1970s, at Cambridge.
the luminous intensity in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian
The SI unit of luminous intensity, defined as "the luminous intensity, in the perpendicular direction, of a surface of 1/600000 square meter of a blackbody at the temperature of freezing platinum under a pressure of 101325 newtons per square meter."
The sixth brightest star in the night sky, consists of two yellow giants. A spectroscopic triple (F8-G0 III, G5 III, M5 V) (1974 parallax 0'.079). It has a high lithium content and a nearly circular orbit. It may be an X-ray source.
The stage when a star fuses carbon into heavier elements, making neon and magnesium. Carbon burning eventually occurs in all stars born with more than eight solar masses.
A series of nuclear reactions in which carbon is used as a catalyst to transform hydrogen into helium. The carbon cycle can take place only if the necessary C and N nuclei are present, and it requires higher temperatures and is far more temperature-dependent than the proton-proton chain. The cycle yields 26.7 MeV of energy. (On the average, 1.7 MeV of this energy is carried away because of neutrino losses.)
An important nuclear fusion process that occurs in stars. Carbon 12 both initiates it and, following interactions with nuclei of nitrogen, hydrogen, oxygen, and other elements, reappears at its conclusion.
One way that a star converts hydrogen into helium. During the CNO cycle, carbon, nitrogen, and oxygen catalyze the nuclear reaction, so the total number of carbon, nitrogen, and oxygen nuclei remains the same. However, carbon and oxygen gradually get converted into nitrogen. The CNO cycle powers the hydrogen burning that occurs in main-sequence stars with more than 1.5 solar masses and in giants and supergiants of all masses.
A supernova model involving the explosive ignition of carbon in the high-density (108 - 1010 g cm-3), electron-degenerate carbon-oxygen core of a 6±2 - 7±2 Msun star by the formation and propagation of a detonation wave. A carbon-detonation supernova seems to leave no dense remnant and converts its C-O core entirely to iron.
A molecule consisting of one carbon atom and one oxygen atom. It is the most abundant interstellar molecule after molecular hydrogen and is especially useful because it radiates at radio wavelengths, so astronomers can use it to map the distribution of molecular hydrogen.
In the HD system, a rather loose category of peculiar red-giant star, usually of spectral types R and N, whose spectra show strong bands of C2, CN, or other carbon compounds and unusually high abundances of lithium. Carbon stars resemble S stars in the relative proportion of heavy and light metals, but they contain so much carbon that these bands dominate their spectra. (C2,0. The number following the comma is an abundance parameter.)
A particle that carries one of the fundamental forces between other interacting particles. For example, the carrier boson for the electromagnetic force is the photon.
Particle that acts as the transmitters of forces. The best known example is the photon, which transmits electromagnetic forces. The gluons are the transmitters of the strong interactions, and the W+, W-, and Z0 particles are the transmitters of the weak interactions. See Table 7.1 on page 120.
An optical arrangement in which light rays striking the parabolic concave primary mirror of a reflecting telescope are reflected to the hyperbolic convex secondary mirror, and re-reflected through a hole bored in the primary to a focus behind it.
Refers to a design of reflecting telescopes in which the light collected and focussed by the large concave primary mirror is refocussed by a smaller convex secondary mirror on the same axis as the primary. The refocussed beam passes through a central hole cut into the primary mirror and emerges behind the primary.
Telescope devised by Cassegrain in which an auxiliary convex mirror reflects the magnified image, upside down, through a hole in the center of the main objective mirror - i.e., through the end of the telescope itself. It was, however, no improvement on the gregorian telescope invented probably slightly earlier.
A gap between the outermost rings of Saturn. The period of a particle in Cassini's division is about two-thirds that of Janus, one-half that of Mimas, one-third that of Enceladus, and one-quarter that of Tethys.
A radio source in Cassiopeia, the strongest extrasolar source in the sky, believed to be the remnant of a Type II supernova whose light reached Earth about 1667. Optically it is a faint nebula. It has a mass of a few solar masses. It is also an extended source of soft X-rays.
A collective name for stars in which the brightness increases suddenly because of an explosive event. The class comprises supernovae, novae, recurrent novae, dwarf novae and flare stars.
A type of variable including flare stars and novae (common, recurrent, and dwarf), all of which are believed to be very close binary systems in which hydrogen-rich matter flows from a late-type star onto a hot white-dwarf primary.
Nineteenth-century hypothesis that depicted the many changes evinced by the geological record as having resulted from cataclysms occurring during a relatively brief period of history.
The doctrine that every new situation must have resulted from a previous state. Causation underlay the original atomic hypothesis of the Greeks, and was popular in classical physics. It is eroded in quantum mechanics and has, in any case, never been proved essential to the scientific world view. See chance, determinism.
The reference pole for nutation and polar motion; the axis of figure for the mean surface of a model Earth in which the free motion has zero amplitude. This pole has no nearly diurnal nutation with respect to a space-fixed or Earth-fixed coordinate system.
Projection of the Earth's equator as a line across the sky (so that to an observer actually on the equator, such a line would pass through the zenith). The directional bearing of a star is given in terms of its right ascension round the celestial equator.
Angular distance on the celestial sphere measured north or south of the ecliptic along the great circle passing through the poles of the ecliptic and the celestial object.
Angular distance on the celestial sphere measured eastward along the ecliptic from the dynamical equinox to the great circle passing through the poles of the ecliptic and the celestial object.
An imaginary sphere of arbitrary radius upon which celestial bodies may be considered to be located. As circumstances require, the celestial sphere may be centered at the observer, at the Earth's center or at any other location.
A mercury-in-glass temperature scale. The zero of the scale represents the melting point of ice and the boiling point of water is taken to be 100 degrees.
A pulsating binary X-ray source in the galactic plane. Optical component is Krzeminski's star, a B0 giant or supergiant. The X-ray component is probably a rotating neutron star of about 0.65-0.83 Msun. Cen X-3 is speeding up at a rate of about 1 part in 103-105 per year and will at this rate fall into its companion in about 1000 years.
A class of stars named after Delta Cephei which vary in brightness over a regular period of time (typically a few days). The period of change is directly related to the true, average brightness or luminosity of the star. Once the period is known the true brightness can be calculated and the distance estimated by observing the "apparent" brightness of the object as seen from Earth.
A yellow supergiant that pulsates, alternately brightening and dimming. Cepheids allow astronomers to measure distances, because the longer a Cepheid's period of variation, the greater the Cepheid's mean intrinsic brightness. To determine a Cepheid's distance, all an astronomer has to do is measure the Cepheid's period; comparing the star's mean intrinsic brightness with the star's mean apparent brightness then yields the distance. Cepheids are so bright that we can see them in other galaxies, allowing us to establish distances to entire galaxies beyond the Milky Way.
One of a group of very luminous supergiant pulsating stars. The luminosities of a Cepheid is proportional to its period, but a different P-L relation applies to each type. No Cepheid is near enough for an accurate trigonometric parallax (Polaris is the nearest). Cepheids are useful distance indicators to about 3 Mpc.
The European Laboratory for Particle Physics (formerly the Conseil Europeen pour la Recherche Nucleaire). Here, the resources of the European member nations are pooled to construct the large particle accelerators needed for high-energy experiments. The major facilities at CERN include the Super Proton Synchrotron (SPS) and the Large Electron-Positron (LEP) collider.
A limiting mass for white dwarfs. If the mass exceeds this critical mass (1.44 solar masses, for the expected mean molecular weight of 2), the load of the overlying layers will be so great that degeneracy pressure will be unable to support it, and no configuration will be stable.
The maximum mass, approximately 1.4 Msun, above which an object cannot support itself by electron degeneracy pressure; hence, the maximum mass of a white dwarf.
Mass above which the helium core of a star begins to contract (eventually to collapse altogether). The limit is now reckoned as 10 to 15 per cent of the star's total mass.
The mass limit for an isothermal core. In order to maintain its luminosity by hydrogen burning just outside the isothermal core, the star must keep a high temperature and a high pressure at the surface of the core. When the helium core exceeds about 12% of the star's total mass, the star can no longer adjust by small changes, but must drastically increase in radius and move rapidly from the main sequence.
A group of particles which differ in electrical charge but which are nearly identical in mass and other respects (such as lifetime and angular momentum) and which seem to experience identical nuclear forces.
A small photoelectronic imaging device (typically 1.5 cm square) made from a crystal of semiconductor silicon in which numerous (at least 250000) individual light-sensitive picture elements (pixels) have been constructed. Each tiny pixel (less than 0.03 mm in size) is capable of storing electronic charges created by the absorption of light. The name derives from the method of extracting the locally stored charges from each pixel which is done by transferring or "coupling" charges from one pixel to the next by the controlled collapse and growth of adjacent storage sites or "potential wells". Each "well" is formed inside the silicon crystal by the electric field generated by voltages applied to tiny, semi-transparent metallic electrodes on the CCD surface.
Highly sensitive photoelectric devices that can electronically record the intensity and point of arrival of tiny amounts of light. CCDs are placed at the receiving end of telescopes, to "take pictures" of very faint astronomical objects; they have almost completely replaced photographic plates.
The fourth flavor (i.e. type) of quark, the discovery of which in 1974 contributed both to the acceptance of the reality of quarks and to our understanding of their dynamics. The charmed quark exhibits a property called "charm" which is conserved in strong interactions.
The property that distinguishes one of the types of quarks. At present, there are six types of quarks known, one of which is the "charmed" quark. (See quark.)
Apparatus through which it is possible to observe the existence and velocity of high-speed particles important in experimental nuclear physics and in the study of cosmic radiation. It was originally built to investigate the Cherenkov radiation effect, in which charged particles travel through a medium at a speed greater than that of light in that medium.
The part of the solar atmosphere between the photosphere and the corona. It consists of two rather well defined zones: the lower chromosphere and the upper chromosphere.
Physics prior to the introduction of the quantum principle. Classical physics incorporates Newtonian mechanics, views energy as a continuum, and is strictly causal.
A homogeneous, isotropic universe is said to be temporally closed if gravity is strong enough to eventually reverse the expansion, causing the universe to recollapse. It is said to be spatially closed if gravity is strong enough to curve the space back on itself, forming a finite volume with no boundary. Triangles would contain more than 180°, the circumference of a circle would be less than π times the diameter, and a traveler intending to travel in a straight line would eventually find herself back at her starting point. If Einstein's cosmological constant is zero, as is frequently assumed, then a universe which is temporally closed is also spatially closed, and vice versa.
A universe that has a finite size. Closed universes expand for a finite time, reach a maximum size, and then collapse. In closed universes, the inward pull of gravity dominates and eventually reverses the outward flying apart of matter; that is, gravitational energy dominates the kinetic energy of expansion. The value of omega is greater than 1 for a closed universe. If a universe begins closed, it remains closed; if it begins open, it remains open; if it begins flat, it remains flat. In the big bang model of the universe, the question of whether the universe is closed, open, or flat is determined by the initial conditions, just as the fate of a rocket launched from earth is determined by its initial upward velocity relative to the strength of earth's gravitational pull. If the initial rate of expansion of the universe was lower than a critical value, determined by the mass density, the universe will expand only for a certain period of time and then collapse, just as a rocket launched with a velocity below a critical value, dependent on the strength of earth's gravity, will reach a maximum height and then fall back to earth. This is the behavior of a closed universe. If the initial rate of expansion of the universe was larger than a critical value, the universe is open and will keep expanding forever. If the initial rate of expansion was precisely the critical value, the universe is flat and will expand forever, but with a rate of expansion that approaches zero. (See flat universe; omega; open universe.)
Big Bang model that was formulated by Friedmann and Lemaitre which has a positive curvature, like the surface of a sphere, in which case the universe is finite, closed, and will eventually recollapse. This space is unbounded.
Cosmological model in which the universe eventually stops expanding and begins to collapse, presumably to end in a fireball like that of the big bang. Compare open universe.
Similar to the CNO cycle, except that it also includes a cycle in which the next-to-last step becomes 15N(p, γ) 16O(p, γ) 17F(β+ν) 17O(p, α) 14N. This reaction occurs once in about 2000 CN cycles. For main-sequence stars greater than a few solar masses, hydrogen burning by the CNO bi-cycle is the main source of energy.
A set of coordinates which do not change in an expanding (or otherwise moving) medium. i.e. the coordinates of a distant galaxy do not change just because of the expansion of space.
A scattering process that leaves atoms in the same energy state after the scattered photon departs in a direction different from that of the incident photon. The energy of the scattered photon is the same (in the rest frame of the atom) as that of the incident photon.
A model of structure formation in which an exotic particle whose energy is low at the time it decouples from other matter is responsible for structure formation.
The spherical region of diffuse gas, about 150000 km in diameter, which surrounds the nucleus (q.v.) of a comet. Together, the coma and the nucleus form the comet's head.
The nearest massive cluster of galaxies. A symmetric cluster with primarily E and S0 galaxies. Luminous mass 4 × 1014Msun = 8 × 1047 g; virial theorem mass about 5 × 1048 g; mass needed to bind the cluster about 4 × 1049 g. R ≈ 9 × 1024 cm.
A diffuse body of gas and solid particles (such as CN, C2, NH3, and OH), which orbits the Sun. The orbit is usually highly elliptical or even parabolic (average perihelion distance less than 1 AU; average aphelion distance, roughly 104 AU). Comets are unstable bodies with masses on the order of 1018 g whose average lifetime is about 100 perihelion passages. Periodic comets comprise only about 4% of all known comets. Comets are obviously related in some manner to meteors, but no meteorites from a comet have ever been recovered. Observations of comets Bennett and Kohoutek have established that a comet is surrounded by a vast hydrogen halo.
A reflection nebula with a fan shape that bears a superficial resemblance to a comet. Classical examples of the heads of cometary nebulae are R Mon, R CrA, and RY Tau. All have A0-G0 type spectra that resemble the spectrum of a T Tauri star, and their brightness varies from year to year.
A galaxy similar to an N galaxy but with no disk or nebulous background. It is an object of high surface brightness which appears slightly nonstellar on photographs and which has a larger redshift than normal stars in our Galaxy. Nearest "compact" galaxy is M32.
The phenomenon in which two bodies have the same apparent celestial longitude (see longitude, celestial) or right ascension as viewed from a third body. Conjunctions are usually tabulated as geocentric phenomena. For Mercury and Venus, geocentric inferior conjunction occurs when the planet is between the Earth and Sun, and superior conjunction occurs when the Sun is between the planet and Earth.
A quantity that remains unchanged in the course of the evolution of a dynamical system. There are seven known quantities that are conserved: energy (including mass), momentum, angular momentum (including spin), charge, electron-family number, muon-family number, and baryon-family number.
Law that identifies a quantity, such as energy, that remains unchanged throughout a transformation. All conservation laws are thought to involve symmetries.
The principle that the total energy of a closed system never changes, that energy is only converted from one form to another. This principle must be enlarged under special relativity to include mass-energy.
The total energy of a system (including kinetic energy and gravitational energy) is conserved and does not vary. Thus, kinetic energy can only increase at the expense of gravitational potential energy. Modern physics has modified the law of conservation of energy, since matter can be created or annihilated; a more general law is the conservation of mass and energy.
Important physical principle and one of the basic laws of physics stating that matter is neither created nor destroyed (although mass may become energy, the energy quantitatively represents the mass). One exception to this principle is a singularity; another follows from the theory of virtual particles.
The principle that matter is neither created nor destroyed. This principle is only approximately true, since special relativity shows that matter and energy are equivalent and interconvertible.
The property possessed by a scientific theory when it contains and extends an earlier well-supported theory; for example, general relativity is consistent with Newtonian gravity.
Precisely defined area of the celestial sphere, associated with a grouping of stars, that the International Astronomical Union has designated as a constellation.
A mass motion in a circulatory pattern. Process in the Sun (and possibly other stars) perhaps caused by solar rotation, which produces the immensely powerful electrical and magnetic fields associated with sunspots.
The view of quantum mechanics holding that prior to the measurement, a system has no physical existence and is describable only in terms of the probability of each possible result of a measurement. After a measurement the physical system exists in one and only one of its possible states.
A class of radio source characterized by an emission "halo" surrounding a more intense "core". About 20% of the known extended radio sources are of the core-halo type.
Outermost atmosphere of the Sun immediately above the chromosphere, consisting of hot (1-2 × 106 K), low-density (about 10-16 g cm-3) gas that extends for millions of miles from the Suns's surface. Ordinarily it can be seen only during a total solar eclipse. Its shape varies from almost spherical at sunspot maximum to unsymmetrical at minimum. Its high temperature is probably caused by MHD shock waves generated below the photosphere. The corona, together with solar flares, is the source of solar X-rays. It is the corona, not the photosphere, that is studied by radio astronomers, except at very short wavelengths.
An area where the extreme-ultraviolet and X-ray coronal emission is abnormally low or absent; a coronal region apparently associated with diverging magnetic fields. A great part, if not all, of the solar wind starts from coronal holes.
The average number of fermions per unit volume of space throughout the universe. Since matter is depicted in general relativity as bending space, the value of the cosmic matter density, if known, could reveal the overall curvature of cosmic space. See critical density, omega.
High-energy charged particles which stream at relativistic velocities down to Earth from space. The Sun ejects low-energy (107 - 1010 eV) cosmic rays during solar flares (those of lower energy than this are unobservable from Earth because of solar system magnetic fields). Those of intermediate energy (1010 - 1016 eV) have an isotropic distribution, and are apparently produced in the Galaxy. Possible sources of acceleration are shock waves accompanying supernovae (although cosmic rays have a higher hydrogen content than would be expected from a star that has processed material to iron), and the rotating magnetic fields of pulsars. The light elements Li, Be, and B have a higher abundance ratio in cosmic rays than in the solar system.
Long, stringlike concentrations of matter-energy that may have formed during symmetry breaking in the first moments of the big bang. If they exist, they would be candidates for the seed perturbations of structure formation.
Microscopically thin, spaghetti-like objects which, according to some theories of elementary particles, could form randomly during a phase transition in the early universe. Cosmic strings could provide the seeds for structure formation in the universe, as an alternative to the possibility that the seeds originated as quantum fluctuations during inflation.
Some contemporary cosmological theories suggest that boundaries were formed between different regions of the universe at the moment of creation. These boundaries survive today as "cosmic strings", incredibly thin but very massive strings many light years in length.
A constant introduced into Einstein's field equations of general relativity in order to provide a supplement to gravity. If positive (repulsive), it counteracts gravity, while if negative (attractive), it augments gravity. It can be interpreted physically as an energy density associated with space itself.
A contribution to gravity that results from the effective mass density, or energy density, in the vacuum. A positive cosmological constant acts as if it were negative gravity - it makes two masses repel each other instead of attract each other. Einstein's first cosmological model contained a cosmological constant, which appeared as an additional term in the equations of general relativity. (See false vacuum; vacuum.)
A parameter that determines the strength of the cosmological term in the equations of general relativity. This term was added by Einstein because he thought the universe was static, and the term provided a repulsive gravitational force that was needed to prevent the universe from collapsing under the force of ordinary gravity. The false vacuum of inflationary models creates a similar repulsive gravitational force, except that it prevails for only a brief period in the early universe. The cosmological constant is often assumed to be zero, but it might make a significant contribution to the evolution equations of our universe.
A possible third parameter in cosmology, in addition to the Hubble constant and omega (Ω). Most cosmologists believe the cosmological constant is zero, but if it is not, it would make the universe older than astronomers calculate from the Hubble constant and Ω. The size of the cosmological constant is designated by the Greek letter lambda (λ).
A term added by Einstein to the gravitational field equations of his theory of general relativity. Such a term would produce a repulsive antigravity force at very large distances and would correspond to energy locked up in the curvature of space-time itself. There is, at present, no evidence for the existence of a cosmological constant (although one may have existed in the past).
A term introduced by Einstein into his field equations of gravitation to permit a static model of the universe. It corresponded, as introduced originally, to a cosmic repulsion force that could withstand the attractive tendency of gravity.
A term sometimes employed in cosmology to express a force of "cosmic repulsion", such as the energy released by the false vacuum thought to power exponential expansion of the universe in the inflationary universe models. Whether any such thing as cosmic repulsion exists or ever played a role in cosmic history remains an open question.
Einstein's general theory of relativity allows for space-time curvature even in an empty universe. The amount of this curvature is given by the cosmological constant. Current indications are that this constant must be zero, but the reason for its vanishing remains a mystery.
The puzzle of why the cosmological constant has a value which is either zero, or in any case roughly 120 orders of magnitude or more smaller than the value that particle theorists would expect. Particle theorists interpret the cosmological constant as a measure of the energy density of the vacuum, which they expect to be large because of the complexity of the vacuum. See vacuum.
The redshift due to the expansion of the Universe. Contrary to popular belief, this is not a Doppler shift. Most galaxies move away from us, but this is not the cause of their redshifts. Instead, as a light wave travels through the fabric of space, the universe expands and the light wave gets stretched and therefore redshifted.
A focus used primarily for spectroscopy. In this arrangement light from the primary mirror is reflected along the polar axis to focus at a fixed place separate from the moving parts of the telescope, where large pieces of equipment can be fitted without interfering with the telescope's balance. (The word comes from a French word meaning "bent like an elbow", not from a man's name)
A stationary focal point in an equatorial mounted telescope obtained by an arrangement of small auxiliary mirrors in the converging beam which eventually directs the light down the hollow polar axle of the telescope.
The practical and the SI unit of charge. It is the quantity of electricity transported in one second by a current of one ampere. From 1908 to 1948 the international coulomb, derived from the international ampere, was in use. Like the other international units it was replaced by the absolute unit on 1 January 1948. The name coulomb was given to the unit at the first meeting of the IEC in Paris in 1881. At this meeting two of the five units which were given definitions were named after French scientists. These were the ampere (A. M. Ampère 1775-1836) and the coulomb (C. A. Coulomb 1736-1806). (1 international coulomb = 0.99985 absolute coulomb.)
A reaction between subatomic particles is said to be a "CP violating" reaction if the reaction produces a different result when the electrical charges of the particles are changed to their opposites and the mirror image of the particle trajectories is used.
A symmetry which is believed to hold true for all particles throughout the course of universal history. It states that matter and antimatter would only react in the same way if the spins of the antimatter particles were reversed and the reaction was caused to run backwards in time.
A theory has "CPT invariance" if for every possible reaction between subatomic particles, a reaction can also occur in which the electrical charges of the particles changed to their opposites, the mirror image of the particle trajectories is used, and the directions of motion are reversed. Assuming general notions of modern physics, all conceivable theories of nature are CPT invariant.
A chaotic, expanding mass of gas in Taurus, the remnant of a Type I supernova whose light reached Earth in 1054. It is an intense radio source, and its visible light is strongly polarized. It is also a source of X-rays and gamma-rays. Its total mass is about 1 Msun, but the total energy radiated by the Crab is 1037-1038 ergs s-1. It is periodically occulted by the Moon, and every June its radio spectrum is occulted by the solar corona.
Belief that the universe was created by God in the relatively recent past, as implied by literal interpretations of biblical chronology, and that the species of terrestrial life did not arise through Darwinian evolution but, rather, all came into existence at once.
If the cosmological constant is assumed to vanish, then the critical mass density is that density which puts the universe just on the border between eternal expansion (open universe) and eventual collapse (closed universe).
The cosmic density of matter required to "close" the universe and so, eventually to halt cosmic expansion. Its value amounts to about ten hydrogen atoms per cubic meter of space. The observed density is so close to the critical value that the question of whether the universe is open or closed has not yet been resolved by observation. See open universe, closed universe.
The density that just stops the expansion of space, after infinite cosmic time has elapsed. In the standard models, the critical density requires that the spatial geometry be flat.
The value of average cosmic mass density above which the universe is closed. The average mass density of the universe is obtained by measuring the mass in a very large volume of space, including many galaxies, and dividing by the size of the volume. The critical mass density is determined by the current rate of expansion of the universe. According to estimates of the current rate of expansion, the current critical mass density is about 10-29 grams per cubic centimeter. According to the best measurements, the average mass density of our universe appears to be about one tenth the critical mass density. (See closed universe; omega; open universe.)
Group characteristics are: strong bands of CN, outstandingly strong absorption near the Na D lines, usually sufficient structure in the 6400-6500 Å region to suggest ZrO.
Passage of a celestial object across the observer's meridian. More precisely, culmination is the passage through the point of greatest altitude in the diurnal path.
A unit of radioactivity which is now defined as the quantity of any radioactive nuclide undergoing 37.00 × 109 disintegrations per second. The unit was adopted at a Radiography Conference in Brussels in 1910 when it was defined as the radioactivity associated with the quantity of radon in equilibrium with one gram of radium. The present definition, which refers to a unit of the same size but described in terms independent of the disintegration of radon, was agreed at the Copenhagen meeting of the International Commission on Radiological Units in July 1953. The unit is named after Pierre Curie (1859-1906), one of the discoverers of radium. The curie is too large for normal laboratory work where the radioactivity is of the order of millicuries. The number of disintegrations occurring per second is called the activity of a sample and a unit for this was originally the reciprocal second but this has been superseded by the curie.
This is sometimes used for indicating temperature in the vicinity of absolute zero. It is based on Curie's law, which states that the susceptibility of a paramagnetic material is approximately proportional to its absolute temperature.
A double radio source, the third strongest radio source in the sky (after the Sun and Cas A), at one time believed to be caused by the collision of two galaxies. It has now been identified with a distant peculiar cD galaxy (z ≈ 0.056).
A supernova remnant, consisting of a large loop of gas ejected from a star. It is 100 pc above the galactic plane. (X-ray observations give a distance of 2-3 kpc.) It is a thermal bremsstrahlung source of soft X-rays with a spectral temperature of 2 × 106 K.
An X-ray source. The visible component is the ninth-magnitude supergiant HDE 226868 (O9.7 Iab). It has rapid night-to-night variations in spectral features.
An X-ray binary. It is also an infrared source, a cosmic ray source, and a strongly variable radio source (interstellar extinction is too high for visible light observations). It is best fitted by a model of an expanding cloud of relativistic electrons emitting synchrotron radiation around a neutron star.
A supergiant radio galaxy (the most common type of radio galaxy) which has an elliptical nucleus surrounded by an extended envelope. Or, an optical galaxy with a very bright nucleus. In the Morgan classification, a galaxy with rotational symmetry but without pronounced spiral or elliptical structure (a dustless galaxy). In the Yerkes 1974 system a galaxy with an elliptical-like nucleus surrounded by an extensive envelope (see also R galaxy).
The massive outer region of the Milky Way that surrounds the disk and stellar halo. The dark halo consists mostly of dark matter, whose form is unknown. Though it emits almost no light, the dark halo outweighs the rest of the Galaxy.
Material astronomers cannot see but whose presence they believe in either because they detect its gravitational influence or because certain theories predict its existence. For example, astronomers believe that the outer part of the Galaxy harbors dark matter, because they notice its gravitational influence on the stars they can see; and inflationary cosmologists believe that the universe is full of dark matter, because inflation predicts that the universe has a large density.
Matter in the universe that we detect by its gravitational influences, yet do not see. Dark matter that has small random speed and is easily concentrated by gravity is called cold dark matter. Dark matter that has large random speed and is thus able to resist gravitational clumping is called hot dark matter. Recent models to explain the observed pattern of galaxy clustering can be characterized, in part, as to whether they invoke hot dark matter or cold dark matter. However, since we do not know what the dark matter is, we do not have any direct evidence of whether it is cold or hot.
Matter that is detected only by its gravitational pull on visible matter. At least 90%, and possibly 99% of the matter in the universe is dark. The composition is unknown; it might consist of very low mass stars or supermassive black holes, but big-bang nucleosynthesis calculations limit the amount of such baryonic matter to a small fraction of the critical mass density. If the mass density is critical, as predicted by the simplest versions of inflation, then the bulk of the dark matter must be a gas of weakly interacting non-baryonic particles, sometimes called WIMPS (Weakly Interacting Massive Particles). Various extensions of the standard model of particle physics suggest specific candidates for the WIMPs.
Matter whose existence is inferred on the basis of dynamical studies - e.g., the orbits of stars, in galaxies - but which does not show up as bright objects such as stars and nebulae. Its composition is unknown: It might consist of subatomic particles, or of dim dwarf stars or black holes, or a combination of various sorts of objects.
Matter whose presence is inferred from dynamical measurements but which has no optical counterpart. The luminous regions of galaxies have mass-luminosity ratios of about 10. However, the mass-luminosity ratio in the outer halos of many spiral galaxies is 100 or more; one sees the brightness fall off with distance from the center of the galaxy but considerable mass is present. A similar situation prevails in galaxy clusters, where nonluminous matter must provide most of the self-gravitational attraction that holds the clusters together. The missing mass is not really missing; it is present but invisible (at least to current detectors). It is generally believed to consist either of the remnants of massive stars or of planetary-sized objects comparable in mass to Jupiter.
A relatively dense cloud of interstellar matter whose dust particles obscure the light from stars beyond it and give the cloud the appearance of a region devoid of stars.
One of a small number of dumbbell-shaped radio galaxies. They might be called D systems with double nuclei, in which two elliptical nuclei share a common extended envelope.
A particular solution to Einstein's cosmological equations, found by Wilhelm de Sitter in 1917, in which space expands at a rapid, exponential rate. This solution was very different from the solutions of Friedmann and of Lemaitre, in which the universe expands at a much slower rate (a rate with the distance between any two points increasing as something between the square root of time and linearly with time). The Friedmann and Lemaitre type solutions became incorporated in the standard big bang model. Recent modifications of the big bang model, such as the inflationary universe model, propose that the universe went through a period of exponential growth, or a de Sitter phase, early in its evolution.
A dimensionless quantity describing the rate at which the expansion of the Universe is slowing down because of self-gravitation: it gives a measure of the matter density. In Friedmann's equation (which describes many cosmological models) q0 = - 1 indicates a steady-state universe, q0 < +1/2 indicates an open universe, q0 = +1/2 indicates a flat Euclidean universe, and q0 > 1/2 indicates a universe that is decelerating and will eventually contract. Sandage and Tammann (1975) obtain q0 = 0.10 for H0 = 55 km s-1 Mpc-1.
A parameter that measures the rate of slowing down of the expansion of the universe. Gravity causes the slowing down. The deceleration parameter equals omega (another cosmological parameter) when the universe is dominated by radiation, approximately the first 100000 years after the big bang, and 1/2 omega when the universe is dominated by matter. Since the deceleration parameter is equivalent to omega (assuming a cosmological constant of zero, as often done), it determines the ultimate fate and spatial geometry of the universe. The deceleration parameter is often denoted by the symbol q0. (See omega.)
Quantity designating the rate at which the expansion of the universe is slowing down, owing to the braking effect of the galaxies' gravitational tug on one another. It is a function of the cosmic matter density.
Angular distance above (positive) or below (negative) the celestial equator. One of the co-ordinates, with right ascension, that defines the position of a heavenly body.
Angular distance north (+) or south (-) of the celestial equator to some object, measured in degrees, minutes, and seconds of arc along an hour circle passing through the object. Declination is analogous to latitude on the Earth's surface.
Angular distance on the celestial sphere north or south of the celestial equator. It is measured along the hour circle passing through the celestial object. Declination is usually given in combination with right ascension or hour angle.
Astronomical coordinate. Equivalent to latitude. The angle in degrees above or below the Celestial Equator, i.e. the projection onto the sky of the Earth's equator. Range of declination is from from zero to ± 90°.
A gas of electrons (or, more generally, fermions) in which all the lowest quantum states are occupied. For such a gas, the pressure in the nonrelativistic limit is proportional to the 5/3 power of the density.
A state of matter found in white dwarfs and other extremely dense objects, in which strong deviations from classical laws of physics occur. As the density increases at a given temperature, the pressure rises more and more rapidly, until it becomes independent of temperature and dependent on density alone. At this point, the gas is said to be degenerate.
A state of matter found in white dwarfs and other ultrahigh-density objects, in which the electrons follow Fermi-Dirac statistics. According to the classical laws of physics, the pressure of a gas is proportional to the temperature and the density. However, in 1926 Fermi and Dirac showed that if the density were high enough, departures from classical laws would occur, in that if at a given temperature the density is increased, the pressure increases more and more rapidly until it becomes independent of the temperature and is a function of the density only. When this point is reached, the gas is said to be degenerate.
Time lapse between the time a signal (e.g., a radar beam) is propagated out to a distant object and the time it is received after the object bounces it back.
An object's mass divided by its volume. Cotton has a low density; lead has a high density. Red giants have a low density; white dwarfs have a high density.
The mean density of a celestial body is generally reckoned as its mass divided by its volume, expressed either in comparison with the density of water, in kilograms per cubic meter, or in relation to some other known density. The mean density of the Earth is thus 5.5 times that of water, i.e. 5.5 × 103 kg m-3 and is just less than four times that of the Sun. Yet the mean density of rocks at the surface is about half the overall mean value, and that of the Earth's central core is perhaps 2 1/2 times the overall value.
Spiral structure is modelled as a small-amplitude wave propagating with fixed angular velocity, as the compression wave goes through, it triggers star formation on the leading edge of the spiral arms.
The German national laboratory for high-energy physics, located near Hamburg. It is the home of the e+e- storage rings DORIS and PETRA, and the electron-proton machine, HERA.
A rare heavy isotope of hydrogen. Believed to be the first compound nucleus formed in the infant universe. It was discovered in interstellar space in 1965. Because deuterium is quickly destroyed in nuclear reactions, one view is that most of the deuterium in the universe is primordial.
Of a stellar cluster or galaxy, the "orbiting" of stars nearer the center faster than those at the edge. Of a single body (such as the Sun or a gaseous planet), the axial rotation of equatorial latitudes faster than polar latitudes.
A property which distinguishes wave-like motions. When a wave is incident upon a barrier which is broken by a narrow slit (of comparable size to the wavelength), then the slit will act as a new isotopic source of secondary waves.
A wave-like property of light which allows it to curl around obstacles whose size is about that of the wavelength of the light. The disturbed waves then interfere to produce ripple-patterns.
A system of parallel slits, where the slit width is of the same order as the wavelength of the incident radiation, which is capable of dispersing light into its spectrum.
An optical device containing thousands of very fine parallel grooves which produce interference patterns in a way which separates out all the components of the light into a spectrum.
Polished metallic surface (usually a metallic mirror on a block of glass or quartz) on which has been ruled a great number (in thousands) of parallel lines, used to split light to produce a spectrum.
For orbital motion in the solar system, motion that is counterclockwise in the orbit as seen from the north pole of the ecliptic; for an object observed on the celestial sphere, motion that is from west to east, resulting from the relative motion of the object and the Earth.
Change in the apparent wavelength of radiation (e.g., light or sound) emitted by a moving body. A star moving away from the observer will appear to be radiating light at a lower frequency than if at rest; consequently, lines in the star's spectrum will be shifted toward the red (lower frequency) end of the spectrum. The existence of a direct relationship between the redshift of light from galaxies and their distances is the fundamental evidence for the expansion of the universe.
Displacement of spectral lines in the radiation received from a source due to its relative motion along the line of sight. A motion of approach results in a blueshift; a motion of recession results in a redshift.
Displacement of spectral lines in the radiation received from a source due to its relative motion in the line of sight. Sources approaching (-) the observer are shifted toward the blue; those receding (+), toward the red. The Doppler shift makes it possible to determine the radial velocity and the rotation of stars.
Effect on the wavelengths of light (or sound) emitted by a source at a distance that is increasing or decreasing in relation to the observer. If the distance is increasing, the wavelengths are "stretched" (the light received shifts towards the red end of the spectrum; sound received goes down in pitch). If the distance is increasing, the wavelengths are "squeezed" (the light received shifts towards the blue end of the spectrum; sound received goes up in pitch).
The apparent change of frequency or wavelength of radiation from an object due to its motion toward or away from us. If the object is receding the frequency is decreased and the wavelength is increased, i.e. becomes red-shifted.
The blueshift or redshift produced by an object's motion toward or away from us. If a star moves toward us, its light waves get compressed and its spectrum is blueshifted; if a star moves away from us, its light waves get stretched and its spectrum is redshifted. The Doppler shift allows astronomers to measure the radial velocities of stars. The Doppler shift is not responsible for the redshifts that most galaxies exhibit; that is a cosmological redshift.
The shift in the received frequency and wavelength of a sound wave or electromagnetic wave that occurs when either the source or the observer are in motion. Approach causes a shift toward shorter wavelengths and higher frequencies, called a blueshift. Recession has the opposite effect, called a redshift.
A radio galaxy, the bulk of whose radio emission comes from two sources on opposite sides of the visual galaxy. The radiation is presumably the result of an explosion in the nucleus of the parent galaxy, which caused the ejection at high speed of energetic particles in two opposite directions. About one-third of all known radio galaxies are double sources.
A "system" of two stars that appear - because of coincidental alignment when viewed from Earth - to be close together; it is, however, an optical effect only, and therefore not the same as a binary star system (although until the twentieth century there were few means of distinguishing double and binary stars).
A slow nova which also happens to be an eclipsing binary. It also has a regular flickering period of 71 seconds, the shortest period of regular variations known, except for pulsars and compact X-ray objects. It is probably composed of an M dwarf and a white dwarf with an accretion disk.
Star, like the Sun, that fuses hydrogen into helium at its core. Ninety percent of all stars are main-sequence stars; examples are Sirius, Vega, Altair, and Alpha Centauri A, B, and C.
A short-period binary system consisting of a hot white dwarf (or a hot blue sdBe subdwarf) and a much cooler and slightly more massive late-type main-sequence companion which fills its Roche lobe and is ejecting mass onto the white dwarf through its inner Lagrangian point. (The light from dwarf novae comes from four sources: a white dwarf, a cool main-sequence star, a hot spot, and a disk.) The outbursts are usually assumed to be caused by the explosive nuclear burning of hydrogen-rich material accreted onto the surface of a degenerate star.
The ascending node of the Earth's mean orbit on the Earth's equator; i.e., the intersection of the ecliptic with the celestial equator at which the Sun's declination is changing from south to north.
The "parallax" (i.e., distance) for a binary star whose orbit is well known, derived by using the mass-luminosity relation and Newton's generalization of Kepler's third law.
The physics that explains how particles and systems move under the influence of forces. The dynamical laws of a theory give a quantitative statement of the response of a particle to an applied force.
A unit of force equal to the force necessary to give an acceleration of 1 cm sec-2 to a mass of 1 gram. 1 dyne of force is roughly equivalent to 1 mg of weight.
A hypothetical group of nuclear reactions by which the iron group is assumed to be synthesized. At temperatures > 5 × 109 K and densities > 3 × 106 g cm-3 there are great numbers of collisions between high-energy photons and nuclei. These collisions break up the nuclei, the fragments of which promptly combine with other particles. Thus, there is in effect an equilibrium between formation and breakup. Since the iron group has the largest binding energies, the particles over the long run will tend to be trapped in these nuclei. The e-process (the e stands for equilibrium) is presumed to occur in a supernova explosion.
A measure of how round or elliptical an orbit is. A perfect circle has an eccentricity of 0 percent, and an extremely elliptical orbit has an eccentricity of just under 100 percent. The Sun has an orbital eccentricity of 6 percent, which means that at perigalacticon the Sun is 6 percent closer to the Galactic center than its mean distance and at apogalacticon the Sun is 6 percent farther from the Galactic center than its mean distance.
In astronomy, the extent to which an elliptical orbit departs from a circular one. It is usually expressed as a decimal fraction, regarding a circle as having an eccentricity of 0.
The amount by which the elliptical orbit deviates from circularity: e = c/a, where c is the distance from the center to a focus and a is the semimajor axis.
Occultation of one celestial body by another which passes between it and the observer. The solar eclipse is caused by the passing of the Moon between the Sun and the Earth in this way; such an eclipse may be complete (total) or incomplete (partial). Eclipsing binary stars also accord with this pattern. Alternatively - and exceptionally - a lunar eclipse is caused by the passage of the Earth between the Sun and the Moon, so that the Earth's shadow falls across the Moon, again either totally or partially, depending upon the position of the observer.
A binary star of which, from the viewpoint of Earth, one of the two bodies regularly passes in front of the other. The resulting variation is perceived luminosity of some eclipsing binaries has led to their classification as variable stars.
Eclipsing variables whose orbital plane lies so nearly in the line of sight that eclipses, as seen from the Earth, can occur and can be detected from their light curves.
Apparent linear path through the 12 constellations of the zodiac that the Sun seems to take during one Earth year, also representing therefore the "edge" of the plane of Earth's orbit. Because the equator of the Earth is at an angle of more than 22° to the plane of its orbit, the ecliptic is at the identical angle to the celestial equator, intersecting it at two points: the vernal and autumnal equinoxes.
Plane of the Earth's orbit. (Strictly speaking, the ecliptic is a mathematical fiction corresponding not to the actual plane of the Earth's orbit, but to one with all minor irregularities smoothed out.)
An approximation used in the study of radiative transfer. It is the assumption that the ratio of the second moment of the radiation field to the mean intensity is everywhere equal to 1/3, the value of this ratio for an isotropic field.
In essence, radiation pressure must not exceed gravity. It is the limit beyond which the radiation force on matter in the emitting region is greater than the gravitational forces that hold the star together. LE = 4πcGM/Ks, where Ks = Thomson and/or Compton scattering opacity. Eddington limit for a 1 Msun star, 1038 ergs s-1.
A stellar model in which energy is transported by radiation throughout the whole star and the ratio of the radiation pressure to the gas pressure is assumed to be constant.
A cosmological model in which the cosmological constant plays a crucial role by allowing an initial phase that is identical to the Einstein static universe. After an arbitrarily long time, the universe begins to expand. The difficulty with this model is that the initiation of galaxy formation may actually cause a collapse rather than initiate an expansion of the universe.
The temperature that a blackbody would have which emitted the same amount of energy per unit area as the star does: it is a temperature characteristic of the surface region. Teff of Sun, 5800 K.
A collision between two particles which conserves the total kinetic energy and momentum of the system. For atomic collisions it is one involving energy less than the excitation potential of the atom.
Particle reactions in which the same particles emerge from the reaction as entered it (e.g. π- p → π- p). In inelastic scattering, where different and/or new particles emerge, energy is used to create new particles.
Fundamental force of nature that acts on all electrically charged particles. Classical electromagnetics is based on Maxwell's and Faraday's equations, quantum electromagnetics on the theory of quantum electrodynamics (QED).
One of the four fundamental forces of nature. Electricity and magnetism arise from the electromagnetic force. The other three fundamental forces are the gravitational force, the weak nuclear force, and the strong nuclear force.
The force between charged particles, which accounts for electricity and magnetism. One of the four fundamental forces of nature, it is carried by photons and is responsible for all observed macroscopic forces, except for gravity.
The phenomena associated with electrical and magnetic forces. Electrical and magnetic forces are intimately related, since a changing electric field produces a magnetic field, and vice versa. Electromagnetic waves are an example of electromagnetism.
A negatively charged spin-1/2 particle, which interacts via the electromagnetic, weak and gravitational forces. It has a mass of 0.511 MeV / c2, some 1800 times lighter than the proton.
A stable, negatively charged elementary particle - the lightest massive particle known. The classical electron radius is 2.82 × 10-13 cm; me = 9.1 × 10-28 g = 5.48597 × 10-4 amu. The electron family (see lepton) includes the electron e-, the positron e+, the electron neutrino νe, and the electron antineutrino νebar. Rest-mass energy of electron 8.186 × 10-7 ergs. Electron charge = 1.60219 × 10-19 coulombs.
Light elementary particle with a negative electrical charge. Electrons are found in shells surrounding the nuclei of atoms; their interactions with the electrons of neighboring atoms create the chemical bonds that link atoms together as molecules.
Negatively charged fundamental particle (also called a beta particle) found in the atoms of all elements, where it "orbits" (at different energy levels and with different directions of spin) round the central nucleus. The combined charge of the orbiting electrons is balanced (in a neutral atom) by the charge of an equal number of positively charged protons in the atomic nucleus. An electron is also the fundamental unit of electricity.
An imaging device containing a thin target material which emits electrons by the photoelectric effect when illuminated and then magnetically focuses these electrons to impact onto a silicon CCD where they generate a large charge.
Unit employed to indicate the energy of a charged particle in terms of the energy received by the charge on an electron due to a potential difference of one volt. An approximate value (1 in 104) for the energy of electromagnetic radiation expressed in electron volts is given by 1234 / λ, where λ is the wavelength in nanometres. In recent years it has become customary to write MeV and GeV for mega (106) and giga (109) electron volts. In the USA 109 electron volts are often written as BeV, the letter B being used in this case as an abbreviation for the American billion (109), but in 1948 the International Union of Pure and Applied Physics disapproved of the use of BeV and expressed a preference for GeV or 109 eV. The electron volt was called the equivalent volt when it was originally introduced in 1912.
Unit of energy used in atomic and nuclear physics; the kinetic energy acquired by one electron in passing through a potential difference of 1 volt in vacuum. Sometimes used as a unit of mass (see rest-mass energy)
The theory that unifies the electromagnetic force and the weak nuclear force into a single force. This theory was developed in the 1960s by Sheldon Glashow, Steven Weinberg, and Abdus Salam and has been subsequently confirmed in the laboratory. One of the mathematical properties of this theory is called the electroweak symmetry.
The unified description of the weak interactions and electromagnetism, developed between 1967 and 1970 by Sheldon Glashow, Steven Weinberg, and Abdus Salam.
Theory demonstrating links between the electromagnetic and the weak nuclear forces. Indicates that in the high energies that characterized the very early universe, electromagnetism and the weak force functioned as a single, electroweak force. Also known as the Weinberg-Salam theory.
Different elements are distinguished by the number of protons in their nuclei. All hydrogen atoms have one proton; all helium atoms have two protons; all oxygen atoms have eight protons.
The building block of matter. The nucleus of an atom consists of one or more protons and may contain neutrons as well; any electrons surround the nucleus. The number of protons in the atom - the atomic number - determines the element.
The fundamental unit of a chemical element. An atom consists of a nucleus, which may contain protons and neutrons, and electrons, which occupy shells that surround the nucleus and are centered on it.
Abstract mathematical relationships that relate elementary particles together and allow them to be grouped into families. A particular symmetry transformation has the effect of, in a theoretical way, transforming one elementary particle into another.
The angle in degrees above the horizon toward the zenith or overhead point. Sometimes loosely called the "altitude" of a star, but not to be confused with height above sea level. Elevation angle is 90° minus the zenith distance (or zenith angle).
The angular distance of a celestial body above or below the horizon, measured along the great circle passing through the body and the zenith. Elevation is 90 deg. minus zenith distance.
Promising higher-dimensional supergravity theory developed in the 1970s, subsequently ignored, and more recently shown to be an important part of string theory.
A galaxy with an ellipsoidal shape, without spiral arms. Ellipticals have little interstellar matter and no blue giants - the only giants are red, and they give ellipticals a slightly redder color than spirals. Ellipticals apparently produce only Type I supernovae.
The process of transition of an electron from an outer orbit to an inner orbit around the nucleus results in a characteristic amount of energy being radiated (as line emission) that corresponds to the lost energy of the electron.
The comet with the shortest known period (3.30 years) (a = 2.21 AU, e = 0.847, i = 12°.4). It has been observed at every apparition since its discovery in 1819. Its period is gradually decreasing. Named after J. F. Encke, who computed its orbit. (It was discovered by Pons.)
Any of the several discrete states of energy in which an atom or ion can exist. For example, an orbital electron can exist only in those energy levels that correspond to an integral number of deBroglie wavelengths in a Bohr atom.
A means of accounting for the apparent motions of the planets in terms of circular motions in a geocentric cosmology. Each planet moves in a circle, the center of which moves in a circle of larger radius, and so on, the largest circles being centered on the earth.
Circular orbit of a body round a point that is itself in a circular orbit round a parent body. Such a system was formulated to explain some planetary orbits in the Solar System before they were known to be elliptical.
A date and time that specifies the reference system to which celestial coordinates are referred. Prior to 1984 coordinates of star catalogs were commonly referred to the mean equator and equinox of the beginning of a Besselian year. Beginning with 1984 the Julian year has been used, as denoted by the prefix J, e.g., J2000.0.
An arbitrary fixed instant of time or date used as a chronological reference datum for calendars (see calendar), celestial reference systems, star catalogs, or orbital motions (see orbit).
An eclipsing binary with an invisible supergiant companion. The primary is an extremely luminous A8 Ia supergiant of 30 Msun in a post-main-sequence stage of evolution; the secondary may be a collapsed star or black hole. It has at least six components.
In 1973 van de Kamp announced that this star has a planet-like object in orbit around it at a distance of about 8 AU and with a period of about 25 years.
The great circle on the surface of a body formed by the intersection of the surface with the plane passing through the center of the body perpendicular to the axis of rotation. (See celestial equator.)
The classic type of telescope mount with one axis parallel to the Earth's polar axis (i.e. pointing at the celestial pole) and the other at right angles. Once the object is located, only the polar axis need be driven by a motor to counteract the Earth's rotation.
Either of the two points on the celestial sphere at which the ecliptic intersects the celestial equator; also the time at which the Sun passes through either of these intersection points; i.e., when the apparent longitude (see apparent place; celestial longitude) of the Sun is 0° or 180°. (See catalog equinox; dynamical equinox for precise usage.)
One of two points in the sky that represent where the Sun appears to cross the plane of the Earth's equator. From the Earth's viewpoint therefore, the Sun reaches one point at a quarter, the other at three quarters of the way through the sidereal year: the vernal (spring) equinox is thus on or around 21 March, the autumnal on or around 22 September. The actual points in the sky change slightly every year through a process called precession.
Core principle of general relativity declaring the indistinguishability of accelerated motion and immersion in a gravitational field (over small enough regions of observation). Generalizes the principle of relativity by showing that all observers, regardless of their state of motion, can claim to be at rest, so long as they acknowledge the presence of a suitable gravitational field.
In a freely falling and nonrotating laboratory the laws of physics, including their numerical content, are the same everywhere including gravity-free space.
The statement that a gravitational force is completely equivalent in all of its physical effects to an overall acceleration in the opposite direction. For example, a person in an elevator in space accelerating upward at 32 feet per second per second would feel the floor pushing upward against her feet in exactly the same way as if the elevator were at rest on earth, where gravity pulls downward with an acceleration of 32 feet per second per second. The "weak equivalence principle," which is not as strong as the equivalence principle, states that all objects, independent of their mass or composition, fall with the same acceleration in the presence of gravity. The Eötvös experiment, and later refinements of this experiment, have proven the weak equivalence principle.
The speed at which an object can leave another object behind, without being recalled by its gravitational force. The escape velocity of Earth - which must, for instance, be attained by a spacecraft if it is to reach another planet - is 25,000 miles per hour.
The velocity that a body requires to achieve a parabolic orbit around its primary (Ve = sqrt(2GM/R)). Escape velocity at Earth's surface is 11.2 km s-1; of Moon, 2.4 km s-1; of Sun, 617.7 km s-1 (cf. orbital velocity).
Essentially an optical filter that operates by multiple-beam interference of light reflected and transmitted by a pair of parallel flat reflecting plates.
Two identical fermions cannot occupy the same quantum state (i.e. cannot have the same charge, spin, momentum, quantum numbers etc. within the same region of space).
A theory of galaxy formation wherein the explosion of a large number of stars creates a giant shock wave that travels outward and compresses the surrounding gas. Galaxies form in the regions of high-density gas.
The nucleosynthesis processes that are believed to occur in supernovae. Explosive carbon burning occurs at a temperature of about 2 × 109 K and produces the nuclei from neon to silicon. Explosive oxygen burning occurs near 4 × 109 K and produces nuclei between silicon and calcium in atomic weight. At higher temperatures, still heavier nuclei, up to and beyond iron, are produced.
The nucleosynthetic processes which are thought to occur in supernovae. These explosive processes are thought to produce the nuclei from neon up to and including the e-process nuclei and possibly the r-process nuclei. Explosive carbon burning occurs for a temperature of about 2 × 109 K and a density of 104-107 g cm-3 and produces nuclei from neon to silicon. Explosive oxygen burning occurs for a temperature of about 4 × 109 K and produces nuclei from silicon to calcium, and the e-process occurs at a temperature greater than 5 × 109 K and produces the iron peak nuclei.
An expansion described by a fixed doubling time. The size doubles after one doubling time, quadruples after two doubling times, octuples after three doubling times, etc.
Extremely rapid expansion. "Exponential" is a mathematical term that precisely defines the rate of expansion. For example, a balloon that doubles its size every second is expanding exponentially. By contrast, a balloon whose radius is one inch after one second, two inches after two seconds, three inches after three seconds, and so on, is expanding linearly with time, rather than exponentially. According to the inflationary universe model, the early universe went through a brief period of exponential expansion, during which its size increased enormously.
The outer part of the solar corona which emits a continuous spectrum in which absorption lines can be seen. The F corona is caused by radiation from the photosphere scattered by interplanetary dust, and it decreases slowly with distance from the Sun. (the F stands for Fraunhofer)
A type of interferometer wherein the beam of light is passed through a series of pairs of partly reflecting surfaces set at various angles to it and spaced at certain prechosen numbers of the wavelength to be examined. It differs from the Michelson interferometer in that it has only one arm.
Bright areas on the face of the Sun, commonly in the vicinity of sunspots. Named by Johannes Hevelius, they are thought to be caused by luminous hydrogen clouds close to the photosphere.
A temperature scale based on three fixed temperature points - the temperature of an ice and salt mixture, the freezing point of water and normal human temperature - which were taken to be 0, 32 and 96 respectively. It is mere coincidence that the temperature interval between the freezing (32 °F) and boiling (212 °F) points of water is 180° when expressed in the Fahrenheit scale.
The maximum energy of any particle in a group of low-temperature subatomic particles called fermions. Fermions, such as electrons, cannot occupy the same space at the same energy. Thus, if many fermions are placed close together, their energies must all be different. The energy of that particle with the largest energy is the Fermi energy of the system.
Home of the Tevatron, the world's most powerful accelerator, a p p bar collider with a maximum collision energy of 1.8 Te V (= 1800 Ge V = 1.8 × 1012 eV).
Nuclei of odd A-number (i.e., nuclei that do not have integral spin) (cf. Bose-Einstein nuclei). Fermi-Dirac nuclei therefore obey the exclusion principle (q.v.).
An elementary particle whose spin is a half-integral multiple of h/2π. Fermions include the baryons, the leptons and their antiparticles, and obey the Pauli exclusion principle (cf. boson).
Particle with half-integral spin. Fermions obey the exclusion principle, which says that no two fermions can exist in an atom in the same quantum state; in practice this restricts the number of electrons, which are fermions, permitted in each electron shell.
Splitting of spectral lines by the spin-orbit energy - i.e., the potential energy of the inherent electron magnetic moment in the atom's own magnetic field.
A "coupling constant," e2 / h bar c, approximately 1/137, that measures the strength of an interaction between a charged particle and the electromagnetic field. It gives a rough measure of the relative importance of relativistic and spin effects in the spectra of atoms.
A coupling constant e2/ h bar c, approximately equal to 1/137 (where e is the electron charge, h bar is Plancks constant, and c is the speed of light), that measures the strength of the interaction between a charged particle and the electromagnetic field.
A parameter that measures the strength of the electromagnetic force. The fine-structure constant is a combination of other fundamental constants of nature - the electrical charge of the electron, the speed of light, and Planck's constant of quantum mechanics.
A phase transition which occurs in a manner similar to the way water boils. Bubbles of the new phase (steam) form in the midst of the old phase (water), so that temporarily the two distinct phases (steam and water) coexist.
In nuclear physics, the splitting of the atomic nucleus of a heavy element, resulting in the emission of nuclear energy and possibly causing a chain reaction (with similar results) within a mass of the element.
A member of a class of dwarf stars (usually dM3e-dM6e) that show sudden, intense outbursts of energy. The flares are usually rare and very short with mean amplitudes of about 0.5-0.6 mag. All known flare stars are intrinsically faint and have emission lines of H I and Ca II. It is generally believed that flares in flare stars have certain properties in common: rapid rise to peak light followed initially by a rapid decline and later by a slower phase that occasionally does not return to a preflare level within practical monitoring times (several hours). An increase in radio emission is often detected simultaneously with the optical outburst. About 30 flare stars are known, all within 20 pc. (In at least one theory, the flare star stage directly follows the T Tauri stage.)
Dim red dwarf star that suddenly lights up with great - but brief - luminosity, corresponding to an equally powerful but short-lived burst of radio emission. The cause is thought to be a sudden and intense outburst of radiation on or above the star's surface.
Stars undergoing erratic jumps in brightness (up to a few magnitudes) on time scales of the order of minutes. During the quiescent phase the spectrum is that of an M dwarf with emissions in the CaII and Balmer lines.
A cosmological model in which a static (neither expanding nor collapsing) universe is maintained by introducing a cosmological repulsion force (in the form of the cosmological constant) to counterbalance the gravitational force.
A homogeneous world model (the simplest relativistic model) of finite density, zero curvature, and nonzero cosmological constant, subject to the field equations of general relativity in an expanding Euclidean space. The radius increases rapidly from zero, and, although it always increases, the rate of increase becomes less as time goes on.
A homogeneous, isotropic universe is called flat if it is just on the borderline between being spatially closed and spatially open, so the geometry is precisely Euclidean. If Einstein's cosmological constant is zero, then a flat universe will go on expanding forever, but the velocity of recession between any two objects would approach zero at large times.
A Universe in which there is no curvature to the spacetime continuum. This means that the kinetic energy of the expansion is exactly balanced by the potential gravitational energy of the matter. Thus, after an infinite amount of time the Universe will stop expanding.
A universe that is at the boundary between an open and closed universe. In a flat universe, the average mass density always has precisely the critical value. A flat universe has zero total energy and an infinite size. Flat universes have the geometry of an infinite, flat surface, that is, Euclidean geometry. The value of omega is 1 for a flat universe. (See closed universe; critical mass density; Euclidean geometry; omega; open universe.)
A problem of the traditional big bang theory (without inflation) related to the precision required for the initial value of omega, the ratio of the actual mass density to the critical mass density. If the description is started at one second after the big bang, for example, omega must have been equal to one to an accuracy of fifteen decimal places, or else the resulting universe would not resemble our own. Yet the traditional big bang theory offers no explanation for this special value, which must be incorporated as an arbitrary postulate about the initial conditions. See also horizon problem.
The puzzle of why the universe today is so close to the boundary between open and closed, that is, why it is almost flat. Equivalently, why should the average mass density today be so close to the critical mass density, but not exactly equal to it? If omega begins bigger than 1, it should get bigger and bigger as time goes on; if it begins smaller than 1, it should get smaller and smaller. For omega to be near 0.1 today, about 10 billion years after the big bang, it had to be extraordinarily close to 1 when the universe was a second old. Some people consider such a fine balance to have been highly unlikely according to the standard big bang model, and thus are puzzled as to why the universe today is almost flat. (See closed universe; critical mass density; flat universe; open universe.)
The known quarks exist in six different types, or flavors: up, down, charmed, strange, top, and bottom. The up and down quarks belong to the first generation, the charmed and strange quarks belong to the second, and the top and bottom quarks belong to the third. The up, charmed, and top quarks each have an electrical charge 2/3 that of a proton, while the down, strange, and bottom quarks have a charge -1/3 that of a proton. See Table 7.1 on page 120.
The absorption of a photon of one wavelength and reemission of one or more photons at longer wavelengths, especially the transformation of ultraviolet radiation into visible light.
The emission of light at one wavelength, the green say, following absorption of light with a much shorter wavelength such as the ultraviolet. The UV photon parts with its energy by ejecting the electron into a high-energy level from which it cascades back down, releasing photons of lower energy and therefore longer wavelengths. A material which has this property is called a phosphor.
An optical component or system for changing the image scale of a telescope to achieve a better match between the seeing disk and the pixel size. See optical matching.
Spectral line emitted from a metastable state (q.v.), or those which have a very low probability (10-9-10-10) of occurrence. They appear at particle densities ≤ 108 cm-3. All forbidden lines have low excitation potentials. Forbidden lines are designated by enclosing them in brackets, e.g., [O II].
Agency responsible for a change in a system. In Newtonian mechanics, gravitational force bends the moon away from the straight trajectory it would otherwise pursue.
An astronomical instrument comprised of four highly sensitive photoelectric cells (CCDs). The four-shooter is placed at the end of a telescope and used to electronically record incoming light.
A geometric figure in which a pattern is repeated ad infinitum on smaller and smaller scales. A classic example is Von Koch's snowflake, for which the construction begins with an equilateral triangle. Trisect each side, and replace the middle section by two sides of a smaller equilateral triangle, bulging outward. The snowflake is obtained by repeating this process for each side of the resulting figure, then for each side of the subsequent figure, and continuing forever.
A pattern that repeats itself or nearly repeats itself on many different scales of magnification. For example, suppose that some ink on a piece of paper appears to form a star. If you look at the piece of paper with a magnifying glass, you see that the dark areas are not solid black, but are formed of tiny stars themselves. If you look at one of these small stars with a microscope, you see that the dark areas of each of the tiny stars is formed from an arrangement of even tinier stars. Such a repeating pattern of stars would be called a fractal.
A CCD construction in which one half of the imaging area of the device is purposely covered with a mask opaque to light to provide a temporary charge storage section.
Absorption line in the spectrum of the Sun, studied by Fraunhofer in 1814. The nine most prominent he labeled with capital letters (from the red end) A. B. C. D. E. F. G. H. and K. The A band and B band are now known to be groups of telluric lines due to O2 absorption in Earth's atmosphere, and C and F are respectively known as Hα and Hβ.
The number of oscillations or wave cycles per second passing a given point. For electromagnetic radiation, the product of the frequency and the wavelength is the speed of light.
The number of peaks (often called crests) of a propagating wave that cross a given point in a unit of time. For example, if 1000 peaks cross a given point in one second, one says that the frequency is 1000 cycles per second or 1000 hertz.
In nuclear physics, the combining of the atomic nuclei of lighter elements to form nuclei of a heavier element. Such a process involving the atomic nuclei of elements lighter than iron is accompanied by the emission of energy; for fusion of heavier elements, energy must be supplied. The process is thought to contribute to the condensation of stars from interstellar gas and dust. See also nuclear fusion.
Process by which the Sun (and other stars) radiates energy. The nucleus of an atom fuses with the nuclei of other atoms to form new, heavier atoms at the same time releasing large amounts of energy. In the Sun, hydrogen atoms are converted into helium by this process, with carbon and nitrogen as intermediates. Cooler stars undergo the proton-proton cycle with a similar result.
Yellowish star in which the H and K lines of Ca II have become dominant and in which a tremendous profusion of spectral lines of both neutral and ionized metals, particularly iron, begins to show. The Balmer lines of hydrogen are still recognizable. Examples are the Sun and Capella.
A system of coordinates based on the mean plane of the Galaxy, which is inclined about 63° to the celestial equator. Galactic latitude (b) is measured from the galactic equator north (+) or south (-); galactic longitude (l) is measured eastward along the galactic plane from the galactic center. In 1958, because of increased precision in determining the location of the galactic center, a new system of galactic coordinates was adopted, with the origin at the galactic center in Sagittarius at α(1950) = 17h42m.4, δ(1950) = - 28°55'. The new system is designated by a superior roman numeral II (i.e., bII, lII) and the old system by a superior roman numeral I: lII ≈ lI + 32°.31. Galactic coordinates are independent of precession.
The angle between the line of sight to a star and the Galactic plane. Galactic latitude ranges from +90 degrees to -90 degrees; the Galactic plane has a Galactic latitude of 0 degrees. Regions north of the Galactic plane have positive Galactic latitude; regions south have negative Galactic latitude. The point with a Galactic latitude of +90 degrees is called the north Galactic pole, and the point with a Galactic latitude of -90 degrees is called the south Galactic pole.
A measure of a star's position with respect to the Sun and Galactic center. Galactic longitude ranges from 0 degrees to 360 degrees. Imagine the Sun at the center of a giant clock, with the Galactic center located in the direction of six o'clock. A Galactic longitude of 0 degrees would correspond to the direction of six o'clock, a Galactic longitude of 90 degrees to the direction of three o'clock, a Galactic longitude of 180 degrees to the direction of twelve o'clock, and a Galactic longitude of 270 degrees to the direction of nine o'clock.
In the innermost region of a galaxy, there is often a concentration of stars and gas, sometimes extending over thousands of light-years from the center of the galaxy.
The plane that contains the disk of the Milky Way. By definition, one direction perpendicular to this plane is called "above" or "north", and the opposite direction, also perpendicular to the Galactic plane, is called "below" or "south". From Earth, due Galactic north is marked by the north Galactic pole, which lies near the bright star Arcturus, and due Galactic south is marked by the south Galactic pole, which lies in the faint constellation Sculptor.
Either of the two points in the sky where we look perpendicular to the disk of the Milky Way. The north Galactic pole is the Galactic pole located above the disk; the south Galactic pole is the Galactic pole located below the disk.
The revolving of a galaxy round its central nucleus even as it continues its proper motion. Such rotation, however, is not uniform but differential. One revolution of the Sun within our own Galaxy takes about 225 million years, or 1 cosmic year.
A huge collection of millions, billions, or trillions of stars. When referring to the Milky Way, "galaxy" is capitalized, otherwise not; thus: "Andromeda is the nearest giant galaxy to the Galaxy".
A large (108-1013Msun), gravitationally bound aggregate of stars and interstellar matter. Galaxy formation is currently believed to have occurred around z ≈ 3-4.
A large aggregation of stars, bound together gravitationally. There are three major classifications of galaxies - spiral, elliptical, and irregular - and several subclassifications. The sun belongs to a spiral galaxy, the Milky Way galaxy.
An isolated aggregation of stars and gas, held together by their mutual gravity. A typical galaxy has about 100 billion stars, has a total mass equal to about a trillion times the mass of the sun, is about 100,000 light years in diameter, and is separated from the nearest galaxy by a distance of about 100 times its own diameter. Thus, galaxies are islands of stars in space. Our galaxy is called the Milky Way. Galaxies come in two major shapes: flattened disks with a central bulge, called spirals, and amorphous, semispherical blobs, called ellipticals. If galaxies are found bunched up next to each other, they are said to lie in groups or clusters. Clusters with a particularly large number of galaxies in them are called rich clusters. Galaxies that do not lie in such groups but rather seem to be scattered uniformly and randomly through space are called field galaxies. Some galaxies are characterized by the dominant type of radiation they emit.
Vast system of celestial objects, typically consisting of between 106 and 1012 stars, plus interstellar gas and dust. There are three basic types: spiral (further subdivided into normal spirals and spirals with a "bar" at the centre, and yet further subdivided according to the "openness" of the spiral arms), elliptical (subdivided according to ellipticity) and irregular (subdivided according to whether they are made up of Population I or Population II stars). Another not uncommon type of galaxy is a lenticular form mid-way between the spiral and the elliptical.
An aggregate of galaxies. Bautz and Morgan divide them into three morphological types: type I contains a supergiant cD galaxy; Coma is type II, type III contains no members significantly brighter than the general bright population. Virgo is type III. 21 known X-ray sources are associated with clusters of galaxies.
An electromagnetic wave with a wavelength in the range of 10-13 to 10-10 meter, corresponding to photons with energy in the range of 104 to 107 electron volts.
Electromagnetic radiation similar to X-radiation, although of shorter wavelength, emitted spontaneously by some radioactive substances from atomic nuclei during radioactive decay.
Electromagnetic radiation with a wavelength less than about 1 Å (10-10 m); blends from the "hard" X-ray region. Photons of energy greater than about 10 keV.
Photon of very high frequency (wavelength shorter than a few tenths of an angstrom); the most energetic form of electromagnetic radiation, although there is no sharp boundary between γ-ray photon and an X-ray photon. Usually γ-ray photons come from the nucleus and X-ray photons come from the inner orbital electrons. Galactic γ-ray photons seem to originate primarily in the spiral arms.
Short, intense, low-energy bursts, first recorded by the Vela satellite system on 1967 July 2. Their isotropic distribution suggests an extragalactic origin, but a galactic disk origin cannot be ruled out: there is a large increase in γ-ray flux in the direction of the galactic center.
The systematic breakup of a gas cloud into smaller and smaller subunits as the gas cools and continues to collapse. The gravitational forces continually overtake the opposing pressure gradients as long as the cloud is able to radiate freely; consequently, the Jeans mass decreases, and fragments divide into smaller subfragments. The process stops only when opacity intervenes to inhibit the cooling and radiation.
An H II region, a supernova remnant, or a planetary nebula. H II regions have an emission-line optical spectrum, and a thermal continuous spectrum declining in intensity as the wavelength increases (from maximum in the ultraviolet) through infrared and radio. Supernova remnants have an emission-line optical spectrum and a nonthermal radio spectrum. Temperatures of planetary nebulae are much higher than those of H II regions.
A property of modern theories of physics, formulated in the 1960s and confirmed by experiment. A symmetry, in general, is a property that allows a system to behave in the same way even though it has undergone some change. For instance, a snowflake has a 6-sided "rotational symmetry" - a snowflake appears identical after every 60 degree rotation. A gauge symmetry is something like a rotation, in which the amount of rotation can vary randomly from one point of space to the next. (See field theory.)
Symmetry principle underlying the quantum-mechanical description of the three nongravitational forces; the symmetry involves the invariance of a physical system under various shifts in the values of force charges, shifts that can change from place to place and from moment to moment.
A theory whose dynamics originate from a symmetry. That is, the formulae describing the theory (in particular, the Lagrangian) are unchanged under certain symmetry transformations, called "gauge" transformations. For example, the equations of classical electrodynamics are invariant under local redefinitions of the electrostatic potential. This symmetry is ultimately responsible for the conservation of electric charge. However, in quantum electrodynamics this gauge symmetry is reinterpreted as invariance under local redefinitions of the phase of the electron wave function.
In 1973 David Gross, Frank Wilczek, and David Politzer showed that these theories possess a property called asymptotic freedom, just what was needed for a theory of how quarks bind to form protons and neutrons. The new theory, dubbed quantum chromodynamics or QCD, proposed that the color of the quarks acts as the charge of the Yang-Mills interactions.
A random distribution of initial conditions is often referred to as a Gaussian distribution. Also, a certain kind of bell-shaped curve is called a Gaussian.
A statistical distribution defined by the equation p = c exp(-k2x2), in which x is the statistical variable. It yields the familiar bell-shaped curve. Accidental errors of measurement and similar phenomena follow this law.
The constant defining the astronomical system of units of length (astronomical unit), mass (solar mass) and time (day), by means of Kepler's third law. The dimensions of k2 are those of Newton's constant of gravitation: L3M-1T-2.
Faint oval patch of light visible from Earth only at certain times of the year, opposite the Sun. Its nature and cause are still not known. It is sometimes known as "counterglow".
The sum of the lunisolar and the planetary precession. It causes the ecliptic longitude to increase at a constant rate but has no effect on ecliptic latitude.
The latitude and longitude of a point on the Earth's surface relative to the center of the Earth; also celestial coordinates given with respect to the center of the Earth.
The outermost part of Earth's atmosphere, a hydrogen halo extending out to perhaps 15 Earth radii, which emits Lyman-α radiation when it is bombarded by sunlight.
The latitude and longitude of a point on the Earth's surface determined from the geodetic vertical (normal to the specified spheroid). (See zenith; latitude, terrestrial; longitude, terrestrial.)
1. A star that has evolved off the main sequence and is roughly a hundred times as luminous as the Sun. Giants can be of any color, but yellow, orange, and red giants are the most common. 2. A planet much more massive than Earth. The solar system has four giant planets, all far from the Sun: Jupiter, Saturn, Uranus, and Neptune.
A tightly packed, symmetrical group of thousands of very old (pure Population II) stars. The stellar density is so great in the center that the nucleus is usually unresolved. The stars within a globular cluster orbit each other because of their mutual gravity.
Theoretical particles made exclusively of gluons. Tentative evidence of the existence of glueballs had been found in accelerator experiments by the mid-1980s.
Gluons are the massless gauge bosons of QCD which mediate the strong color force between quarks. Because of the non-Abelian structure of the theory, gluons can interact with themselves, and may form particles consisting of gluons bound together. The existence of these "glueballs" has yet to be confirmed.
Quanta that carry the strong nuclear force. Like photons, vector bosons, and gravitons - the carriers respectively of electromagnetism, the weak force, and gravitation - gluons are massless bosons. Consequently, for simplicity's sake, some physicists lump together all the force-carrying quanta under the term "gluons".
The force-carrying particles associated with the strong interactions, the forces which bind quarks inside of protons and neutrons. For more details, see Yang-Mills theories.
A speculative class of theories of particle interactions, first developed in 1974, which attempt to describe electromagnetism, the weak interactions, and the strong interactions in a fully unified theory. Of the known forces, only gravitation is omitted.
An attempt to produce a unification of all the forces of nature. While some success was made in unifying the gluon force between quarks with the electroweak force, problems always arose when gravity was included. Grand unification eventually gave way to superstring theory.
Class of theories that purport to reveal identities linking the strong and electroweak forces. The differences between these forces in nature today is attributed to the breaking of symmetrical relationships among force-carrying particles as the very early universe expanded and cooled.
Convective cell in the solar photosphere. A granule represents a temperature roughly 300° higher than the surrounding dark areas. At any one time, granules cover about one-third of the solar photosphere.
The sudden collapse of a massive star when the radiation pressure outward is no longer sufficient to balance the gravitational pressure inward. In gravitational collapse there is a sudden, catastrophic release of great quantities of gravitational potential energy, and this release has been postulated as the cause of supernovae, neutron stars, and black holes.
A galaxy that intervenes between us and a distant astronomical object and that gravitationally deflects the light from that distant object. (Light, like matter, is attracted by gravity.) Gravitational lenses can focus, distort, and split light beams in the same way that ordinary glass lenses do.
Deflection of electromagnetic radiation from a distant background source by a strong gravitational field associated with a foreground source resulting in more than one image of the original source. Many double-quasars are produced by this phenomenon.
Gravitational effect that bends a ray of light. Such an effect was predicted within the general theory of relativity, although previously considered impossible.
The apparent path of a photon is altered from a straight line by the gravitational field of the Sun. The path is deflected radially away from the Sun by up to 1".75 at the Sun's limb. Correction for this effect, which is independent of wavelength, is included in the reduction from mean place to apparent place.
The bending of light caused by the gravity of an object lying between us and the light source. This may cause the light source to look brighter than it normally does.
The effect of matter in curved spacetime, which tends to focus any beam of radiation from a distant source. In effect, the spacetime curvature is a lens of great focal length. At z ≈ 1, the angular size of an object starts increasing with distance.
Energy that a body can acquire by falling through a gravitational field and that decreases as the kinetic energy increases. There is no general reference level (analogous to the state of rest of a body in defining kinetic energy), and so we customarily define the change in gravitational potential energy as the negative of the work done by the gravitational forces during the bodys change of position.
When we lift a weight from the floor to a tabletop, we clearly put energy into it. The energy is not lost, however, because we can retrieve it by allowing the weight to fall back to the floor. While the weight is on the table, we say that the energy is stored as gravitational potential energy. The energy is stored in the gravitational field.
The rate at which a clock keeps time when it is in a gravitational field is slower than the rate at which it will keep time in the absence of a gravitational field. (The gravitational redshift was experimentally verified by Pound and Rebka in 1960.) The amount of redshift is directly proportional to the mass of the emitting body and inversely proportional to its radius.
A hypothetical elementary particle associated with the gravitational interaction. It is a stable particle with zero rest mass, zero charge, and a spin of ± 2, and travels with the speed of light.
A massless spin-2 particle which is the hypothetical quantum of the gravitational field. It mediates the force of gravity in a similar way to that in which the spin-1 gauge bosons (i.e. the photon, W±, Z0, and gluons) mediate the other forces.
The quanta thought to convey gravitational force; analogous to the photons, gluons, and intermediate vector bosons of electromagnetism and the strong and weak nuclear forces. Predicted by quantum theory of gravity, gravitons have not yet been detected.
As described first by Isaac Newton, gravity is a force that exists between bodies of any mass whatever (from particles to stars) in proportion to the product of their masses, and in inverse proportion to the square of the distance between them. The weakest of the four natural forces (the other three being the electromagnetic and the two nuclear interactive forces), its real nature is still not fully understood. Einstein's General Theory of Relativity presented another viewpoint.
Fundamental force of nature, generated by all particles that possess mass. Interpreted by means of Newtonian mechanics or by the general theory of relativity.
In Aristotelian physics, an innate tendency of the elements earth and water to fall. In Newtonian physics, the universal, mutual, attraction of all massive objects for one another; its force is directly proportional to the mass of each object, and decreases by the square of the distance separating the objects involved.
In Einstein's general relativity, gravity is viewed as a consequence of the curvature of space induced by the presence of a massive object. In quantum mechanics the gravitational field is said to be conveyed by quanta called gravitons.
The mutual attraction between any two masses, as was first described accurately by Newton. Gravity appears strong because it has infinite range and it is always attractive (except for a false vacuum), but on a subatomic level gravity is the weakest of the known interactions; the gravitational force between a proton and an electron is 2 × 1039 times weaker than the electrical attraction.
The weakest of the four fundamental forces of nature, the gravitational force between any two masses is proportional to the product of the masses and varies inversely as the square of the distance between them. The other three fundamental forces are the electromagnetic force and two kinds of nuclear forces. (See electromagnetic force; nuclear forces.)
The weakest of the four fundamental forces of nature. Described by Newton's universal theory of gravity, and subsequently by Einstein's general relativity.
A telescope used in X-ray and gamma-ray astronomy. It focuses these rays by making use of the fact that they behave like light rays if they strike surfaces at a shallow enough angle.
A split in the Milky Way between Cygnus and Sagittarius caused by a succession of large, overlapping dark clouds in the equatorial plane of the Galaxy.
A class of reflecting telescope which uses a concave secondary mirror placed after the prime focus is reached instead of a convex secondary placed before the prime focus.
Telescope devised - but never constructed - by James Gregory, in which an auxiliary concave mirror reflects the magnified image, the right way up, through a hole in the centre of the main objective mirror, i.e., through the end of the telescope itself. The Cassegrain telescope is similar but produces an inverted image.
This is a right-angled glass prism with a transmission diffraction grating deposited on the hypotenuse surface. The spectrum produced by the grating is deflected by the prism to remain on the optical axis and the apex angle of the prism is chosen to get a certain wavelength in the center of the detector. Grisms can be placed in a filter wheel.
An area of ionized hydrogen. Most H II regions are red and arise from hot blue O and B stars, whose ultraviolet light can ionize all the hydrogen for dozens or even hundreds of light-years in every direction. The most famous H II region is the Orion Nebula.
Region of ionized hydrogen in interstellar space. H II regions occur near stars with high luminosities and high surface temperatures. The kinetic temperature of H II regions is about 10,000-20,000 K, and the density is about 10 atoms per cm3. Ionized hydrogen, of course, having no electron, does not produce spectral lines; however, occasionally a free electron will be captured by a free proton and the resulting radiation can be studied optically (see also radio recombination lines).
The Big Bang era when the Universe was matter-dominated, containing many hadrons in equilibrium with the radiation field and when kT ≈ mπ. The hadron era ended when the characteristic photon energy fell below the rest mass of a pion or π-meson (270 electron masses), and very few hadrons remained (about one hadron for every 108 photons).
The angle across the main lobe of an antenna pattern between the two directions where the sensitivity of the antenna is half the value at the center of the lobe. This is the nominal resolving power of the antenna system.
Probably the best known of all comets. Its orbit was computed by Edmund Halley in 1704, at which time he predicted that the bright comet of 1682 would return in 1758 (Halley died in 1742, before he could see his prediction verified). Records of Halley's comet (a = 17.8 AU, e = 0.967, i = 162°.3, P = 76.2 yr perihelion distance 0.587 AU) have been traced back to 240 B.C. Last appearance 1910, next appearance 1986.
A spherical aggregation of stars, globular star clusters, and thin gas clouds, centered on the nucleus of the galaxy and extending beyond the known extremities of the galactic disk.
The somewhat round population of old, metal-poor stars in the Milky Way. Also, the huge entity that surrounds the disk and contains most of the Galaxy's dark matter. To distinguish between the two, astronomers call the former the stellar halo and the latter the dark halo. Most of the stellar halo lies closer to the Galactic center than the Sun, while most of the dark halo lies farther from the Galactic center than the Sun.
A class of relatively weak radio sources associated with clusters of galaxies and characterized by a high-brightness "head" close to the optical galaxy and a long low-brightness "tail".
Colourless, odorless gas obtained mainly from gas wells. Element which, after hydrogen, is the second lightest and second most abundant in the Universe. Its atom comprises two protons and two electrons. The nucleus of helium 4 is sometimes called an alpha particle. Helium is the product of the nuclear fusion of hydrogen in most stars, but this does not explain the overall helium abundance. Most of it was produced by the big bang, with main-sequence stars making an additional contribution.
The onset of runaway helium burning under degenerate conditions. The helium flash occurs in the hydrogen-exhausted core of a star in the red-giant phase of evolution. When gravitational pressure has brought the degenerate core to a temperature of about 108 K, the helium nuclei can start to undergo thermonuclear reactions. Once the helium burning has started, the temperature builds up rapidly (without a cooling, stabilizing expansion), and the extreme sensitivity of the nuclear reaction rate to temperature causes the helium-burning process to accelerate. This in turn raises the temperature, which further accelerates the helium burning, until a point is reached where the thermal pressure expands the core and thus removes the degeneracy and limits the flash. The helium flash can only occur when the helium core is less than the 1.4 Msun Chandrasekhar mass limit and thus it is restricted to low-mass stars.
It has been shown that helium shell burning outside a degenerate core is unstable; the helium-burning shell does not generate energy at a constant rate but instead produces energy primarily during short flashes. During a flash, the region just outside the helium-burning shell becomes unstable to convection. The resultant mixing probably leads to the s-process as well as to the upward movement of carbon produced by helium burning.
Bp star in which the strength of the helium lines varies periodically. At the extreme phases the objects appear as helium-rich, whereas at other phases He can be very weak or absent.
B star in which the helium lines are stronger than in normal stars. One distinguishes usually the extreme helium stars (also called hydrogen-deficient stars), in which no trace of hydrogen is seen, and the intermediate helium-rich stars, in which the hydrogen lines are still visible, but weaker than in normal stars. Related to these objects are the hydrogen deficient C stars.
An unsymmetrical cluster. Half of the galaxies are spiral or irregular and about half elliptical or S0. It contains a rather large number of disturbed and peculiar galaxies. The "missing mass", if present, must constitute more than 95% of the total.
An X-ray pulsar, a member of an occulting binary system. The visible component has been identified as the blue variable HZ Herculis, whose spectrum varies from late A or early F to B. Her X- l has a pulsation period of 1.2378 seconds, presumably its rotation period, and exhibits a 35-day quasi-periodicity in the X-ray region (but not in the optical). It is probably a rotating neutron star in a circular orbit with a mass of about 0.7 Msun, which is accreting matter from HZ Her. The orbital period is stable, but the pulsation period is speeding up at a rate of about 1 part in 105 per year. The X-ray eclipse lasts 0.24 days.
One of the five superstring theories; involves closed strings whose right-moving vibrations resemble those of the Type II string and whose left-moving vibrations involve those of the bosonic string. Differs in important but subtle ways from the Heterotic-O string theory.
One of the five superstring theories; involves closed strings whose right-moving vibrations resemble those of the Type II string and whose left-moving vibrations involve those of the bosonic string. Differs in important but subtle ways from the Heterotic-E string theory.
In the context of grand unified theories, the hierarchy problem is our inability to understand theoretically why the energy scale at which the unification becomes apparent, about 1016 GeV (billion electron volts), is so much higher than other energy scales of relevance to particle physics, such as the mass/energy of a proton, which is only 1 GeV.
A hypothetical, spinless particle that plays an important role in the Glashow-Weinberg-Salam electroweak theory (and in other theories involving spontaneous symmetry breaking, e.g. GUTs).
The particle or particles associated with the bundles of energy in the Higgs field. Such particles are analogous to the photons that are associated with the electromagnetic field. The standard model of particle physics predicts one electrically neutral Higgs particle which has not yet been found, but which will be sought in upcoming particle accelerator experiments. The grand unified theories predict many Higgs particles, but they are too massive to be accessible at existing or foreseeable accelerators.
Device for recording the presence of subatomic particles. A typical modem detector consists of an array of electronic sensors connected to a computer, capable of recording the paths of the particles as they fly out from the collision site in a particle accelerator.
Late type stars whose spatial velocities are greater than 100 km s-1. Other authors prefer the definition, with radial velocities greater than 60 km s-1.
A group of asteroids with similar orbital elements. The members of a given family are widely believed to have resulted from collisions between larger parent bodies.
The radius of an external galaxy at which the surface brightness is 26.6 mag arcsec-2. This criterion was developed by Holmberg in 1958 to estimate the actual dimensions of the major and minor axes of a galaxy without regard to its orientation in space.
An interferometric method of recording information about the three-dimensional nature of an object which relies on preserving both the amplitudes and phases of the wavefronts which reach the detector, instead of merely the amplitudes. Hologram means "whole record". The basic principle was outlined by D. Gabor in 1948.
A plane perpendicular to the line from an observer to the zenith. The great circle formed by the intersection of the celestial sphere with a plane perpendicular to the line from an observer to the zenith is called the astronomical horizon.
The maximum distance that an observer can see. In cosmology, our horizon is the distance from us that light has traveled since the beginning of the universe. Objects more distant than our horizon are invisible to us because there hasn't been enough time for light to have traveled from there to here.
A problem of the traditional big bang theory (without inflation) related to the large scale uniformity of the observed universe. The problem is seen most clearly in the cosmic background radiation, which is believed to have been released at about 300000 years after the big bang, and has been observed to have the same temperature in all directions to an accuracy of one part in 100,000. Calculations in the traditional big bang theory show that the sources of the background radiation arriving today from two opposite directions in the sky were separated from each other, at 300000 years after the big bang, by about 100 horizon distances. Since no energy or information can be transported further than one horizon distance, the observed uniformity can be reconciled only by postulating that the universe began in a state of near-perfect uniformity. See also flatness problem.
A quandary in standard big bang theory, which indicates that few of the particles of the early universe would have had time to be in causal contact with one another at the outset of cosmic expansion. It appears to have been resolved in the inflationary universe theory.
Cosmological puzzle associated with the fact that regions of the universe that are separated by vast distances nevertheless have nearly identical properties such as temperature. Inflationary cosmology offers a solution.
The puzzle that widely separated regions of the universe are observed to share the same physical properties, such as temperature, even though these regions were too far apart when they emitted their radiation to have exchanged heat and homogenized during the time since the beginning of the universe. In particular, we detect the same intensity of cosmic radio waves (cosmic background radiation) from all directions of space, suggesting that the regions that emitted that radiation had the same temperature at the time of emission. However, at the time of emission, when the universe was about 1 million years old, those regions were separated by roughly 100 million light years, much exceeding the distance light or heat could have traveled since the big bang. The horizon problem is also called the causality puzzle. (See horizon.)
A metal-poor star after it has undergone the helium flash and begins to quietly burn helium into carbon and oxygen in its core and hydrogen in a surrounding envelope.
A reformulation of the general theory of relativity that incorporates and extends Mach's principle (q.v.). In this theory, the inertial mass of a particle is a function of the masses of all other particles, multiplied by a coupling constant which is a function of cosmic epoch. In cosmologies based on this theory, the gravitational constant G decreases strongly with time.
According to Hubble's law, discovered by Edwin Hubble in 1929, distant galaxies are receding from us, on average, with a speed equal to the product of the Hubble constant and the distance to the galaxy. Hubble's "constant" is independent of distance, but actually decreases slowly in time as the expansion is slowed by the gravitational pull of each galaxy on all the others. The present value is somewhere between 15 and 30 kilometers per second per million light-years.
The constant of proportionality in the Hubble law. Its value must vary with time, so it is often referred to as the Hubble parameter. The Hubble constant is generally used to mean the value of the Hubble parameter at the current epoch, and is somewhere between 50 and 100 km/s/Mpc with possibly a value close to 75 km/s/Mpc.
The present expansion rate of the universe, in units of kilometers per second per megaparsec. The larger the Hubble constant, the younger the universe.
The rate of expansion of the universe. The Hubble constant is equal to the recessional speed of a distant galaxy, divided by its distance from us. Assuming a homogeneous and isotropic universe, the recessional speed of a distant galaxy is proportional to its distance; thus the Hubble constant as determined by any receding galaxy should be the same, yielding a universal rate of expansion of the universe. According to estimates, the current value of the Hubble constant is approximately 100 km/s/Mpc, meaning that the distance between any two distant galaxies will double in about 10 billion years at the current rate of expansion.
A relation which states that recessional speed is proportional to distance for a homogeneous and isotropic universe. Galaxies moving away from us with a speed precisely following this relation are said to follow the Hubble flow. Because the actual universe is not precisely homogeneous, with lumpiness arising from clustering of galaxies and voids of empty space, the motions of actual galaxies deviate somewhat from the Hubble flow.
Hubble Space Telescope. A space-based reflecting telescope with a primary mirror diameter of 2.4 m (94 in) capable of high-resolution imaging from the far ultraviolet to the near infrared. A joint NASA/ESA mission. Launched in 1990 with a planned lifetime of 15 years. Encountered reduced performance when the mirror was found to have spherical aberration. Solved by the installation of corrective optics (COSTAR) in 1994.
The Hubble time is one divided by the Hubble constant, which gives a number from 10 to 20 billion years. For a flat universe with no cosmological constant, the age of the universe is two-thirds of the Hubble time.
The inverse of the Hubble constant and a crude measure of the universe's age. For a Hubble constant of 50, one can calculate that the Hubble time is 19.6 billion years; for a Hubble constant of 80, the Hubble time is 12.2 billion years. If there is no cosmological constant, the universe is younger than the Hubble time. In particular, if the mass density of the universe (designated Ω) is 0.1, the universe's age is 90 percent of the Hubble time; if Ω is 1.0, the universe's age is 67 percent of the Hubble time.
A device in which the roles of radiation (infrared mostly) detector and signal multiplexer are separated. The device is a sandwich of two slabs. Other names include focal plane array (FPA) and sensor chip assembly (SCA).
Colourless, odourless gas, insuluable in water. Lightest, most abundant element in the Universe. Its atom comprises one proton and one electron. The element occurs both in stars and as interstellar clouds, in regions where it may be neutral (H I regions) or ionized (H II regions). It was produced by the big bang. Hydrogen 1 is the most common isotope; deuterium, is rarer; and tritium, is radioactive.
The fusion of hydrogen into helium and the process by which all main-sequence stars generate energy. Every star born with more than 0.08 solar masses burns hydrogen.
An interstellar molecule first detected in 1963 at a wavelength of 18 cm. The four transitions that occur near 18 cm are caused by the splitting of the ground level. Galactic OH sources have been divided into three classes according to whether the OH emission is strongest in the main lines, particularly at 1665 MHz (Class 1), whether the emission and absorption are highly anomalous only in the satellite lines (Class 2) (Class 2a, 1720-line emitters; Class 2b, 1612-line emitters), or whether there is only absorption in all four lines (Class 3).
Baryons heavier than the neutron (this term is seldom used today). They have non-zero strangeness. Free hyperons are unstable and decay into end products, one of which is a proton.
The ninth satellite of Saturn. It has the most extreme variation in albedo of any satellite in the solar system (0.04 for the leading side, 0.28 for the trailing side). Discovered by Cassini in 1671.
A nondegenerate gas in which the individual molecules are assumed to occupy mathematical points and to have zero volume, and in which the mutual attraction of neighboring molecules is zero.
he pressure of a gas is directly proportional to the product of its temperature and density (p = CρT). The higher the temperature and the more rarefied a gas, the more closely it obeys the ideal gas laws, so the gases in most stars closely approximate ideal gases. For a degenerate gas, the pressure depends only on the density and is independent of the temperature.
An electronic device for increasing the brightness of a faint optical image. The image is first formed on a thin metallic surface called a photocathode from which electrons are then ejected. The stream of electrons is accelerated and focussed onto a phosphorescent screen which glows brightly as a result of the impact.
A form of very low light level detector used in astronomy. By means of an image intensifier the IPCS is capable of counting individual photons of light.
An electronic camera in which electrons, emitted from a photocathode surface exposed to light, are focused electronically onto a phosphor or photographic plate.
Class of instruments which preserve the image field while also determining the spectrum. Integral Field Unit (IFU). Usually implies some kind of image slicing either with facets or fiber optics.
The angle between one plane and another. The (equatorial) inclination of a planet is the angle between the plane of its equator and that of its orbit. The inclination of the orbit of a planet in the Solar System other than Earth is the angle between the plane of that orbit and the ecliptic.
A reaction involving a change in the kinetic energy of the system, as in ionization, excitation, or capture; or a process which changes the energy level of the system.
Big Bang era in which the infant universe went through a brief period of extremely rapid (exponential) expansion, after which it settled back into the more leisurely rate of expansion of the standard model. The period of rapid expansion began and ended when the universe was still much less than a second old, yet it provides a physical explanation for the flatness and horizon puzzles. The inflationary universe model also suggests that the universe is vastly larger than the portion of it that is visible to us. (See exponential expansion.)
Big Bang era in which the spacetime continuum underwent an intense period of exponential expansion, in response to the separation of the strong nuclear force from the electroweak force. This idea solves the flatness and horizon problems.
Big Bang era in which the universe is driven into exponential expansion by the repulsive gravitational field created by a false vacuum. The inflation would end with the decay of the false vacuum. Although the inflation would occur in far less than a second, it could account for the "bang" of the big bang theory, it could explain the origin of essentially all the matter in the observed universe, and it can solve the horizon problem and the flatness problem. It could also generate the density perturbations that would later become the seeds for galaxy formation.
The idea that, when it was a fraction of a second old, the universe expanded dramatically. If inflation is correct, then the mass density of the universe (Ω) should be 1.0, if there is no cosmological constant; if there is a cosmological constant and inflation is correct, the sum of Ω and the cosmological constant (λ) should be 1.0.
That part of the electromagnetic spectrum that lies beyond the red, having wavelengths from about 7500 Å to a few millimeters (about 1011-1014 Hz). Infrared radiation is caused by atomic transitions, or by vibrational (near-IR) and rotational (far-IR) transitions in molecules. (IRe1, IRc1, IRs1: the e sources are extended; the c sources are unresolved; the s indicates an infrared nebula surrounding a visible star.)
The region of the electromagnetic spectrum from a wavelength of about 1 μm (10-6 m) to about 200 μm. The region from 1 to 5 µm is the near infrared: 5-30 is the mid infrared and 30-200 µm is the far infrared.
An unstable Lagrangian point between the two bodies on the line passing through the centers of mass of the two bodies. Mass transfer occurs through this point in a close binary star system.
Any imaging device, like a photographic emulsion or CCD, which can build up more signal and contrast by a longer exposure to light or other electromagnetic energy.
A method of constructing an optical filter to select a particular wavelength band for transmission and reject wavelengths outside this band. Similar to a Fabry-Perot etalon. The construction relies on constructive and destructive interference effects in a multilayer stack of quarter-wave reflective layers and half-wave spacer layers.
A CCD construction consisting of vertical strips which are alternately opaque and light sensitive. The opaque strips conceal charge transfer registers.
The properties of different elementary particles can be related to each other by mathematical transformations that look very much like the more familiar symmetry properties of our own physical space. Physicists have therefore hypothesized an abstract internal space in which these internal symmetries are defined. With the help of these internal symmetries, the elementary particles can be gathered into families. The relationship between space-time and these internal symmetries remains to be fully explained.
A temperature scale defined between 0.65 K and the highest temperatures practicably measurable in terms of the Planck radiation law using monochromatic radiation. There are 17 fixed temperature points :
Below 5 K are defined in terms of the vapour pressures of helium 3 and helium 4
Between 5 K and the triple point of water are marked by the triple points of certain elements, e.g. neon (24.5561 K)
Above 0.01 °C are defined with reference to the freezing points of specified metallic elements
Upper limit is 1064.43 °C (1337.58 K), the freezing point of gold
Sparse, cool gas (mainly hydrogen) in interstellar space. Dust absorbs and scatters radiation; gas does not interact directly with radiation but is coupled to the dust by collisions. Interstellar gas affects only light of certain wavelengths. Temperature 10-100 K.
Sharp, distinct absorption lines superposed on stellar spectra, produced by the interstellar gas located between the source and the observer. Strongest are the D lines, followed by the H and K lines, and the K I doublet at 7699 and 7644 Å.
The amount of light an object actually emits, as opposed to how bright the object looks from Earth. An apparently bright star can be intrinsically bright and far away or intrinsically faint and nearby.
The innermost Galilean satellite of Jupiter, similar in size and density to the Moon. Jupiter's decametric radiation has been linked at least partially to Io. It has the highest mean density of any of the Galilean satellites. Pioneer 10 also detected the presence of an ionosphere, and Na D emission. It also has the highest albedo in the solar system.
Loss or gain by an atom of one or more electrons, by which process the atom becomes an ion and instead of being neutral, has a charge: positive if it has lost an electron, negative if it has gained one. High temperature is particularly conducive to ionization.
The minimum energy required to remove an electron from an atom. It always takes a higher energy to remove a second electron from a singly ionized atom, a still higher energy to remove a third, etc. The ionization potential for hydrogen is 13.596 eV, which corresponds to a wavelength of 912 Å.
The region of Earth's atmosphere (80-500 km), immediately above the stratosphere. The ionosphere consists of the D layer, the E layer, and the F layers (q.v.). It is strongest at the end of the day.
The angular region on the sky over which the wavefront correction applied by an adaptive optics system remains valid. It is relatively small and therefore a nearby reference star is also required.
An atomic nucleus having the same number of protons as a more commonly found atomic nucleus but a different number of neutrons. For example, the hydrogen nucleus has a single proton; deuterium has one proton and one neutron and would be called an isotope of hydrogen. (see neutron; proton.)
Two nuclei with the same number of protons but different numbers of neutrons are said to represent the same element, but different isotopes. For example, helium-3, with two protons and one neutron in each nucleus, and helium-4, with two protons and two neutrons, are two different isotopes of helium. For another example, see deuterium.
A universe is said to be isotropic, from the point of view of a given observer, if it looks the same in all directions. The isotropy of the real universe is seen most strikingly in the cosmic background radiation, which has the same temperature in all directions to an accuracy of about one part in 100,000. See also homogeneous.
In cosmology, the property that the universe appears the same in all directions. The uniformity of the cosmic background radiation, coming from all directions of space, suggests that on the large scale the universe is isotropic about our position. If we then assume that our position is not unique, we conclude that: the universe appears isotropic about all points. This last result requires that the universe be homogeneous. (See cosmic background radiation; homogeneity.)
The property of being unchanged by a rotation. A sphere is rotationally invariant, but a rectangle is not. As far as we know the fundamental laws of physics are rotationally invariant.
Unit of work which is equal to a force of one newton acting over a distance of one metre in the direction of the force. It was recognized by the IEC in 1889. The specific heat of water at 15 °C is 4185.5 joule (kg °C)-1, a figure previously always associated with J, the mechanical equivalent of heat.
Fifth planet from the Sun. It is more massive than all other planets and satellites combined; if it were about 80 times more massive, it would become self-luminous due to fusion reactions. The heat flux to from the center to the surface is mainly convective. For both Jupiter and Saturn it is necessary to invoke a substantial source of internal heating (presumably gravitational contraction) to account for the surface temperature (Jupiter radiates about 2 1/2 times as much heat as it receives from the Sun). Jupiter's surface shows pronounced horizontal striations: the light layers (zones) are at a slightly higher altitude and about 15° cooler than the dark layers (belts). It is surrounded by a partial torus of atomic H in the orbit of Io. Thirteen satellites, the four outermost of which have retrograde motion, high eccentricity, and high inclination. (Jupiter XIII, discovered in 1974, has a period of 239 days; i = 26°.7, e = 0.147.)
A spectral line in the X-ray region (α = 0.334 Å), produced by the transition between the lowest level of the K shell and the lowest level of the L shell.
The inner part of the Solar corona (the gaseous phase) which emits a continuous spectrum without absorption lines. Physically, the K component results from Thomson scattering of photospheric radiation by free electrons in the corona. The K component is polarized and decreases rapidly with distance from the Sun. (from the German Kontinuum)
An electron in the K shell, the innermost shell, or energy level, of an atom. All elements heavier than hydrogen have a filled K shell, which consists of two 1s electrons orbiting the nucleus.
Cool star with spectral type K with spectra resembling those of sunspots, in which the hydrogen lines have been greatly weakened. The HK lines (q.v.) reach their greatest intensity. Strongest lines are Ca I (4227 Å) and the G-band (4303 Å).
An early attempt to unify general relativity and electromagnetism by working in five dimensions. The electromagnetic field was obtained by curling up or compactifying the extra dimension. With the advent of higher dimensional theories such as superstrings, the Kaluza-Klein approach came back into fashion.
The fraction 1/273.16 of the thermodynamic temperature of the triple point of water (the point at which the solid, liquid and gaseous phase of pure water are in equilibrium). Absolute zero corresponds to zero energy. Represents the same temperature interval as those on the Celsius scale.
The contraction of a star contemplated by Kelvin and Helmholtz as a consequence of a star's radiating its thermal energy. It is currently believed that the contraction of a star occurs in this manner in its pre-main-sequence evolution.
A Type I supernova (SN Oph 1604) whose light reached Earth in 1604. If H0 = 50, then Kepler's supernova is out in the galactic halo at a distance of 12.1 kpc and 1.4 kpc above the galactic plane, according to van den Bergh. Kepler's supernova is the prototype of Type I supernovae; at its brightest it reached an apparent magnitude of about -2.2. (3C 358)
The orbit of a spherical particle of a finite mass around another spherical particle, also of finite mass, by virtue of the gravitational attraction between them. In the Bohr-Sommerfeld picture of atoms, the electrons are considered as describing Keplerian orbits in the field of the positive nucleus by virtue of the inverse square electric attraction between the electrons and the nuclei.
The path that an orbital body (e.g., a planet) would follow if it were subject only to the inverse-square attraction of the Sun or other central body. In practiced secondary bodies, such as Jupiter, produce perturbations.
Unit of mass (not of weight or of force). Kilogram is equal to the mass of 1.000028 cubic decimeters of water at the temperature of its maximum density.
Theory proposed by Edward Milne as a viable alternative to Einstein's general theory of relativity, and based generally on kinematics (the science of pure motion, without reference to matter or force), from which Milne successfully derived new systems of dynamics and electrodynamics.
The branch of mechanics that studies bodies undergoing continuous change of position. Whereas dynamics takes into account mass, force, distance, and time, kinematics is concerned only with distance and time.
The energy associated with motion; the work that must be done to change a body from a state of rest to a state of motion, equal to 1/2 mv2 for a body of mass m moving at velocity v.
(1) To each chemical species there corresponds a characteristic spectrum. (2) Every element is capable of absorbing the radiation which it is able to emit; this is the phenomenon of the reversal of the lines.
A cosmological model in which the early universe is depicted as a giant collapsing spherical cloud of matter and antimatter. When a critical density is reached, the matter and antimatter begin to annihilate, the resulting release of radiation and energy causing the universe to expand. There are many difficulties with this model of the expanding universe, which is largely discredited on observational grounds.
A cosmology in which the observed expansion of the Universe results from the bounce of an originally collapsing cloud of matter and antimatter. The bounce is caused by radiation pressure generated by annihilations when the cloud reaches high density (10-2 cm-3).
A nonparametric test used in statistics. The Kolmogorov statistic is simply the magnitude of the maximum deviation between the integral distribution function of a sample and the theoretical distribution one wishes to test.
One of five points in the orbital plane of two massive particles in circular orbits around a common center of gravity, where a third particle of negligible mass can remain in equilibrium.
A tiny change in the energy of an electron orbiting the nucleus of an atom, caused by the interaction of the electron with other particles that appear and quickly disappear in "quantum fluctuations". (See quantum fluctuations.)
Clockwork conception of the universe in which complete knowledge of the state of the universe at one moment completely determines its state at all future and past moments.
The frequency of precession of a charged particle orbiting in a uniform magnetic field. It is equal to eH / 4πme, where e is the electron charge, me is the electron mass, and H is the magnetic field strength.
A second (see second, Système International) added between 60s and 0s at announced times to keep UTC within 0s.90 of UT1. Generally, leap seconds are added at the end of June or December.
The precession of the plane of the geodesic orbit of a test particle around a rotating mass in general relativity. It arises from the coupling of the rotation of the central mass with the orbital angular momentum of the test particle. This precession is described as resulting from the dragging of inertial frames.
Two dwarf elliptical galaxies (about 220-250 kpc distant) belonging to the Local Group. Leo I (dE3), Mv ≈ - 11, diameter 1.8 kpc; Leo II, Mv = -9.5, diameter 1.3 kpc.
A fundamental particle of nature, which may interact via all of the fundamental forces except the strong nuclear force. The electron is an example of a lepton.
Elementary particle like the electron and neutrino that do not experience the strong nuclear force. Unlike the strongly interacting hadrons, the leptons have small masses.
The generic name for any spin-1/2 particle which does not feel the strong nuclear force. The six known leptons are the electron, the muon, the tau lepton, and their respective neutrinos. The name was originally coined to refer to light particles.
The Big Bang era when the temperature had dropped to about 1012 K and when the Universe consisted mainly of leptons and photons. It lasted until the temperature fell below 1010 K. At this stage, the characteristic photon energy fell below the rest mass energy of an electron, and the abundance of electron-positron pairs fell by many orders of magnitude. Only one electron survived for every 108 photons. The universe was subsequently radiation-dominated (substantial numbers of neutrinos were also present, but they did not interact directly with the matter or the radiation).
Any of several oscillations in the apparent aspect of the Moon as seen from Earth, which, when combined, enable Earth-based observers over a period of time to see altogether about 59 percent of the Moon's surface. Physical librations are angular motions about the center of mass due to gravitational torques on the Moon. Optical librations are the apparent rotations of the Moon, caused by our observing it from somewhat different directions at different times.
The "turning" of the Moon so that although the same face is presented to Earth at all times, the overall surface of the Moon visible is 59% of the total. Libration is described as latitudinal, longitudinal and diurnal.
Variations in the orientation of the Moon's surface with respect to an observer on the Earth. Physical librations are due to variations in the rate at which the Moon rotates on its axis. The much larger optical librations are due to variations in the rate of the Moon's orbital motion, the obliquity of the Moon's equator to its orbital plane, and the diurnal changes of geometric perspective of an observer on the Earth's surface.
The distance light travels in a vacuum in 1 year. 1 lt-yr = 9.4605 × 1012 km = 0.307 pc (c = 299792.46 km s-1 = 186274 miles s-1). (1 lt-min ≈ 0.13 AU.)
Soft, silvery-white, metal. Lightest of all solid elements, third in the periodic table after hydrogen and helium. Its atom comprises one proton and three electrons. One of the electrons is at a higher energy level than the other two. Some lithium formed in the big bang, along with huge amounts of hydrogen and helium.
Peculiar giant star (spectral types G-M) whose spectra show high abundances of lithium. They are primarily S stars and carbon stars, although Li is also found in T Tauri stars, and is sometimes observed in normal late-type giants. Interstellar Li / H ≈ 2 × 10-10. (Solar system Li / H ≈ 10-9.) Lithium is destroyed rapidly (in about 7500 years) at typical nuclear burning temperatures. Spallation is the only production mechanism known for 6Li, but 7Li can be transported from the core in the form of 7Be and converted in the envelope to 7Li by electron capture. Observed 7Li / 6Li > 10.
The cluster of galaxies to which our Galaxy belongs. It is a poor, irregular cluster with some 20 certain members: three spirals (the Galaxy, M31, and M33); four irregulars (LMC, SMC, IC 1613, and NGC 6822); and about 13 intermediate or dwarf ellipticals (NGC 147; NGC 185; NGC 205; M32; the Sculptor, Fornax, Leo I and II, UMi, and Draco systems; and three companions to M31 discovered by van den Bergh in 1972). It may also include several other dwarf galaxies as well as the giant elliptical Maffei 1. The total mass of the Local Group is less than 1.5 times the combined masses of the Galaxy and M31.
The gravitationally bound collection of nearby galaxies ruled by the Andromeda galaxy and the Milky Way, which are its largest members. The Local Group has about 30 known galaxies.
The system of galaxies to which our Milky Way galaxy belongs is a small group, containing only two large spirals (our galaxy and the Andromeda galaxy, Messier 31) and twenty or more smaller systems.
A coordinate system or frame of reference defined in the vicinity of the earth in which Newtons first law of motion is valid; that is, a nonrotating and nonaccelerating reference frame.
A frame of reference in which the mean motion of stars in the immediate neighborhood is zero. In such a reference system, the motions of stars in the solar neighborhood (a volume of space about 100 pc in diameter) average out to zero (cf. solar motion). It is a coordinate system in which the origin is a point in the galactic plane moving in a circular orbit around the galactic center, and in which the three velocity components are Π, in the direction from the galactic center to the origin; θ, in the direction of galactic rotation; and Z, perpendicular to the galactic plane.
An imaginary point, located at the Sun's distance from the Galactic center, that revolves clockwise around the Galaxy on a circular orbit. Astronomers measure a star's U, V, and W velocities with respect to the local standard of rest rather than with respect to the Sun, because the Sun has a slightly noncircular orbit. The orbital velocity of the local standard of rest around the Galaxy is about 220 kilometers per second.
The supercluster of galaxies to which the Local Group may belong (see Virgo Supercluster). De Vaucouleurs lists 54 groups of galaxies in the LSC (R ≈ 17 Mpc).
The supercluster to which the Local Group belongs. It is shaped like a cigar, with the Virgo cluster of galaxies at its center and the Local Group near one end.
Pulsating red giant or supergiant. Population I typically have periods greater than 200 days; Population II, periods less than 200 days. Long-period variables emit most of their radiation in the infrared.
Phenomenon that, owing to the finite velocity of light, the more distant an object being observed, the older is the information received from it. A galaxy one billion light-years away, for instance, is seen as it looked one billion years ago.
The time in the past at which the light we now receive from a distant object was emitted. Galaxies of a certain type (redshift and luminosity) can be seen only at a certain distance.
A transformation which enables one to relate the physical parameters describing an object when viewed in one frame of reference to those which are appropriate to an observer moving with a uniform velocity in that frame.
This is the number of molecules in a cubic metre of an ideal gas at s.t.p. It is equal to the Avogadro constant divided by the molar volume. In Germany it is sometimes called Avogadro's constant.
The lower part of the chromosphere and consists of cool neutral hydrogen in which radiation at certain wavelengths is absorbed from the continuous spectrum emitted from the Sun's photosphere.
Brightness of a celestial body, measured in terms of (apparent) magnitude, absolute magnitude, or using the Sun's brightness as 1.0 on a solar scale. The luminosity of a star corresponds with its internal radiation pressure, which in turn depends on its mass.
Fluorescent UV coating which improves the blue sensitivity of a CCD by emitting light at approximately 540 to 580 nm when excited with light of wavelengths shorter than 450 nm.
A giant spiral galaxy 11 million light-years away in the constellation Ursa Major. It rules the M81 group, the second nearest galaxy group to the Local Group.
A strong radio source. Optically, it is an elliptical galaxy with a luminous blue jet about 1500 pc long. It is also one of the most powerful extragalactic sources of radiation at infrared wavelengths.
Star of spectral type M are cool red stars with surface temperatures of less than 3600 K whose spectra are dominated by molecular bands, especially those of TiO. M dwarfs are the most numerous type in our galaxy.
Theory emerging from the second superstring revolution that unites the previous five superstring theories within a single overarching framework. M-theory appears to be a theory involving eleven spacetime dimensions, although many of its detailed properties have yet to be understood.
A pre-relativity statement to the effect that the local inertial frame is determined by some average of the motion of all the matter in the universe. In essence, mach's principle says that space, which is the arena in which matter interacts, is itself an aspect of that matter.
The hypothesis that the inertia of bodies - that is, their resistance to acceleration by applied forces - is determined not by any absolute properties of space but by the effects of distant matter in the universe. equivalently, Mach's principle proposes that the distinction between accelerated and nonaccelerated frames of reference is determined by the effects of distant matter.
The precept that the inertia of objects results not from their relationship to Newtonian absolute space, but to the rest of the mass and energy distributed throughout the universe. Though unproved and perhaps unprovable, Mach's principle inspired einstein, who sought with partial success to incorporate it into the general theory of relativity.
The Large and Small Magellanic Clouds, the two nearest and largest of the galaxies that orbit the milky way. The Magellanic Clouds lie in the southern sky and cannot be seen from the United States.
Two relatively small, nebulous stellar systems visible only in the southern hemisphere; the larger is, however, the brightest "nebular" object in the sky. both are members of the local group of galaxies, and in fact seem to be associated, though detached, parts of the milky way system.
Two small irregular (or possibly barred spiral) galaxies (satellites of the Milky Way galaxy) about 50-60 kpc (LMC, in Dorado) and 60-70 kpc (SMC, in Toucana) distant, visible to the naked eye from the southern hemisphere. Both clouds contain mainly Population I stars. The LMC contains numerous ob stars and at least 10 stars that are an order of magnitude brighter (mv = - 9) than any supergiants known in our galaxy. It also contains several times our galaxy's concentration of interstellar matter.
A name given to the long H I filament that extends from the region between the Magellanic Clouds down to the south galactic pole and which appears to make a 180° arc of a great circle across the sky.
A hypothetical particle that carries an isolated north or south magnetic pole. This is in contrast to magnets which are north-south pole pairs. If magnetic monopoles exist, they must be very massive.
A magnet with an isolated north (or south) pole, rather than a pair of equal-strength north and south poles, as in conventional magnets. Magnetic monopoles have never been observed, but they are predicted to exist by grand unified theories.
An hypothesized particle that would have either a magnetic north pole or a magnetic south pole but not both. all magnetic particles and magnets ever observed have both poles. Magnetic monopoles are predicted by grand unified theories of physics. that grand unified theories predict the existence of large numbers of magnetic monopoles, when none have been discovered, is called the monopole problem. (see grand unified theories.)
has definition A hypothetical quantum object being a single, isolated magnetic pole. Normally, magnetic poles, the sources of a magnetic field, occur in pairs as north and south poles.
A problem, discovered by John Preskill in 1979, concerning the compatibility of grand unified theories with standard cosmology. Preskill showed that if standard cosmology were combined with grand unified theories, far too many magnetic monopoles would have been produced in the early universe.
The pressure exerted by a magnetic field on the material that contains the field. in gaussian units it is given by pm = b2 / 8π, where b is the magnetic field strength.
The region in earth's ionosphere where the magnetosphere meets the solar wind. Essentially, it is the place where earth's magnetic field stops; the region above the magnetopause is no longer part of earth's atmosphere, but is part of interplanetary space.
The region of space surrounding a rotating, magnetized sphere. Specifically, the outer region of earth's ionosphere, starting at about 1000 km above earth's surface, and extending to about 60,000 km (or considerably farther, on the side away from the sun).
A measure. on a logarithmic scale, used to indicate the brightness of a celestial object. A 1-magnitude difference in brightness between two stars corresponds to a difference in luminosity by 100.4 or 2.51; 5 magnitudes corresponds to factor of 100 in luminosity.
An arbitrary number, measured on a logarithmic scale, used to indicate the brightness of an object. Two stars differing by 5 mag differ in luminosity by 100. 1 magnitude is the fifth root of 100, or about 2.512. The brighter the star, the lower the numerical value of the magnitude (see also Pogson's ratio).
The brightness of a star or planet, expressed on a scale in which lower numbers mean greater brightness. Apparent magnitude indicates the brightness of objects as we see them from earth, regardless of their distance. Absolute magnitude is defined as the apparent magnitude a star would have if viewed from a distance of ten parsecs, or 32.6 light-years. each step in magnitude equals a difference of 2.5 times in brightness: the brightest stars in the sky are apparent magnitude 1; the dimmest, 6. The magnitudes of extremely bright objects are expressed in negative values - e.g., the apparent magnitude of the Sun is about -26.
The measure of a star's brightness. apparent magnitude measures a star's apparent brightness - that is, how bright a star looks from Earth. absolute magnitude measures a star's intrinsic brightness - that is, how much light the star actually emits.
The measured brightness of a celestial body. dim objects have magnitudes of high numbers, bright objects have magnitudes of low or even negative numbers. Seen from earth, stars of (apparent) magnitude 6 or higher cannot be detected with the naked eye. The Full Moon has a magnitude of -11, and the Sun one of -26.8. in order to standardize measurements of the brightness of more distant objects, the system of absolute magnitude is used. A measure of the radiation at all wavelengths emitted by a star is known as the bolometric magnitude.
A reflector whose primary mirror is spheroidal instead of parabolic. The light initially passes through a large concave lens to remove the spherical aberration.
Multi-Anode Microchannel Analyzer. A detection system used with microchannel plates to detect events. Used as an imaging system in the ultraviolet. See microchannel plates.
The view of quantum mechanics holding that a physical system simultaneously exists in all of its possible states prior to and after a measurement of the system. In the many-worlds interpretation, each of these simultaneous existences is part of a separate universe. Every time we make a measurement of a physical system and find it to be in a particular one of its possible states, our universe branches off to one of the universes in which the system is in that particular state at that moment. The system, however, continues to exist in its other possible states, in parallel universes.
A galaxy in Markarian's list of galaxies with abnormally strong ultraviolet continua. They have broad emission lines arising in a bright, semi-stellar nucleus. Markarian 231 is the most luminous galaxy known if it is at its Hubble distance.
Abbreviated form of mass concentrations: apparent regions on the lunar surface where gravity is somehow stronger. The effect is presumed to be due to localized areas of denser rock strata.
Measure of the amount of matter in an object. Inertial mass indicates the object's resistance to changes in its state of motion. Gravitational mass indicates its response to the gravitational force. In the general theory of relativity, gravitational and inertial mass are revealed to be aspects of the same quantity.
The measure of the inertia of an object, determined by observing the acceleration when a known force is applied. The gravitational force acting on an object is found to be proportional to its mass, as is the gravitational force that it exerts on other objects.
The quantitative property of an object due to the matter it contains. (Weight, in contrast, describes a force with which a body is attracted towards a gravitational focus.) Units of mass are grams and kilograms.
Utilized in a theoretical model for quasars and active galactic nuclei, according to which the energy source is due to infall (and resultant heating) of gas and stars onto a supermassive central black hole.
These are black holes, neutron stars and brown dwarfs, none of which are luminous and all of which are postulated to exist in the halos of galaxies. They are a form of dark matter.
In string theory, a particular kind of black hole that may have large mass initially, but that becomes ever lighter as a piece of the Calabi-Yau portion of space shrinks. When the portion of space has shrunk down to a point, the initially massive black hole has no remaining mass - it is massless. In this state, it no longer manifests such usual black hole properties as an event horizon.
Belief that material objects and their interactions constitute the complete reality of all phenomena, including such seemingly insubstantial phenomena as thoughts and dreams. Compare spiritualism.
A cosmic epoch during which the matter content of the Universe ceased to be ionized. This led to a decrease in the optical depth of the Universe, and the photons of radiation (which we now observe as the cosmic microwave background) became able to travel large distances without interacting with matter.
Separation of classes of particles from regular interaction with one another, as in the decoupling of photons from particles of matter that produced the cosmic background radiation.
The Big Bang era when the temperature had dropped to 3000 K, at which time the recombination of hydrogen became possible. The plasma of free electrons and nuclei condensed to form a neutral gas. Matter and radiation consequently decouple from one another because no further scattering of the radiation occurs.
The era some 3 × 105 years after the Big Bang when the cosmic blackbody radiation was last scattered by the matter. At this era, at a redshift of about 1,000 and a temperature of about 3,000 K, the protons and electrons combined to form hydrogen atoms, which are effectively transparent to the radiation.
The rapid transition from an ionized state at a redshift of 1000, when the blackbody radiation is scattered by the free electrons, to an unionized state, when the matter is predominantly in the form of hydrogen atoms that do not scatter the radiation appreciably. Radiation subsequently does not interact with matter unless the matter becomes reionized at a later epoch by radiation from quasars or forming galaxies.
The distribution function that any species of particle will have if it is in thermodynamic equilibrium. This distribution function describes both the equilibrium in velocity space or kinetic energy, and the equilibrium in potential energy.
One of several elements of an adopted reference orbit (see elements, orbital) that approximates the actual, perturbed orbit. Mean elements may serve as the basis for calculating perturbations.
Literally the middle value in a sequence of values arranged in increasing size order. A useful mathematical estimator of the true value from a set of values when one of these values is contaminated, i.e. known to be much larger than the average.
A very thin mirror with a high curvature. A method of constructing very large mirrors which assumes from the outset that the mirror is too thin to hold its shape against gravity and will require an active control system.
A great circle passing through the celestial poles and through the zenith of any location on Earth. For planetary observations a meridian is half the great circle passing through the planet's poles and through any location on the planet.
Theoretical north-south line on the Earth's surface, or an extension of that line onto the night sky, connecting the observer's zenith with the celestial pole and the horizon. The meridian is used to state directional bearings. Devices and structures - such as meridian arcs - marking the meridian were once common in observatories.
Flow between the poles, or between the equator and the poles. A positive value indicates flow away from the equator: a negative value, flow toward the equator.
A strongly interacting particle consisting of a quark and an antiquark. Its mass is intermediate between that of a proton and an electron, which is believed to be responsible for the strong nuclear force. In contrast to the case of baryons or leptons, meson number is not conserved: like photons, mesons can be created or destroyed in arbitrary numbers.
List of the locations in the sky of more than 100 galaxies and nebulae, compiled by Charles Messier between 1760 and 1784. Some designations he originated are still used in identification; M1 is the Crab Nebula (in Taurus).
One of the earliest catalogues of nebulous-appearing astronomical objects, compiled in 1781 by the French astronomer Charles Messier. Messier's catalogue included many objects that were later realized to be galaxies.
To an astronomer, a metal is any element heavier than hydrogen and helium; thus, not only are iron and copper metals, but so are elements like oxygen and neon.
A hypothetical form of hydrogen in which the molecules have been forced by extremely high pressures to assume the lattice structure typical of metals. It is estimated that as much as 40% of Jupiter's mass (but not more than 3% of Saturn's) may be in the form of metallic hydrogen.
A "shooting star" - the streak of light in the sky produced by the transit of a meteoroid through the Earth's atmosphere; also the glowing meteoroid itself. The term "fireball" is sometimes used for a meteor approaching the brightness of Venus; the term "bolide" for one approaching the brightness of the full Moon.
Fragment or particle that enters the Earth's atmosphere and is then destroyed through friction, becoming visible as this occurs as a momentary streak of light. At certain times of the year, meteors apparently emanating from a single area of the sky (a radiant) form meteor showers. They are thought to originate within the Solar System. See also meteorite.
A profusion of meteors that fall within a period of a few hours and that appear to radiate from a common point in the sky. Shower meteors are usually low-density material and have high eccentricities.
A unit of length. The meter is the length equal to 1650763.73 wavelengths in vacuum of the radiation corresponding to the transition between the levels 2p10 and 5d5 of the Krypton-86 atom.
A molecule discovered in interstellar apace in 1974, in Sgr B2, at a frequency of 87.77 GHz. Methylamine can react with formic acid to produce glycine, the simplest amino acid.
A compact electrostatic high-voltage electron multiplier with a very large number of narrow pores or channels. A photoelectron generated at the entrance face (photocathode) stimulates a cascade of secondary electrons down the nearest channel to produce a huge cloud of charge at the output face. The output pulse can be used in many different ways to record the event. If it impacts a phosphor screen then light emission can be detected with a CCD. Direct electrical detection can be obtained using a Multi Anode Microchannel Analyzer.
Radiation in the electromagnetic spectrum between infrared and radio waves. This range has wavelengths of between about 20 cm and about 1 mm. Radiation of this type was detected as background radiation.
System of approximately 100000 million stars, of which our Sun is one. It is a normal spiral galaxy of class Sb, with a diameter now reckoned to be probably less than 100,000 light-years, and a strong but obscure energy source at the center (emitting infrared radiation). It is undergoing galactic rotation. Possibly one tenth of the galaxy's total mass - estimated at 1.8 × 1011 solar masses - comprises interstellar gas and dust.
The galaxy to which the Sun belongs. Our Galaxy is about 1010 years old and contains about 1011 stars. Its mass is at least 1011Msun, about 5-10 percent of which is in the form of gas and dust. Diameter ~ 30 kpc; thickness of nuclear bulge about 4 kpc; thickness of disk about 700-800 pc; distance between spiral arms about 1.4 kpc. Mv = - 20.5. Mean density about 0.1 Msun per cubic parsec. Magnetic field about 3-5 × 10-6 gauss. Total luminosity about 1044 ergs s-1
In a chaotic early universe, black holes may form at eras as early as the Planck time. The characteristic size of these mini black holes is 10-6 gram, the minimum mass of a collapsing inhomogeneity at that time. Larger mini black holes may form at later eras. Since conventional theories of stellar evolution show that only very massive stars can form black holes, the possible formation of mini black holes is a unique characteristic of the very early universe.
A irregular long-period intrinsic variable. It was named Mira ("wonderful") in 1596 by Fabricius, who made the first recorded observations of its brightness fluctuations. Mira is a double star with a faint B companion which is itself variable.
A red giant that varies in brightness as it pulsates. When brightest, Mira is visible to the naked eye; when dimmest, Mira can be viewed only with optical aid. Mira is the prototype of all pulsating red giants, which are called Miras in its honor.
Poses the question: why does the Universe seem to have much more mass in it than can be seen with a telescope? Dynamical and theoretical constraints place the proportion of missing mass to be somewhere between 90-99 per cent of the total mass of the Universe.
The cosmic mass that some scientists hypothesize so that the universe will have the critical density of matter, with an exact balance between gravitational energy and kinetic energy of expansion. Such mass is called missing because it represents about 10 times as much mass as has actually been detected. (See closed universe; critical mass density; dark matter.)
The SI unit of the amount of substance, defined as the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilograms of carbon 12. One mole, which is equal to gram multiplied by the molecular weight, contains 6.02 × 1023 molecules (see Avogadro's number). In general, 1 mole of any gas occupies a volume of 22.4 liters.
A series of closely spaced, often unresolved, emission or absorption lines found in the spectra of molecules. Each line represents an increment of energy due to a change in the rotational state of the molecule.
A molecule of hydrogen, discovered in interstellar space in 1970. H2 is a very hard molecule to detect. None of its transitions lie in the visible part of the spectrum. Second, being a symmetric homonuclear molecule, it does not have an electric-dipole rotation-vibration spectrum, and detection must be based on the weak quadrupole spectrum. Third, ultraviolet radiation is a very efficient dissociator of H2, so any H2 that survived would presumably be located inside very dense interstellar clouds. So far observations have borne out this supposition. Measurements of the region within about 1 kpc of the Sun suggest that H2 is about twice as abundant as atomic H.
The period of one complete synodic or sidereal revolution of the Moon around the Earth; also a calendrical unit that approximates the period of revolution.
Natural satellite of Earth. Studies of lunar rocks have shown that melting and separation must have begun at least 4.5 × 109 years ago,so the crust of the Moon was beginning to form a very short time after the solar system itself. It would have taken only 107 years to slow the Moon's rotation into its present lock with its orbital period. The Moon's orbit is always concave toward the Sun.
The time at which the apparent upper limb of the Moon is on the astronomical horizon just before it rises; i.e., when the true zenith distance, referred to the center of the Earth, of the central point of the disk is 90°34' + s - π, where s is the Moon's semidiameter, π is the horizontal parallax, and 34' is the adopted value of horizontal refraction.
The times at which the apparent upper limb of the Moon is on the astronomical horizon just as it sets; i.e., when the true zenith distance, referred to the center of the Earth, of the central point of the disk is 90°34' + s - π, where s is the Moon's semidiameter, π is the horizontal parallax, and 34' is the adopted value of horizontal refraction.
A group of spectral lines arising from transitions having a common lower energy level. The group of lines have the same values of L and S but different values of J.
An elementary particle, formerly called a mu-meson but now classified with the leptons because it seems to be identical with the electron except for its much greater mass (207 times that of an electron). The muon family includes the muons and their neutrinos (and their antiparticles). Muons may have a positive or a negative charge.
A galaxy with a small, bright, blue nucleus superposed on a considerably fainter red background. (In the Yerkes 1974 system, a galaxy with a small nucleus containing a considerable fraction of the luminosity; N-, less pronounced N galaxies; N+, extreme examples of N galaxies.) Also, a type of radio galaxy having a brilliant, starlike nucleus containing most of the luminosity of the system. N galaxies are compact galaxies, and as a class are intermediate between Seyfert galaxies and quasars in properties of form, color, spectra, redshift, and optical and radio variability.
Red star similar to M stars except that bands of C2, CN, and CH are present instead of those of TiO. N stars are strongly concentrated toward the Galactic plane.
US government body set up in 1958, under which the Space Center at Houston, Texas, and the Space Center at Cape Canaveral, Florida, are responsible for manned and unmanned space flights.
Line broadening resulting from the fact that excited levels have certain mean lives, and these mean lives, by virtue of the uncertainty principle, imply a spread in the energy values.
Body orbiting a planet. Since 1957 the term has also been applied to man-made (artificial) satellites; many astronomers make the distinction by calling natural satellites moons (and the Earth's natural satellite the Moon).
Unit of length, time, mass, etc. in which the fundamental constants c (the speed of light), h bar (Planck's constant) and kB (Boltzmann's constant) are equal to unity. That is, c, h bar and kB have the numerical value 1. (For example, if we measure length in light-years and time in years, then c = 1 light-year per year.) The use of natural units allows these constants to be omitted from mathematical equations, leading to less-cluttered calculations. In natural units, E = mc2 becomes E = m and E = kBT becomes E = T, so that both mass and temperature can be expressed in units of energy. (Of course, the correct factors of c, h bar and kB must be inserted at the end of a calculation to obtain measurable quantities.)
An irregularly shaped cloud of interstellar gas or dust whose spectrum may contain emission lines (emission nebula) or absorption lines characteristic of the spectrum of nearby illuminating stars (reflection nebula).
Indistinct, nonterrestrial objects visible in the night sky. "Bright" nebulae glow with light emitted by the gas of which they are composed ("emission" nebulae) or by reflected starlight ("reflection" nebulae) or both. "Dark" nebulae consist of clouds of gas and dust that are not so illuminated. "Planetary" nebulae are shells of gas ejected by stars. Spiral nebulae are galaxies.
The term "nebula" was previously applied to all kinds of hazy patches in the sky, many of which are now recognized to be clusters or galaxies. (See also diffuse nebula, gaseous nebula, dark nebula.)
Hypothesis, maintained in the nineteenth and early twentieth century, that the spiral nebulae are not galaxies but are instead whirlpools of gas from which new systems of stars and planets are condensing. Compare island universe theory.
Eighth major planet out from the Sun. Discovered in by following predictions calculated by Urbain Le Verrier. Similar predictions had been made earlier by John Couch Adams but were not followed up.
A fundamental elementary particle with no electric charge and very small if any rest mass. Believed to be exceedingly abundant in the universe. The neutrino has a very low cross-section for interaction with matter and is almost impossible to detect, hence the uncertainty over its rest mass. The Sun produces neutrinos from thermonuclear reactions in its core, and a large flux of neutrinos carries away most of the energy, of a supernova. Neutrinos are one candidate for Dark Matter. Experiments to detect cosmic neutrinos involve large masses of "stopping" material and indirect detection of the effects of neutrino absorption.
A stable particle with no charge, a rest mass of zero, and a spin of 1/2, that carries away energy in the course of nuclear reactions. Its main characteristic is the weakness of its interactions with all other particles. Since the wavelengths of neutrinos at the energies at which they are normally emitted from unstable nuclei are only a few thousandths of an angstrom (compared with the wavelength of a light photon which is several thousand angstroms), they have negligible probability (10-19 that of a light photon) of interacting with matter and escape at the speed of light. Neutrinos arise only in the energy-producing regions of stars and therefore, unlike light photons, provide direct evidence of conditions in stellar cores. There are two types of neutrinos, those associated with electrons (ve) and those associated with muons (vµ).
An electrically neutral, massless particle of spin-1/2, which interacts only by the weak force and gravity. It was first postulated by Pauli in 1930 to ensure conservation of energy and angular momentum in nuclear β decay. Three different types of neutrinos are known to exist corresponding to the three massive leptons: νe, νµ and ν τ.
An electrically neutral, very weakly interacting particle, with a rest energy which is either zero or very small. The particle was predicted in 1931 as a means of reconciling the measurements of beta decays with the conservation of energy, but it was not directly detected until 1956.
A nuclear particle with a mass slightly greater than that of a proton. A free neutron decays, after a half-life of about 10.6 minutes, into a proton, an electron, and an antineutrino. The neutron is probably made up of still more fundamental particles having both positive and negative charges. The charges balance to give a net charge of zero, but the motions of the charges are such that their magnetic contributions do not cancel and consequently the neutron is magnetic.
One of the constituents of the atomic nucleus discovered in 1932. It is bound into atomic nuclei by the strong nuclear force. Free neutrons decay slowly via the weak nuclear force. Despite being electrically neutral, the neutron possesses both an electric dipole moment (as if it were made of positive and negative charges separated a minute distance) and a magnetic moment, indicating some internal substructure.
Particle in the nucleus of all atoms except hydrogen. Through beta decay, a neutron may become a proton and an electron; the process occurs in reverse during the formation of a neutron star.
A star whose core is composed primarily of neutrons, as is expected to occur when the mean density is in the range 1013-1015 g cm-3. Under current theories pulsars are assumed to be rotating magnetic neutron stars. A neutron star would probably be only 10-15 km in diameter with a magnetic field of about 1012 gauss, a density of 1013-1015 g cm-3 (compared with a white dwarf's maximum density of about 108 g cm-3) and a central temperature of about 109 K and thus would be both bluer and dimmer than a white dwarf.
Remnant of a star after it has exploded as a supernova. Usually optically dim, a neutron star sends out regular or irregular radio emissions and is therefore also called a pulsar. The density of such a star may be unimaginably great although the diameter is generally around only 10 km; the gravitational and magnetic forces are correspondingly vast. It is called a neutron star because in such density, protons fuse with electrons to form neutrons, of which the star is almost entirely composed.
One of three laws describing the motion of bodies based on the conception of an absolute and immutable space and time; these laws held sway until Einstein's discovery of special relativity.
A force law that applies to the gravitational and electromagnetic forces in which the magnitude of the force decreases in proportion to the inverse of the square of the distance.
Theory of gravity declaring that the force of attraction between two bodies is proportional to the product of their masses and inversely proportional to the square of the distance between them. Subsequently supplanted by Einstein's general relativity.
Newton's law which states that when two bodies interact, the force on the first due to the second is equal and opposite to the force on the second due to the first.
A class of reflecting telescope developed by Sir Issac Newton with a paraboloidal primary mirror and a small, plane secondary mirror at 45°; to deflect the focus of the primary to a position outside the tube near the top of the telescope.
The strongest known extragalactic X-ray source. Also a strong radio source. Optically it is a Seyfert galaxy with a huge amount (about 108Msun) of ionized gas receding from it.
An infrared star. Its surface temperature is about the same as the surface temperature of Venus. It is a strong OH emitter, and CO has been identified in its spectrum.
Absorption of a photon and reemission at a different frequency (as seen by an observer) by scattering atoms. The natural width of the lines, Doppler broadening, and pressure broadening are the main processes that give rise to noncoherent scattering.
A point in the constellation Coma Berenices where we look perpendicular to and above the Galactic Plane. The nearest bright star to the North Galactic Pole is Arcturus, in the neighboring constellation Bootes.
A radio continuum feature extending from the galactic plane to the vicinity of the North Galactic Pole. It is believed to be a supernova remnant. It is also an X-ray source.
A star that brightens suddenly and to an unprecedented degree, creating the impression that a new star has appeared where none was before. Hence the name, from nova for "new". See supernova.
A star that exhibits a sudden surge of energy, temporarily increasing its luminosity by as much as 14 mag. (Since 1925 novae have been given variable star designations.) Novae are old disk-population stars. Unlike supernovae, novae retain their stellar form and most of their substance after the outburst. All known common novae are found in close binary systems with one component a cool red giant and the other a hot, less massive object which is the seat of the instability.
A stars that undergoes an explosion during which its brightness increases by up to ten magnitudes. Usually the following phases are distinguished (in order of time): pre-maximum, principal, diffuse enhanced, Orion, nebular and post-nova.
Time required for a star to evolve a significant distance off the main sequence; the time it takes a star to convert all its available hydrogen into helium.
The Big Bang era when neutrons were produced and helium and deuterium were synthesized. At t = 200 sec, nucleosynthesis began rather abruptly and virtually all deuterium was synthesized to helium.
The massive, positively charged central part of an atom, composed mainly of protons and neutrons, around which the electrons revolve. The radius of an atomic nucleus is directly proportional to the cube root of its mass. Density at least 1014 g cm-3. Radius 10-12-10-13 cm.
A species of atomic nucleus, analogous to the word "isotope" for a species of atom. The word is also used to distinguish between atomic nuclei that are in different energy states.
A small, irregular oscillation in the precessional motion of Earth's rotational axis, caused primarily by lunar perturbations. It has a principal period of 18.6 years, and moves the equinox as much as 17" ahead of or behind its mean position.
Slight but recurrent oscillation of the axis of the Earth, caused by the Moon's minutely greater gravitational effect on the Earth's equatorial "bulge".
Very hot blue star, whose spectra is dominated by the lines of singly ionized helium (see Pickering series). (Most other lines are from at least doubly ionized elements, though H and He I lines are also present.) O stars are useful because they are found in dust clouds and virtually define the spiral arms.
Any hypothesis should be shorn of all unnecessary assumptions; if two hypotheses fit the observations equally well, the one that makes the fewest assumptions should be chosen.
Peculiar O stars in which emission features at λλ4634-4641 from N III and 4686 from He II are present. They have a well-developed absorption spectrum, which implies that the excitation mechanism of the emission lines is selective, unlike that of Wolf-Rayet stars. The spectra of Of stars are usually variable, and the intensities of their emission lines vary in an irregular manner. Of stars belong to extreme Population I. All O stars earlier than 05 are Of.
A paradox formulated by the German astronomer Heinrich Olbers in 1826 that can be traced back to the writings of others, such as de Cheseaux, a century or more earlier. The paradox is: Why is the sky dark at night, if the universe is infinite? We now know that several of the assumptions made by Olbers (explicitly or implicitly) are incorrect.
A paradox formulated by the German astronomer Heinrich Olbers in 1826: Why is the sky dark at night? The amount of light we receive from a star decreases as the square of the distance from us. On the other hand, if we assume a uniform distribution of stars in space, the number of stars increases as the square of their distance from us, so the two factors should cancel out. In theory, then, the night sky should be a blazing mass of light, and obviously it is not. This self-contradictory statement is Olbers' paradox. In seeking to resolve it, astronomers noted that, besides the assumption of uniformity or homogeneity, Olbers made four other assumptions: (1) space is Euclidean; (2) the laws of physics that apply on Earth apply to the Universe as a whole; (3) the Universe is static (i.e., neither expanding nor contracting); (4) the Universe is spatially and temporally infinite. It is now known that all four of these assumptions are either incorrect or inaccurate.
The puzzle of why the sky is dark at night. If the universe extends infinitely in space, as it might, then the accumulated light from an infinite number of distant stars should seemingly cause the sky to be bright at all times, whether our sun is visible or not. This paradox, first posed in the eighteenth century, has been resolved by the big bang theory. In a universe with a beginning, we can receive light only from that part of the universe close enough so that light has had time to travel from there to here since the big bang (about 10 billion years ago). Thus, even if space extends infinitely far, only a limited region, and a limited number of stars, are visible to us. And the accumulated light from this limited number of stars is not sufficient to spoil the darkness of the night sky.
Star that, according to contemporary stellar evolution theory, have an age comparable to that of the galaxy to which they belong. This is not an observational definition.
The older part of the thin-disk population, ranging in age from about 1 to 10 billion years. The Sun and most other nearby stars belong to the old thin disk. The scale height of the old thin disk is about 1000 light-years.
The ratio of the average density of mass in the universe to the critical mass density, the latter being the density of mass needed to eventually halt the outward expansion of the universe. In an open universe, omega is always less than 1; in a closed universe, it is always greater than 1; in a flat universe it is always exactly equal to 1. Unless omega is exactly equal to 1, it changes in time, constantly decreasing in an open universe and constantly increasing in a closed universe. Omega has been measured to be about 0.1, although such measurements are difficult and uncertain. (See critical mass density; closed universe; flat universe; open universe.)
Globular cluster which differs in the period of transition between Bailey type ab and type c variables, the ratio of type c to type ab stars, in the metallicity of RR Lyrae stars, and in the mean period of the ab variables.
A comparatively loose grouping (mass range 102-103M)sun of Population I stars, strongly concentrated in the spiral arms or the disk of the Galaxy (in fact, open clusters give a good indication of where the spiral arms are). Unlike associations, open clusters are dynamically stable. Depending on their age, stars in open clusters "peel off" from the main sequence at different points (the higher the turnoff point, the younger the cluster). (Sometimes called Galactic cluster; NGC 188 is the oldest known open cluster.)
A small, loose cluster of stars that typically contains several hundred members. The best examples are the Hyades and the Pleiades, both in the constellation Taurus. Open clusters line the Galactic plane, in contrast with globular clusters, which are members of the Galaxy's halo or thick disk.
A homogeneous, isotropic Universe is said to be temporally open if gravity is not strong enough to eventually reverse the expansion, so the universe goes on expanding forever. It is said to be spatially open if it curves the opposite way from a closed universe, so that triangles would contain less than 180°, the circumference of a circle would be more than π times the diameter, and the volume would be infinite. If Einstein's Cosmological Constant is zero, as is frequently assumed, then a universe which is temporally open is also spatially open, and vice versa.
Big Bang model that was formulated by Friedmann and Lemaitre which has a negative curvature, like a saddle-shaped surface, in which case the universe is infinite, open, and will expand forever. This space is unbounded.
A configuration of the Sun, Earth and a planet in which the apparent geocentric longitude of the planet differs by 180 degrees from the apparent geocentric longitude of the Sun.
Glass and transparent plastics can be made into a very thin wire or fiber. Typical dimensions are 10-50 µm. If a ray enters one end of a fiber at the appropriate angle, it will undergo total internal reflection and travel down the fiber without much loss through the sides.
The use of lenses or other optical devices to match the size of the image of the seeing disk, as it appears in the focal plane of the telescope, to the physical size of the CCD pixels. If the telescope yields 10 arcseconds per millimeter and the seeing is 1 arcsecond then the image is 0.1 mm in size. But a typical CCD pixel is 0.022 mm, five times smaller.
A collision in which an ion and an atom approach each other very closely and spend a long time (several orbits of the atomic electrons) in close proximity.
A radio continuum feature (an H II region) centered on the Trapezium, and excited by θ1 Ori C. The Orion A molecular cloud, which lies beyond it, is a rich source of molecules CO, OH, HCN, and probably NO, HCO, and H2CO have been observed.
A large cloud of gas and dust giving birth to young stars in the constellation Orion and visible to the naked eye. It is an HII region 1500 light-years away.
An HII region about 500 pc distant, barely visible to the naked eye in the center of Orion's sword. It is undoubtedly a region where stars are being born; young O stars and many T Tauri variables are associated with it, and its members are extreme Population I. Probably no more than 20000 years old. It is also an X-ray source (3U 0527-05 and M42, NGC 1976).
Molecular hydrogen in which the two protons of the diatomic molecule have the same direction of spin. It is a higher energy state than the para form. Terrestrial H2 is 75% ortho-hydrogen, 25% para-hydrogen.
Cosmological model in which the Universe is "closed" and its expansion is destined to stop, to be succeeded by collapse and "then" (if ordinary temporal terms may be said to apply) by a rebound into a new expansion phase.
One of several parameters that specifies the instantaneous position and velocity of a celestial body in its perturbed orbit. Osculating elements describe the unperturbed (two-body) orbit that the body would follow if perturbations were to cease instantaneously.
The Sun and all objects gravitationally bound to it. The solar system is roughly a sphere with a radius greater than 100,000 AU, with the Sun at the center. The Sun is overwhelmingly the dominant object. Planets, satellites, and all interplanetary material together comprise only about 1/750 of the total mass. Geochemical dating methods show that the solar system chemically isolated itself from the rest of the Galaxy (4.7 ± 0.1) × 109years ago.
A layer in the lower part of Earth's stratosphere where the greatest concentration of ozone (03) appears. This is the layer responsible for the absorption of ultraviolet radiation.
A type of star whose spectrum shows strong emission lines, like those of the Be and Wolf-Rayet stars, with blueshifted absorption components which are presumed to come from an expanding shell of low-density matter. A P Cygni profile is taken as an indication of mass loss.
The name of the hypothetical nucleosynthetic process thought to be responsible for the synthesis of the rare heavy proton-rich nuclei which are bypassed by the r-process and s-processes. It is manifestly less efficient (and therefore rarer) than the s- or r-process, since protons must overcome the Coulomb barrier, and may in fact work as a secondary process on the r-process and s-process nuclei. It seems to involve primarily (p, γ) reactions below cerium (where neutron separation energies are high) and the (γ, n) reactions above cerium (where neutron separation energies are low). The p-process is assumed to occur in supernova envelopes at a temperature greater than about 109 K and at densities less than about 104 g cm-3.
A quantity which expresses the fact that nuclei with odd numbers of neutrons and protons have less energy and are less stable than those with even numbers of neutrons and protons.
The mountain in California upon which sits what was the largest telescope in the United States, 200 inches in diameter. The telescope itself is sometimes referred to as the Mt. Palomar telescope.
A model of galaxy formation in which the first structures to condense out of the smooth background of primordial gas were very large in size. These large masses then collapsed into thin sheets (pancakes) and fragmented into many smaller pieces the size of galaxies. A competing theory, sometimes called the hierarchical clustering model, proposes that the first structures to form were the size of galaxies. As galaxies clustered together, due to gravity, larger and larger structures were formed. (See hierarchical clustering model.)
A self-contradictory proposition. Paradoxes are most useful when they seem most likely to be true, for it is then that they best serve to expose flaws in the data or reasoning that led to their appearance.
Angle subtended by the apparent difference in a star's position when viewed from the Earth either simultaneously from opposite sides of the planet, or half such an angle, measured after a gap of six months from opposite sides of the planet's orbit; the nearer the celestial body, the greater the parallax.
The apparent displacement in the position of a star or planet occasioned by its being viewed from two different locations - e.g., by observing it from two widely separated stations on Earth, or at intervals of six months, when the earth is at either extreme of its orbit around the sun. The resulting angle can be used, by triangulation, to determine the distance of the star or planet.
The difference in apparent direction of an object as seen from two different locations; conversely, the angle at the object that is subtended by the line joining two designated points. Geocentric (diurnal) parallax is the difference in direction between a topocentric observation and a hypothetical geocentric observation. Heliocentric or annual parallax is the difference between hypothetical geocentric and heliocentric observations; it is the angle subtended at the observed object by the semi-major axis of the Earth's orbit. First trigonometric parallax was obtained in 1838. (eee also horizontal parallax.)
The tiny shift in a star's apparent position that occurs when the star is viewed from slightly different perspectives as the Earth revolves around the Sun. The larger a star's parallax, the closer the star is to Earth.
The principle of space-inversion invariance; i.e., no experiment can differentiate between the behavior of a system and that of its mirror image. Parity is conserved in strong interactions, but not in weak ones.
A unit used by astronomers to describe stellar distance. It is the distance from which the radius of the earth's orbit would subtend an angle of one second of arc. Alternatively one parsec is the distance at which one astronomical unit subtends one second of arc. The name was proposed by Professor H. H. Turner (1861-1930), Savilian Professor of Astronomy at Oxford.
The distance at which one astronomical unit subtends an angle of one second of arc; equivalently, the distance to an object having an annual parallax of one second of arc. (abbreviation for parallax second)
Fundamental unit of matter and energy. All may be classed as fermions, which have half-integral spin and obey the exclusion principle, and bosons, which have integral spin and do not obey the exclusion principle. The term particle is metaphoric, in that all subatomic particles also evince aspects of wave-like behavior.
A device using electric and magnetic fields to accelerate beams of particles-usually electrons, positrons, protons, or antiprotons-to high energies for experimental purposes. Modern accelerators are often very large: the main ring at Fermilab, for example, is 4 miles in circumference.
A machine for speeding subatomic particles to high velocity, then colliding them with a stationary target or with another beam of particles moving in the opposite direction. (In the latter instance, the machine may be called a collider.) At velocities approaching that of light the mass of the particles increases dramatically, adding greatly to the energy released on impact. The resulting explosion promotes the production of exotic particles, which are analyzed according to their behavior as they fly away through a particle detector.
The branch of science that deals with the smallest known structures of matter and energy. As their experimental investigation usually involves the application of considerable energy, particle physics overlaps with high-energy physics.
An effect on spectral lines obtained when the light source is located in a strong magnetic field, so that the magnetic splitting becomes greater than the multiplet splitting.
A reaction occurring in the proton-proton chain which occurs only once in 400 p-p reactions but produces far more energetic neutrinos (1.44 MeV as against 0.42 MeV).
The point at which a body in orbit around the Earth most closely approaches the Earth. Perigee is sometimes used with reference to the apparent orbit of the Sun around the Earth.
An orbital element representing the time required to complete an orbit. This parameter is required when determining the orbit of a binary star system in which the mass is not known.
A diffuse, irregular cluster (richness class 2) dominated by and centered on the Seyfert galaxy NGC 1275 (Perseus A). Mass required to bind the cluster, greater than 1015Msun.
A region of space containing a huge congregation of galaxies called a supercluster. The galaxies in this supercluster appear to be distributed in a long chain.
Very hot star with strong O VI and C IV lines, which is an X-ray emitter. Probably these stars are the central stars of planetary nebulae that have dissipated their envelopes.
When used in reference to matter, describes its possible states: solid phase, liquid phase, gas phase. More generally, refers to the possible descriptions of a physical system as features on which it depends (temperature, string coupling constant values, form of spacetime, etc.) are varied.
An abrupt change in the equilibrium state of a system. A sudden transition between one state of matter or energy and another state. For example, when hot water turns to steam or when ice crystallizes out of a liquid that has been cooled to below freezing, a phase transition has occurred. According to the grand unified theories of particle physics, the infant universe may have undergone one or more overall phase transitions. In this case, the energy uniformly filling all space corresponded to the supercooled liquid.
A thin metallic plate housed inside an evacuated tube capable of releasing electrons through the "photoelectric effect" when illuminated by light. These surfaces are best for optical and ultraviolet light.
A light-sensitive device made from the junction of two differently doped species of a semiconductor such as silicon. Also known as a pn junction. An internal electric field is generated at the junction of p and n type material. Photons absorbed in the junction create electron-hole pairs which are separated by the field and create a current.
The magnitude of an object as measured on the traditional photographic emulsions, which are sensitive to a slightly bluer region of the spectrum than is the human eye. (mph)(antiquated term)
A vacuum encapsulated photocathode from which electrons are ejected by the photoelectric effect followed by multiple cathodes from which many additional electrons are emitted in a cascade. When finally collected, the original single electron may have generated a pulse of over one million electrons.
Device used in photometry for the amplification of light by the release and acceleration of electrons from a sensitive surface. The result is a measurable electric current that is proportional to the intensity of received radiation.
The quantum of the electromagnetic field. It is the massless spin-1 gauge boson of QED. Virtual photons mediate the electromagnetic force between charged particles. Virtual photons can also adopt a mass for a short period, in accordance with Heisenberg's uncertainty principle.
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon for a range of applications ranging from detection to laser energy production to communications and information processing.
The region of a star which gives rise to the continuum radiation emitted by the star. The visible surface of the Sun (temperature about 6000K), just below the chromosphere and just above the convective zone. The photosphere ends (and the chromosphere begins) at about the place where the density of negative hydrogen ions has dropped to too low a value to result in appreciable opacity. The spectrum of the photosphere consists of absorption lines (unlike that of the chromosphere, which consists of emission lines).
The visible surface of the Sun, or more generally, the layer of a star that gives rise to the continuum (as opposed to spectral-line) radiation emitted by the star. photosphere of the Sun has definition
The magnitude of an object as measured photographically by filters and emulsions that are sensitive to the same region of the spectrum as the human eye. (antiquated)
An unstable nuclear particle of mass intermediate between that of a proton and an electron. The pions are believed to be the particles exchanged by nucleons, resulting in the strong nuclear force; they play a role in the strong interactions analogous to that of the photons in electromagnetic interactions.
Angle specifying the direction of electron velocity; or the angle between a tangent to a spiral arm and the perpendicular to the direction of the galactic center.
The bright rim of a sunspot, observed in emission in monochromatic light of some spectral line (Halpha or Ca II). It is a chromospheric phenomenon associated with and often confused with a facula. (sometimes called flocculus)
Planck's constant is a fundamental parameter in quantum mechanics. It determines the size of the discrete units of energy, mass, spin, etc. into which the microscopic world is partitioned.
About 1000 kilowatt hours. The energy necessary to probe to distances as small as the Planck length. The typical energy of a vibrating string in string theory.
An energy of 1.22 × 1019 GeV (billion electron volts), at which the strength of the gravitational interactions of fundamental particles becomes comparable to that of the other interactions. It is believed that the quantum effects of gravity become important at approximately this energy.
The Big Bang era, prior to which Einstein's theory of gravitation breaks down and a quantized theory of gravity is needed. Density was so high that gravitational force acted as strongly as the other fundamental forces on the sub-atomic scale.
About ten billion billion times the mass of a proton; about one-hundredth of a thousandth of a gram; about the mass of a small grain of dust. The typical mass equivalent of a vibrating string in string theory.
An object that formed in the disk surrounding a star. Unlike stars, planets do not produce light of their own but merely reflect that of the star(s) they orbit. Planets can have natural satellites.
The angle planet-Earth-Sun. Eastern elongations appear east of the Sun in the evening; western elongations, west of the Sun in the morning. An elongation of 0° is called conjunction; one of 180° is called opposition: and one of 90° is called quadrature.
The geocentric angle between a planet and the Sun, measured in the plane of the planet, Earth and Sun. Planetary elongations are measured from 0° to 180°, east or west of the Sun.
A bubble of gas surrounding a hot, dying star. The star is so hot that it makes the planetary nebula glow, which allows astronomers to see it. The star was once the core of a red giant, which ejected its outer atmosphere and created the planetary. A planetary nebula has nothing to do with a planet, but through a small telescope, it looks like a planet's disk, hence the misleading name.
An expanding envelope of rarefied ionized gas surrounding a hot white dwarf. The envelope receives ultraviolet radiation from the central star and reemits it as visible light by the process of fluorescence. The planetary nebula stage lasts for less than 50,000 years. During the core contraction that terminates the red-giant stage, the helium-burning shell is ejected at a velocity so high that it becomes separated from the core. Under current theories, a star with a carbon core and a mass greater than 0.6 Msun (but less than 4 Msun) will become a planetary nebula and leave behind a white dwarf. Planetary nebulae are now known to occur in stars less than 4 Msun whose envelope becomes unstable during the hydrogen shell burning stage.
The component of general precession caused by the gravitational coupling between the center of mass of the Earth and that of the other planets. The effect of planetary precession is to move the equinox eastward by ≈ 0".11 / year and to diminish the angle between the ecliptic and the equator by about 0".47 / year.
Coordinates for general use, where the z-axis is the mean axis of rotation; the x-axis is the intersection of the planetary equator (normal to the z-axis through the center of mass) and an arbitrary prime meridian; and the y-axis completes a right-hand coordinate system. Longitude (see longitude, celestial) of a point is measured positive to the prime meridian as defined by rotational elements. Latitude (see latitude, celestial) of a point is the angle between the planetary equator and a line to the center of mass. The radius is measured from the center of mass to the surface point.
Coordinates for cartographic purposes dependent on an equipotential surface as a reference surface. Longitude (see longitude, celestial) of a point is measured in the direction opposite to the rotation (positive to the west for direct rotation) from the cartographic position of the prime meridian defined by a clearly observable surface feature. Latitude (see latitude, celestial) of a point is the angle between the planetary equator (normal to the z-axis and through the center of mass) and normal to the reference surface at the point. The height of a point is specified as the distance above a point with the same longitude and latitude on the reference surface.
A very massive O-type giant with known anomalies in its spectrum. It is a spectroscopic binary in which mass exchange is occurring. Its spectrum can be interpreted to mean that each component has a mass of 75 Msun.
A completely ionized gas; the so-called fourth state of matter (besides solid, liquid, and gas) in which the temperature is too high for atoms as such to exist and which consists of free electrons and free atomic nuclei.
The region in Earth's ionosphere where the particle density (100 particles per cm3 just below the plasmapause) drops off very rapidly. It marks the transition from high to low density.
A B8pe star, one of the brightest stars in the Pleiades, which developed an envelope or shell first observed in 1938. The shell increased in strength and attained its maximum intensity in 1945; thereafter it weakened and was scarcely visible by 1954. In 1972 it developed another shell. It is rotating so fast that it is unstable.
The most distant known planet from the Sun. Its orbit has the highest eccentricity and highest inclination to the ecliptic of any planet and some astronomers suggest that it may be an escaped satellite of Neptune. In the mid-1970s Pluto crosses Neptune's orbit on its way in, and for the rest of this century Pluto will be closer to the Sun than Neptune (Pluto and Neptune, however, are never less than 2.6 AU apart). Its mass and radius have not been determined with any great certainty.
An approximation to the binomial distribution used when the probability of success in a single trial is very small and the number of trials is very large.
A supergiant F8 Ib, F3 V visual binary, with an orbital period of thousands of years. The primary (a Cepheid with a pulsation period of 3.97 days) is itself a single-lined spectroscopic double with a period of 29.6 years. There are at least two more faint (12th mag) components of the system.
A Cepheid which is about 4 times more luminous than Population II Cepheids, probably because of their higher metal content (although mass may also be a factor).
Young star with relatively high abundances of metals, and are usually found in the disk of a galaxy, especially the spiral arms, in dense regions of interstellar gas.
Younger stars, generally formed towards the edge of a galaxy, of the dusty material in the spiral arms, including the heavy elements. The brightest of this Population are hot, white stars.
Youngest observed stars, like our sun, formed from hydrogen, helium, and a large range of heavier elements (like carbon and oxygen) believed to have been created in the interiors of earlier Population II stars and Population III stars and then blown out into space.
Older star with relatively low abundances of metals, usually found in the nucleus of a galaxy or in globular clusters. The Sun is a rather old Population I star.
The antiparticle of the electron, discovered by Anderson in 1934. It has the same mass and spin as the electron, but opposite charge and magnetic moment.
A thermonuclear reaction in which hydrogen nuclei are transformed into helium nuclei producing 4 × 10-5 ergs of energy. Although the neutrinos from this reaction are detectable, they have not been observed.
A thermonuclear reaction in which hydrogen nuclei are transformed into helium nuclei. Although the neutrinos from this reaction are detectable, they have not been observed.
A slow, periodic conical motion of the rotation axis of a spinning body. In the case of Earth's precession it is due to the fact that Earth's axis of rotation is not perpendicular to the ecliptic but is inclined about 23°.5 and is thus affected by gravitational perturbations from other bodies in the solar system. The Moon and Sun pull harder on that part of the Earth's equatorial bulge nearest them than on that farthest away; this causes a torque which precesses the Earth's rotational axis.
The uniformly progressing motion of the pole of rotation of a freely rotating body undergoing torque from external gravitational forces. In the case of the Earth, the component of precession caused by the Sun and Moon acting on the Earth's equatorial bulge is called lunisolar precession; the component caused by the action of the planets is called planetary precession. The sum of lunisolar and planetary precession is called general precession. (See nutation.)
A state found in white dwarfs and other degenerate matter in which the atoms are packed so tightly that the electron orbits encroach on each other to the point where an electron can no longer be regarded as belonging to any particular nucleus and must be considered free.
The focal point of the large primary reflecting mirror in astronomical telescopes when the light source is extremely distant. This focus actually falls at a point just within the upper structure of the telescope itself and is therefore accessible to CCD cameras and other instruments; it provides a large field of view.
The creation of elements that occurred just minutes after the Big Bang. According to standard theory, primordial nucleosynthesis gave the universe only five nuclei, all lightweight: hydrogen-1, hydrogen-2 (or deuterium), helium-3, helium-4, and lithium-7.
The process, which took place between one second and 3-4 minutes after the beginning, in which the protons and neutrons of the primordial soup condensed to form the lightest atomic nuclei: Deuterium, Helium-3, Helium-4, and Lithium-7. See isotope and Lithium.
The production of heavy nuclei from the fusion of lighter ones during the Big Bang. The infant universe consisted of only hydrogen, the lightest of all atomic nuclei, because any heavier nuclei would have come apart in the intense heat. All other elements would have to be formed later, in nucleosynthesis processes. It is believed that most of the helium, the next lightest element after hydrogen, was formed when the universe was a few minutes old
All baryons and mesons are believed to be composed of quarks, which are elementary particles of fractional charge. In the high-density, hot-temperature phase of the very early universe, prolific numbers of quarks would have been present in equilibrium with the other elementary particles. As the universe expanded and cooled, some of these quarks may have been frozen out. To what extent independent free quarks could survive is an unresolved issue of elementary particle physics.
A region of cool, high-density gas embedded in the hot (106 K), low-density solar corona. Prominences are the flamelike tongues of gas that appear above the limb of the Sun.
Mass of hot, hydrogen rising from the Sun's chromosphere, best observed indirectly during a total eclipse. There are two kinds of prominence : erruptive prominence and quiescent prominence.
The apparent movement of a star, year after year, caused by the star's velocity across the line of sight. If the star's distance is known, this velocity-called the tangential velocity, can be computed. The star with the largest proper motion is Barnard's Star, whose proper motion is 10.3 arc-seconds per year.
The projection onto the celestial sphere of the space motion of a star relative to the solar system; thus the transverse component of the space motion of a star with respect to the solar system. Proper motion is usually tabulated in star catalogs as changes in right ascension and declination per year or century.
Early stage in the formation of planets according to the theory by which planetary systems evolve through the condensation of gas clouds surrounding a young star. The theory is not, however, generally accepted.
A baryon made of two up quarks and a down quark. It possesses a positive electromagnetic charge and can only be found in atomic nuclei. A single proton is a hydrogen nucleus.
A massive particle with positive electrical charge found in the nuclei of atoms. Composed of two up quarks and one down quark. The proton's mass is 938.3 MeV, slightly less than that of the neutron.
A subatomic particle with positive electric charge. Every atom has at least one proton in its nucleus; the number of protons determines the element. For example, all atoms with one proton are hydrogen, all atoms with two protons are helium, and so on.
One of the constituents of the atomic nucleus. It is a spin-1/2 particle carrying positive electric charge. The proton is the lightest baryon and, as a result, is the particle into which all other baryons eventually decay. It is believed to be absolutely stable, but certain theories (GUTs) predict it will decay very, very slowly.
A series of thermonuclear reactions in which hydrogen nuclei are transformed into helium nuclei. The temperature and density required are about 107 K and 100 g cm-3. It is the main source of energy in the Sun, where 1038 of these reactions occur every second. All parts of this reaction have been observed in the laboratory, except for the first step 1H(p, β+v)2D, which occurs only a few times in 1012 collisions of protons. But the first two reactions provide about one-third of the Sun's total energy release. The p-p chain divides into three main branches: PP I, PP II and PP III.
An important nuclear fusion reaction that occurs in stars. It begins with the fusion of two hydrogen nuclei, each of which consists of a single proton.
Process of nuclear fusion by which relatively cooler stars produce and radiate energy; hotter stars commonly achieve the same result by means of the carbon-nitrogen cycle.
thermonuclear reaction in which two protons collide a very high velocities and combine to form deuteurium, the deuteurium can capture a proton to form tritium and tritium can capture a proton to form helium
An object discovered at Cambridge University in 1967 which has the mass of a star and a radius no larger than that of Earth and which emits radio pulses with a very high degree of regularity (periods range from 0.03 s for the youngest to more than 3 s for the oldest). All pulsars are characterized by the general properties of dispersion, periodicity, and short duty cycle. Pulsars are believed to be rotating, magnetic (surface magnetic fields of 1010 to 1014 gauss are estimated) neutron stars which are the end products of supernovae. Type S pulsars have a simple pulse shape: Type C, complex: Type D have drifting subpulses.
Elongation of a planet when it makes a 90° angle with the Sun as seen from Earth. (b) A configuration in which two celestial bodies have apparent longitudes (see longitude, celestial) that differ by 90° as viewed from a third body. Quadratures are usually tabulated with respect to the Sun as viewed from the center of the Earth.
Relativistically invariant version of quantum mechanics used to describe the physics of elementary particles. The action of forces is a result of the exchange of sub-atomic particles.
A theory of gravity that would properly include quantum mechanics. To date, there is no complete and self-consistent theory of quantum gravity, although successful quantum theories have been found for all the forces of nature except gravity. (See quantum mechanics.)
A theory that successfully mergers quantum mechanics and general relativity, possibly involving modifications of one or both. String theory is an example of a theory of quantum gravity.
A particle that is acted upon by the strong force. Quarks exist in six varieties: up, down, charm, strange, top, bottom and three "colors" (red, green, blue).
Fundamental particle of which protons, neutrons and electrons are now thought perhaps to be made up. There are possibly three or four types of quark. It is even possible that quarks themselves may be made up of still more fundamental particles.
Fundamental particles from which all hadrons are made. According to the theory of quantum chromodynamics, protons, neutrons, and their higher-energy cousins are composed of trios of quarks, while the mesons are each made of one quark and one antiquark. Held together by the strong nuclear force, quarks are not found in isolation in nature today; see asymptotic freedom.
One of the fundamental, indestructible particles of nature, out of which many other subatomic particles are made. Five types of quarks have been discovered, and it is believed that a sixth also exists. Quarks interact mainly via the strong nuclear force and the electromagnetic force.
The hypothetical constituent of the elementary particles that interacts via glue forces. Originally only three quarks were hypothesized; today it appears that six are required. For a variety of theoretical reasons, free quarks can never be seen.
An attribute which distinguishes otherwise identical quarks of the same flavor. Three colors red, green and blue - are required to distinguish the three valence quarks of which baryons are composed. It must be stressed that these colors are just labels and have nothing to do with ordinary color. Color is the source of the strong force which binds quarks together inside baryons and mesons, and so the three colors (r, g, b) can be thought of as three different color charges analogous to electric charge.
Each flavor of quark can exist in three variations, called colors, usually labeled as red, green, and blue. The color of a quark has no relation to its visual appearance, but the word color is used because there are three variations, in analogy with the three primary colors. Measurable properties of the quarks, such as electric charge and mass, depend on the flavor but not the color, but the color is responsible for the interactions that bind the quarks together (see Yang-Mills theories). Individual quarks cannot exist independently, but are forever confined within baryons or mesons, each of which is colorless. Baryons achieve colorlessness by being composed of three quarks, one of each color, while mesons achieve colorlessness by pairing each colored quark with its corresponding antiquark.
Property of quarks that expresses their behavior under the strong force. Analogous to the concept of charge in electromagnetism, except that, whereas there are two electrical charges (plus and minus), the strong force involves three color charges - red, green, and blue. The term is whimsical, and has nothing to do with color in the conventional sense, any more than quark "flavor", which determines the weak force behavior of quarks, has anything to do with taste.
An intensely bright extragalactic object which superficially resembles a star. Most exist at very high redshifts and are therefore thought to be the nuclei of active galaxies.
An object with a dominant starlike (i.e., diameter less than 1") component, with an emission line spectrum showing a large redshift - up to z = 3.53 (0.91c) for OQ 172. (The largest redshift known for a normal radio galaxy is z = 0.637 for 3C 123.) Many have multiple absorption redshifts; a few have multiple emission redshifts. (Bahcall system: class I, zabs ≈ zem; class II, zabs significantly less than zem.) The light of most if not all quasars is variable over time intervals between a few days and several years, so their diameters must not be much larger than the diameter of the solar system; yet they are the intrinsically brightest objects known (for 3C 273 (z = 0.158), Mv = -27.5 if its redshift is cosmological). The energy output of a typical quasar at "cosmological" distance is of the order of 1047 ergs per second - which would require a mass of 1010Msun if it derives its energy solely from nuclear fusion. (Energy requirement under the "local" hypothesis is on the order of 1042 ergs per second.) The basic problem of quasars is that they emit too much radiation in too short a time from too small an area.
Compact-looking objects, often radio sources, with emission lines in their spectrum which are displaced by very large amounts towards the red. These redshifts correspond to velocities which are a large fraction of the speed of light, and hence these objects are believed to lie at great distances.
Extremely distant and luminous astronomical objects that are much smaller than a galaxy and much more luminous. Quasars may be the central regions of certain very energetic galaxies at an early stage of their evolution. It is believed that the power of a quasar derives from a massive black hole at its center.
The brightest objects in the universe, quasars can generate over a trillion times as much light as the Sun from a region little larger than the solar system. Most are extremely distant, which means that they existed long ago.
The capture of neutrons on a very rapid time scale (i.e., one in which a nucleus can absorb neutrons in rapid succession, so that regions of great nuclear instability are bridged), a theory advanced to account for the existence of all elements heavier than bismuth (up to A ≈ 298) as well as the neutron-rich isotopes heavier than iron. The essential feature of the r-process is the release of great numbers of neutrons in a very short time (less than 100 seconds). The presumed source for such a large flux of neutrons is a supernova, at the boundary between the collapsing neutron star and the ejected material. However, other proposed sources have included such things as supernova shocks and black-hole-neutron-star collisions. The heavier r-process elements are synthesized at a temperature of about 109 K and an assumed neutron density of 1020-1030 per cm3. The r-process is terminated by neutron-induced fission. The existence of 244Pu (half-life 82 million years) in the early solar system shows that at least one r-process event had occurred in the Galaxy just before the formation of the solar system.
The creation of elements heavier than zinc through the rapid bombardment of other elements by neutrons. The r process occurs in supernovae. Examples of reprocess elements are gold, iodine, and europium.
The speed at which an object moves toward or away from us. It can be measured from a star's spectrum: a star moving toward us has a blueshifted spectrum, and a star moving away from us has a redshifted spectrum. The larger the blueshift or redshift, the larger the radial velocity. The present radial-velocity champion is a star in the constellation Lacerta named Giclas 233-27, which moves toward us at 583 kilometers per second.
Unit of angular measure equal to the angle subtended at the centre of a circle by an arc the length of which is equal to the radius. There are 2π radians in a circle. The angle π/4 is called an octant and a thousandth of a radian is sometimes called a mil, which is equal to 3 minutes 26.5 seconds of arc.
The Big Bang era when the temperature had dropped to 109K and the rate of electron-positron pair annihilation exceeded the rate of their production, leaving radiation the dominant constituent of the universe.
Electromagnetic radiation with the lowest energy and longest wavelength. Unlike visible light, radio waves penetrate dust and can be detected from throughout the Galaxy.
A galaxy that is extremely luminous at radio wavelengths. A radio galaxy is usually a giant elliptical - the largest galaxy in a cluster - and is a strong emitter of synchrotron radiation. M87 and M82 are examples.
Type of radio telescope that relies on the use of two or more aerials at a distance from each other to provide a combination of signals from one source which can be analyzed by computer. Such an analysis results in a resolution that is considerably better than that of a parabolic dish aerial by itself because of the greater effective diameter.
A source of extraterrestrial radio radiation. The strongest known is Cassiopeia A, followed by Cyg A and the Crab Nebula (Tau A) (the capital letters following the name of a constellation refer to the radio sources of the constellation, A being the strongest source). Radio sources are divided into two main categories: Class I, those associated with our Galaxy (which is a weak radio source), and Class II, extragalactic sources. Most radio sources are galaxies, supernova remnants, or H II regions.
Non-optical telescope (of various types) which, instead of focusing light received from a distant object, focuses radio signals onto a receiver-amplifier.
Motion of a blunt body at supersonic velocity through an ambient gaseous medium causes a strong drag or ram pressure to be exerted on the body. In the case of a galaxy moving through the intergalactic gas, the ram pressure is capable of stripping the galaxy of much of its interstellar gas.
Selective scattering (i.e., preferential scattering of shorter wavelengths) of light by very small particles suspended in the Earth's atmosphere, or by molecules of the air itself. The scattering is inversely proportional to the fourth power of the wavelength.
An approximation of Planck's blackbody formula valid at long wavelengths (hv << kT). It is often used in radio astronomy; it gives the brightness temperature of a radio telescope.
An arbitrary scale in which the freezing and boiling points of water are taken to be 0 and 80°R respectively. The scale is based on the thermal expansion of an alcohol and water mixture. If the 'length' is 1000 units at the ice point the length expands to 1080 units at the boiling point, hence the peculiar figure of 80 in this scale.
A main-sequence star with spectral type M. Red dwarfs are much fainter, cooler, and smaller than the Sun but are the most common type of star in the Galaxy, accounting for 70 percent of all stars.
A giant star with spectral type M. Such stars are in a more advanced state of evolution than the Sun, for they do not burn hydrogen into helium at their cores. Instead, they may fuse hydrogen into helium in a layer surrounding their cores, or they may fuse helium into carbon and oxygen, or they may do both. Often, astronomers use "red giant" loosely, to include not only M giants but G and K giants, too.
A late-type (K or M) high-luminosity (brighter than Mv = 0) star that occupies the upper right portion of the H-R diagram. Red giants are post-main-sequence stars that have exhausted the nuclear fuel in their cores. The red-giant phase corresponds to the establishment of a deep convective envelope. Red giants in a globular cluster are about 3 times more luminous than RR Lyrae stars in the same cluster.
Large, highly luminous but relatively cool star that has reached a late stage in its "life". It is running out of nuclear "fuel" and has accordingly expanded greatly and become less dense. Many also become variable stars of long periodicity. Its next evolutionary stage is to become a white dwarf, in developing into which the star has to cross the main sequence on the Hertzsprung-Russell diagram.
An elliptical spot on Jupiter. Its color and intensity vary with time. It has been observed for at least a century, and an examination of earlier records shows that Cassini had sketched it in the seventeenth century.
A supergiant with spectral type M. Red supergiants are the largest stars in the universe: if put in place of the Sun, some would touch Saturn. The two brightest red supergiants in Earth's sky are Betelgeuse and Antares.
A cloud of interstellar gas and dust whose spectrum contains absorption lines characteristic of the spectrum of nearby illuminating stars. The emission component of its spectrum is due to gas; the reflection component, to dust (see also diffuse nebula).
Deflection (or "bending") of light - or any ray as it passes from one medium into another of greater or lesser density, representing a change in overall speed of the ray. Refracting telescopes rely on the refraction of light through lenses. The refractive index of a medium (e.g., glass) is a measure of the medium's "bending" power.
A telescope in which the light is focused by a lens at the viewing side of the telescope. By contrast, a reflecting telescope is one in which light is focused by a mirror.
The energy which a particle has even when it is at rest. According to the famous relation E = mc2 of special relativity, this rest energy is equal to the rest mass of the particle-the mass it has when a rest-times the square of the speed of light. If the mass is in grams and the speed of light in centimeters per second (c = 2.998 × 1010 centimeters per second), then the energy is given in ergs.
Angular distance on the celestial sphere measured eastward along the celestial equator from the equinox to the hour circle passing through the celestial object. Right ascension is usually given in combination with declination.
A system of four concentric rings, only about 2-4 km thick. The outermost ring is ring A, then comes Cassini's division, then ring B (also called the bright ring), then Lyot's division, then ring C (the crepe ring), then ring D (discovered in 1969). The rings are a swarm of solid particles, probably jagged rocks about 1 meter to 1 km across (1973), not ice as previously had been assumed, inside the Roche limit. Bobrov (1969) estimates the total mass of the rings to be about 0.01 the lunar mass.
A galaxy with a ring-like appearance. The ring contains luminous blue stars, but relatively little luminous matter is present in the central regions. It is believed that such a system was an ordinary galaxy that recently suffered a head-on collision with another galaxy.
ROentgen SATellite, was an X-ray observatory developed through a cooperative program between the Germany, the United States, and the United Kingdom. The satellite was designed and operated by Germany, and was launched by the United States on June 1, 1990. It was turned off on February 12, 1999.
Primary national Observatory in Great Britain, first sited at Greenwich in 1675, but in 1958 moved to Herstmonceux, Sussex. From the first, Directorship of the Observatory has entailed appointment as Astronomer Royal. In the 1980s the Observatory will lose its primary national status with the completion of the Northern Hemisphere Observatory in Las Palmas, the Canary Islands.
An old metal-poor white or yellow-white giant star that pulsates like a Cepheid and therefore varies in brightness. Most RR Lyrae stars have periods of under one day, which is shorter than periods for Cepheids. RR Lyrae stars are also fainter than Cepheids, with absolute magnitudes around +0.7, corresponding to a luminosity about 45 times the Sun's. RR Lyrae stars are excellent distance indicators because they all have nearly the same intrinsic brightness. They take their name from the star RR Lyrae, in the constellation Lyra.
A large class of pulsating (amplitude variation about 1 mag) blue giants of anomalous spectral type (A2-F6) with periods of less than 1 day. Their average absolute magnitude is about +0.8. which makes them almost 50 times more luminous than the Sun. They are Population II objects often (but not always) present in globular clusters. RR Lyrae stars are valid distance indicators out to more than 200 kpc.
Star of spectral type O or early B with unusually high space velocities. Runaway stars are thought to be produced when there is a supernova explosion in a close binary system.
A class of about 100 semiregular variable yellow supergiants of late spectral type (G-K), similar to W Virginis stars but with longer periods. Their spectra often contain emission lines, and their light curves have alternating deep and shallow minima. They have a large infrared flux. RVa stars maintain an approximately constant mean brightness; RVb stars have long-term (on the order of 1000 days) periodicity.
Late type giants (K5 to M) showing distinct bands of ZrO. Jaschek, C., Jaschek, M. 1995 The Behavior of Chemical Elements in Stars, Cambridge University Press
Red-giant star of spectral type S are similar to M stars except that the dominant oxides are those of the metals of the fifth period (Zr, Y, etc.) instead of the third (Ti, Sc, V). They also have strong CN bands and contain spectral lines of lithium and technetium. Pure S stars are those in which ZrO bands are very strong and TiO bands are either absent or only barely detectable. Almost all S stars are LPVs. (S1,0. The number following the comma is an abundance parameter.)
Red-giant stars of spectral type S are similar to M stars except that the dominant oxides are those of the metals of the fifth period (Zr, Y, etc.) instead of the third (Ti, Sc, V). They also have strong CN bands and contain spectral lines of lithium and technetium. Pure S stars are those in which ZrO bands are very strong and TiO bands are either absent or only barely detectable. Almost all S stars are LPVs. (S1,0. The number following the comma is an abundance parameter.) Hopkins, J. 1976 Glossary of Astronomy and Astrophysics, University of Chicago Press
The process by which elements heavier than copper are formed through a slow flux of neutrons. The s-process operates in red giant stars; prominent s-process elements include barium, zirconium, yttrium, and lanthanum.
A massive (3 × 106Msun), dense (up to 108 particles per cm3) H II region and molecular cloud complex - the richest molecular source in the Galaxy. It is in the galactic plane, near the galactic center.
The geocentric angle between a satellite and its primary, measured in the plane of the satellite, planet and Earth. Satellite elongations are measured from 0° east or west of the planet.
Sixth major planet out from the Sun. The most spectacular of the Solar System, it is circled by a series of concentric rings. All the satellites of Saturn are locked in synchronous rotation.
The process whereby light is absorbed and reemitted in all directions, with essentially no change in frequency. Scattering by free electrons was the dominant source of opacity in the early universe.
A telescope with a spherical primary mirror and a thin refractive corrector plate with a complex, non-spherical shape. Very wide-field performance for surveys.
A type of reflecting telescope (more accurately, a large camera) in which the coma produced by a spherical concave mirror is compensated for by a thin correcting lens placed at the opening of the telescope tube. The Schmidt has a usable field of 0°.6.
Telescopic camera incorporating an internal corrective lens or plate that compensates for optical defects and chromatic faults in the main mirror. The system was invented by Bernhard Schmidt.
Adherents to the philosophy and cosmology of Aristotle. Their dominance in the universities, which had been founded largely to study Aristotle, constituted an obstacle to acceptance of the Copernican system advocated by Kepler and Galileo.
The center of a black hole. According to Einstein's theory of general relativity, the entire mass of a black hole is concentrated at a point at its center, the "singularity". It is believed that quantum mechanical effects, not included in the theory, would cause the mass to spread out over a tiny but nonzero region, thus preventing an infinite density of matter and doing away with the singularity.
Systematic study of Nature, based upon the presumption that the universe is based upon rationally intelligible principles and that its behavior can therefore be predicted by subjecting observational data to logical analysis.
A compact eclipsing X-ray source. It has day-to-day variations (period about 0.78 days?) of as much as 1 mag; it also has optical and radio counterparts but no correlation has been found among the flares observed at the three different wavelengths. It is a thermal X-ray source, probably associated with a rotating collapsed star surrounded by an extensive envelope. Tentative optical identification with the 13th mag blue variable V818 Sco. The spectrum of Sco X-1 is similar to that of an old nova. (3U 1617-15)
A unit of time defined as the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom. In 1967 the General Conference of Weights and Measures (CGPM) adopted this as the tentative definition of the second in SI units, replacing the ephemeris second, which remains in the IAU system of astronomical constants.
duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom
The second reflecting surface encountered by the light in a telescope. The secondary is usually suspended in the beam and therefore obstructs part of the primary.
A parallax based on solar motion; i.e., the baseline is the distance the Sun moves in a given interval of time with respect to the local standard of rest.
A large mirror construction technique in which many smaller elements are built and then actively controlled to conform to the shape of the required large mirror.
The process by which light from an astronomical object grows red as the light travels through interstellar dust. Dust scatters blue light more than red, thus leaving predominantly red light transmitted.
Spectral lines from "semiforbidden" transitions, i.e., those whose transition probabilities are perhaps 1 in 106 instead of about 1 in 109 for forbidden transitions. One bracket - e.g., [C III] - is used to indicate semiforbidden lines.
A type of spiral galaxy first discovered by Karl Seyfert in the 1940s. The central region of a Seyfert galaxy is distinguished by powerful radiation, much of it focused into narrow frequencies.
One of a small class of galaxies (many of which are spirals) of very high luminosity and very blue continuum radiation with small, intensely bright nuclei whose spectra show strong, broad, high-excitation emission lines probably caused by discrete clouds moving at velocities that are higher than the escape velocity. Seyferts possess many of the properties of QSOs, such as the ultraviolet excess of the continuum, the wide emission lines, and the strong infrared luminosity. The energy sources in their nuclei are unexplained; presumably the energy input can be associated with some process that liberates gravitational binding energy to accelerate relativistic particles. Seyferts comprise about 1 percent of the bright galaxies. The brightest Seyfert known is NGC 1068. Weedman-Khachikian classification: class 1 Seyferts have broad Balmer line wings; class 2 have no obvious Balmer line wings.
Theoretical classes of particles, their existence intimated by supersymmetry, theory, that participate in few if any of the four known fundamental forces. Planets, stars, and galaxies made of shadow matter could conceivably exist in the same space and time we occupy without our sensing their presence.
A catalog of all galaxies brighter than seventeenth magnitude (a measure of brightness). There are about a million galaxies in the Shane-Wirtanen catalogue.
A hot main-sequence star, usually of spectral class B-F, whose spectrum shows bright emission lines presumed to be due to a gaseous ring or shell surrounding the star.
International System of Units. A practical system of units of measurement adopted in 1969 by the 11th International General Conference of Weights and Measures (CGPM). The seven base units are the meter, the kilogram, the second, the ampere, the kelvin, the mole, and the candela.
The length of time between two successive meridian transits of the vernal equinox (cf. mean solar day). Because of precession the sidereal day is about 0.0084 second shorter than the period of rotation of Earth relative to a fixed direction (23h56m4s.099).
A period of time based on the revolution of the Earth around the Sun, where a year is defined as the mean period of revolution with respect to the background stars.
A place, either in space or in time at which some quantity, such as density, becomes infinite. The laws of physics cannot describe infinite quantities and, in fact, physicists believe that infinities do not exist in nature. All singularities, such as the Schwarzschild singularity, are therefore probably the artifacts of inadequate theories rather than real properties of nature. According to Einstein's theory of general relativity, the universe began in a singularity of infinite density, the big bang. Physicists believe that an improved and yet-to-be discovered modification of general relativity, incorporating quantum mechanics, will show that the universe did not begin as a singularity. (See Schwarzschild singularity.)
A point of infinite curvature of space where the equations of general relativity break down. A black hole represents a singularity; so, perhaps, did the universe at the first moment of time.
Anomaly in space-time at which a state not in accord with the classical laws of physics obtains. An example is a black hole; another is the moment of the big bang.
If the standard big bang theory is extrapolated all the way back to time zero, one reaches an instant of infinite density, infinite pressure, and infinite temperature - an instant that is frequently called the initial singularity. This singularity is sometimes said to mark the beginning of time, but it is more realistic to recognize that an extrapolation to infinite density cannot be trusted.
The brightest star in the sky. Its companion (Sirius B) is a white dwarf of about 0.96 Msun but only about 0.03 Rsun, the nearest white dwarf to Earth.
A nova whose light curve shows a much more gradual development - i.e., rise time of several days, maximum of several weeks, slower decline, amplitude only about 10 mag.
An X-ray source in the Small Magellanic Cloud. It is a binary system. Identified with Sanduleak No. 160, a B0 I supergiant (mv = + 13.6). Because no radial-velocity variations are apparent in Sk 160, the mass of the X-ray emitter must be small relative to Sk 160 (about 2 Msun if Sk 160 is 20 Msun), unlike the compact member of CygX-1.
For a refracted light beam, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant. (also called the Law of Refraction)
sodium (Na, 2S1/2 in ground state, natrium from Latin)
soft, silvery-white metal which oxidizes rapidly when cut
A point on the celestial sphere toward which the Sun and the solar system are moving with respect to the local standard of rest at a rate of about 19.4 km per second (about 4.09 AU per year).
The gaseous outer layers of the Sun, including, from the deeper layers outward, the photosphere, the chromosphere, and the corona. The atmosphere constitutes those layers of the Sun that can be observed directly.
Suddenly enhanced nonthermal radio emission from the high solar corona immediately following a solar flare, probably due to energetic electrons trapped in the coronal magnetic field. Bursts are divided into several types, depending on their time frequency characteristics (type III is the most common). They are classified on a scale of importance ranging from -1 (least important) to +3. Bursts are generally attributed to a sudden acceleration of some 1035-36 electrons to energies greater than 100 keV in less than 1 second.
The 11-year period between maxima (or minima) of solar activity. Every 11 years the magnetic field of the Sun reverses polarity; hence the more basic period may be 22 years.
An eclipse in which the Earth passes through the shadow cast by the Moon. It may be total (observer in the Moon's umbra), partial (observer in the Moon's penumbra), or annular. (See eclipse, annular.)
The velocity of the Sun through space, relative to the local standard of rest. The solar motion is U = -9 kilometers per second, V = +12 kilometers per second, and W = +7 kilometers per second.
A radial outflow of hot plasma from the solar corona which carries both mass and angular momentum away from the sun. It is the effects of the solar wind that produce aurorae in the Earth's upper atmosphere, that cause the tails of comets to stream back from the Sun, and that distort the symmetry of planetary magnetospheres.
One of the two points on the ecliptic at which the Sun appears to be farthest away from the celestial equator (representing therefore mid-summer or mid-winter).
A disturbance in the geomagnetic field (a region of intense charged-particle fluxes) over the south part of the Atlantic Ocean. It was discovered in early OAO (Orbiting Atronomical Observatory) flights that when the detector passed over that area, the data it collected were not valid.
Velocity of a star with respect to the Sun; hypotenuse of the right triangle formed by its radial and tangential velocities (cf. peculiar velocity). Space motion vectors are U (in the direction of the galactic anticenter), V (in the direction of galactic rotation), and W (in the direction of the galactic north pole).
A star's total velocity with respect to the local standard of rest. This is the combination of the star's U, V, and W velocities: space velocity = sqrt (U2 + V2 + W2). For example, the Sun (U = -9, V = +12, W = +7) has a space velocity of 17 kilometers per second.
A line corresponding to an image of the entrance slit of the spectrometer, seen when light is either emitted by or interrupted by a hot rarefied gas such as hydrogen. The pattern is characteristic of the gas and the wavelength at which the features are observed to occur is indicative of the velocity of the object.
Dark lines visible in an absorption spectrum, or bright lines that make up an emission spectrum. They are caused by the transference of an electron in an atom from one energy level to another; strong lines are produced at levels at which such transference occurs easily, weak where it occurs with difficulty. Ionization of certain elements can affect such transferences and cause problems in spectral analysis.
Discrete emissions (or absorptions) in frequency, usually formed by atomic transitions. The essential difference between optical line spectra and X-ray spectra is that the former correspond to energy changes in the outer electrons in an atom, and the latter to energy changes in the inner electron orbitals. Gamma rays usually correspond to energy changes in the nucleus. Infrared radiation is produced by high-n transitions of atoms or by the vibration or rotation of molecules. Thermal radio emission is usually produced by still higher-n transitions (the notation 109α corresponds to a transition in a hydrogenic atom between the principal quantum number n = 109 and n' = Δn = n + 1 = 110; similarly, a β-transition indicates Δn = 2. etc.).
Emission or absorption at a discrete wavelength or frequency, caused by atomic or molecular transitions. In the case of atoms, the transitions involve the jump of an electron from one orbit to another; a quantum of light is emitted if the electron jumps toward the nucleus and absorbed if it jumps outward.
A device, usually based on a finely etched grate that performs the function of a prism, for breaking up light into its constituent parts and making a photographic or electronic record of the resulting spectrum. When lacking a means for recording the spectrum, the device is called a spectroscope.
An instrument that records the amount of light in each range of wavelength, that is, in each range of color. In general, each type of astronomical object, such as a star or a galaxy, will emit a characteristic spectrum of light. (See spectrum.)
Star whose binary nature can be detected from the periodic Doppler shifts of their spectra, owing to their varying velocities in the line of sight. Double-lined spectroscopic binaries have two sets of spectral features, oscillating with opposite phases. Single-lined spectroscopic binaries have only one set of oscillating spectral lines, owing to the dimness of the secondary component. Spectroscopic binaries are typically of spectral type B, with almost circular orbits (whereas long-period M-type binaries have highly eccentric orbits).
Main-sequence Am or Ap stars whose spectra show anomalously strong lines of metals and rare earths which vary in intensity by about 0.1 mag over periods of about 1-25 days. They are characterized by large magnetic fields (103-104 gauss) at the surface, small variations in light and color, and small projected rotational velocities. These peculiarities are sometimes interpreted in terms of an oblique rotator.
The outer surface of a ball. The surface of a familiar three-dimensional ball has two dimensions (which can be labeled by two numbers such as "latitude" and "longitude," as on the surface of the earth). The concept of a sphere, though, applies more generally to balls and hence their surfaces, in any number of dimensions. A one-dimensional sphere is a fancy name for a circle; a zero-dimensional sphere is two points (as explained in the text). A three-dimensional sphere is harder to picture; it is the surface of a four-dimensional ball.
Concept probably older than the ancient Greeks, in which the Sun, Moon, planets and the stars were thought to orbit the Earth travelling on their own crystalline but - except for that of the stars - transparent spheres.
A quantum-mechanical version of the familiar notion of the same name; particles have an intrinsic amount of spin that is either a whole number or half a whole number (in multiples of Plancks constant), and which never changes.
The intrinsic angular momentum of an elementary particle, as by the particle's spinning on its axis. Spin is quantized in units of Planck's constant of action, h, so that, e.g., "spin 1," means spin = 1h. Particles with integral spin (0, 1) are called bosons; those with half spin are fermions.
The intrinsic angular momentum possessed by many particles. It can be thought of as resulting from the particles spinning about an axis through their centers. In contrast to orbital angular momentum, spin is quantized in integer and half-integer units of h bar. Fundamentally, spin describes how quantum fields transform under the transformations of special relativity.
Collision between particles in which the direction of the spin angular momentum changes. Since the total angular momentum is conserved, the orbital angular momentum must be changed in magnitude or direction or both.
A wave, due to a local increase in the gravitational field, that produces a series of alternate compressions and rarefactions as it propagates with fixed angular velocity in a rotating galaxy. The compression also acts on interstellar gas in the galaxy, which is triggered to form stars on the leading edges of the spiral arms. The large-scale structure of spiral galaxies can be understood in this way.
Belief that material interactions alone cannot account for all phenomena, and that some - e.g., thought - are due to the fundamentally insensible actions of intangibles.
Any situation in physics in which the ground state (i.e. the state of minimum energy) of a system has less symmetry than the system itself. For example, the state of minimum energy for an iron magnet is that in which the atomic spins are all aligned in the same direction, giving rise to a net macroscopic magnetic field. By selecting a particular direction in space. the magnetic field has broken the rotational symmetry of the system. However, if the energy of the system is raised, the symmetry may be restored (e.g. the application of heat to an iron magnet destroys the magnetic field and restores rotational symmetry).
The breaking of an exact symmetry of the underlying laws of physics by the random formation of some object. For example, the rotational in variance of the laws of physics can be broken by the randomly chosen orientation of an orthorhombic crystal that condenses as the material is cooled. In the standard model of particle physics, the symmetry between electrons and neutrinos is spontaneously broken by the values that are randomly chosen by the Higgs fields. In grand unified theories, the symmetry between electrons, neutrinos, and quarks is spontaneously broken by the values chosen randomly by the Higgs fields.
A subclass of dwarf nova. SS Cyg is a double-lined, noneclipsing spectroscopic binary (sdBe, dG5) with an orbital period of 6h38m. Mean time between eruptions, 54 days. It may be a sporadic source of soft X-rays.
The standard deviation of a distribution of means or any other statistical measure computed from samples. It is equal to 1.4826 times the probable error.
A pattern of oscillations in space in which the regions of maximum displacement and of zero displacement (the nodes) remain fixed in position. This pattern is formed when two waves of the same amplitude and frequency move simultaneously through a medium in opposite directions.
It is distinguished by having a 2-mile-long linear accelerator in which electrons and positrons can be accelerated for subsequent injection into storage rings such as PEP, an e+e- collider which was commissioned in 1980. It was in the SPEAR rings at SLAC that the J / ψ meson and the τ lepton were first observed in the mid-1970s. However, the most fascinating of SLAC's facilities is the novel SLC (Stanford Linear Collider), consisting of the old linear accelerator together with two new collider arcs.
A celestial object that generates energy by means of nuclear fusion at its core. To do this it must have more than about 0.08 the sun's mass. If, for instance, the planet Jupiter were some fifty to one hundred times more massive than it is, fusion reactions would transpire in its core and it would be a star. See planet.
The range of variation of some quantity in a population, obtained by sampling many members of the population. For example, the statistical distribution of the height of American males could be obtained by sampling 10,000 randomly chosen males and counting the number of them within each range of heights. In cosmology, the distance between pairs of galaxies, averaged over a large number of galaxies, would constitute a statistical distribution.
The uncertainty resulting from a measurement of purely random events. Such an uncertainty is defined as bracketing a range of values within which the correct value has a 66% chance of lying. For example, a value of (100 ± 10) obtained from a given measurement means that the true value has a 66% chance of lying between 90 and 110, and a 34% chance of being either above or below this range.
A cosmological theory propounded by Bondi, Gold, and Hoyle in which the Universe has no beginning and no end and maintains the same mean density, in the face of its observed expansion, by the continuous creation of matter at the current rate of 2.8 × 10-46 g cm-3 s-1 (or roughly one nucleon per cubic kilometer per year). Discovery of the microwave background has persuaded most astronomers to reject the steady-state theory.
Theory that the expanding universe was never in a state of appreciably higher density - i.e., that there was no "big bang" - and that matter is constantly being created out of empty space in order to maintain the cosmic matter density.
A constant relating the energy radiated by a black body to its absolute temperature and incorporated in the Stefan-Boltzmann radiation law which states that the energy radiated per unit time is given by σ(T4 - T40), where σ is the Stefan-Boltzmann constant and T and T0 are the absolute temperatures of the body and its surroundings expressed in kelvins.
A type of plasma machine. It has a twisted-field configuration in the form of a figure 8 to fold the plasma back on itself; therefore, unlike a pinch machine, it has no ends where the plasma can leak out. Stellarators and tokomaks resemble each other in that both are toroidal devices that attain equilibrium and MHD stability through rotational transform and shear; they differ mainly in the way they attain these properties.
A highly disturbed cluster of five peculiar galaxies (NGC 7317, NGC 7318A, NGC 7318B, NGC 7319, NGC 7320) in Pegasus which seem to exhibit gaseous connecting bridges. Four have large redshifts (of the order of 5700-6700 km s-1), but the fifth member (NGC 7320) has a much smaller redshift (800 km s-1). Discovered in 1877 by M. E. Stephan.
Unit of solid (three-dimensional) angular measure. One steradian is equal to the angle subtended at the centre of a sphere by an area of surface equal to the square of the radius. The surface of a sphere subtends an angle of 4π steradians at its centre.
Radiation emitted by a body, such as an atom, when it is bombarded by radiation. The stimulated radiation has the same wavelength and direction as the bombarding radiation.
A property ascribed to certain hyperons whose lifetimes before decay are abnormally long (about 10-8 to 10-10 seconds) relative to their rates of production (about one every 10-23 seconds). Like parity, strangeness is conserved in strong interactions but not in weak ones.
A theory in which the fundamental constituents of matter are not particles but tiny one-dimensional objects, which we can think of as strings. These strings are so minute (only 10-33 cm long) that, even at current experimental energies, they seem to behave just like particles. So, according to string theory, what we call "elementary particles" are actually tiny strings. each of which is vibrating in a way characteristic of the particular "elementary particle".
Nambu's original idea that the elementary particles could be described as extended, one-dimensional objects was called string theory. Since the ends of Nambu's strings whipped around at the speed of light they were also called light strings. Later attempts to include the spin half fermions within a string theory led to the term spinning strings. Strings that possess supersymmetry are called superstrings. Heterotic strings combine spaces of two different dimensionalities. The term string is used in a generic way to describe all these different variations, including superstrings.
The hypothesized, basic constituents of matter, according to new theories of physics. In earlier theories of physics, the basic constituents of matter were point-like particles, such as electrons, which interacted with other particles at a point. According to the string theory, the basic constituents are 1-dimensional structures called strings. There are completely different strings, called cosmic strings, which can form according to some theories and which may extend for great distances in space. Postulated to have formed as a result of processes in the early universe, cosmic strings are 1-dimensional structures of enormous energy, extending for perhaps thousands or millions of light years in space. There is no good observational evidence that either kind of strings exist. (See superstring theory.)
Theory that subatomic particles actually have extension along one axis, and that their properties are determined by the arrangement and vibration of the strings.
Unified theory of the universe postulating that fundamental ingredients of nature are not zero-dimensional point particles but tiny one-dimensional filaments called strings. String theory harmoniously unites quantum mechanics and general relativity the previously known laws of the small and the large, that are otherwise incompatible. Often short for superstring theory.
Fundamental force of nature that binds quarks together, and holds nucleons (which are comprised of quarks) together as the nuclei of atoms. Portrayed in quantum chromodynamics as conveyed by quanta called gluons.
Strongest of the four fundamental forces, responsible for keeping quarks locked inside protons and neutrons and for keeping protons and neutrons crammed inside of atomic nuclei.
The interactions which bind quarks together to form rotons, neutrons, and other particles. The residual effects of these forces are responsible for the forces between protons and neutrons. See Yang-Mills theories.
The short-range nuclear force which is assumed to be responsible for binding the nucleus together. Strong interactions are so called because they occur in the extremely short time of about 10-23 seconds. Strong interactions can occur only when the particles involved are less than 3 fermis apart.
The short-range nuclear interactions responsible for holding nuclei together. The characteristic range of the strong interaction is 10-13 cm, and the time scale over which it operates is 10-33 second.
Symmetrical Unitary of Order 3: A symmetry found in sub-nuclear spectra. It is a concept in group theory, by which Gell-Mann and others, using eight quantum numbers, have been able to combine particles into family groups or supermultiplets, as the lowest-lying eightfold group of the nucleon doublet, the Λ singlet, the Σ triplet, and the Ξ doublet. The SU(3) theory applies only to the strongly interacting particles.
One of the suggested symmetries of the grand unified theory in which the gluon and electroweak forces are united. It includes the group SU(3) × SU(2) × U(1).
A metal-poor main-sequence star. On the H-R diagram, subdwarfs lie slightly below the metal-rich Main Sequence, because they are fainter than metal-rich main-sequence stars of the same color.
Star whose luminosity is 1.5 to 2 magnitudes lower than that of main-sequence stars of the same spectral type. Subdwarfs are primarily Population II and lie just below the Main Sequence on the H-R diagram.
Star that Earth orbits. Central body of solar system. It takes about 1-10 million years for photons to diffuse from the Sun's interior to its surface. About 3% of the energy radiated is in the form of neutrinos. Every second about 655 million tons of H are being converted into 650 million tons of He. A grazing light ray is deflected 1".7 by the Sun. If the total angular momentum of the solar system were concentrated in the Sun, its equatorial rotation speed would be about 100 km s-1.
Time required for the Sun to evolve a significant distance off the main sequence; the time it takes the Sun to convert all its available hydrogen into helium.
The times at which the apparent upper limb of the Sun is on the astronomical horizon; i.e., when the true zenith distance, referred to the center of the Earth, of the central point of the disk is 90°50', based on adopted values of 34' for horizontal refraction and 16' for the Sun's semidiameter.
The times at which the apparent upper limb of the Sun is on the astronomical horizon; i.e., when the true zenith distance, referred to the center of the Earth, of the central point of the disk is 90°50', based on adopted values of 34' for horizontal refraction and 16' for the Sun's semidiameter.
A temporary disturbed area in the solar photosphere that appears dark because it is cooler than the surrounding areas. Sunspots usually occur in pairs of opposite polarity about 30° N and S of the equator, and move in unison across the face of the Sun as it rotates. The leading (or preceding) spot is called the p-spot; the following, the f-spot. Some sunspots have magnetic fields as high as 1000 gauss (highest observed was 5000 gauss. Typical diameter, 109 cm.
Comparatively dark spot on the Sun's photosphere, commonly one of a (not always obvious) group of two. The center of a vast electrostatic field and a magnetic field of a single polarity (up to 4000 gauss), a sunspot represents a comparatively cool depression (at a temperature of approximately 4500 °C). Sunspots occur in cycles of about 11 Earth-years in period although their individual duration - a matter of Earth-days only - is affected by the differential rotation of the Sun; they tend to form at high latitudes and drift towards the solar equator. They are also sources of strong ultra-shortwave radio emissions.
A cluster of clusters of galaxies. Superclusters are typically about one hundred million (108) light-years in diameter and contain tens of thousands of galaxies.
An apparent plane of symmetry, passing through the Virgo cluster of galaxies, about which many of the brightest galaxies in the sky are concentrated. These galaxies form the Local Supercluster.
Convective cell in the solar photosphere, distributed fairly uniformly over the solar disk, that last as long as a day. New sunspots develop in the intersections of adjacent supergranulation cells. Most of the magnetic flux through the photosphere is concentrated in the supergranule boundaries.
A supersymmetric theory of gravity in which the graviton is accompanied by a spin-3/2 particle called the "gravitino". In supergravity theories, supersymmetry has been promoted to the status of a local gauge symmetry.
A gigantic stellar explosion in which the star's luminosity suddenly increases by as much as a billion times. Most of the star's substance is blown off, leaving behind, at least in some cases, an extremely dense core which may be a neutron star.
A gaseous nebula, the expanding shell ejected by a supernova, and deriving its energy (at least in some cases) from the conversion by the remanent neutron star of its rotational energy into a stream of high-energy particles being continually accelerated in the SNR. About 100 SNRs are known in our Galaxy. Supernova remnants are usually powerful radio sources.
The expanding shell of gas ejected at high speed by a supernova explosion, observed as an expanding diffuse gaseous nebula, often with a shell-like structure.
A new type of theory in physics that unifies all the forces of nature, including the gravitational force, and that may be capable of explaining all of the fundamental laws and particles of nature. In superstring theories, the basic constituent of matter is a 1-dimensional structure, called a string, rather than a point-particle structure. According to superstring theory, space has more than 3 dimensions.
A proposal for the ultimate laws of nature, a "theory of everything," stemming primarily from discoveries in the mid 1980's. The fundamental entity in this theory is an ultramicroscopic string-like object, with a length of typically 10-33 centimeters and effectively zero thickness. At present our understanding of string theory is very limited. The simplest predictions of superstring theory concern processes at the Planck energy, and so far very little is known about the consequences of string theory at lower energies.
A mathematical property of some theories of physics proposing that every particle of integer spin (intrinsic angular momentum) has a partner of half integer spin. For example, the photon, which is the particle of light, has a spin of 1 unit. Its hypothesized super symmetric partner is called the photino, which would have a spin of 1/2 units.
A symmetry principle that relates the properties of particles with a whole number amount of spin (bosons) to those with half a whole (odd) number amount of spin (fermions).
A symmetry relating fermions and bosons. If supersymmetry is a true symmetry of nature, then every "ordinary" particle has a corresponding "superpartner" which differs in spin by half a unit.
Class of theories that seek to identify symmetrical relationships linking fermions and bosons - i.e., particles of half integer spin, like electrons, protoins, and neutrinos, with those of integral spin, like photons and gluons. If attainable, a fully realized supersymmetry theory would provide a unified account of all four fundamental forces, and might well shed light on the very early evolution of the universe as well.
Hypothetical theory that presumably would show how all four fundamental forces of nature functioned as a single force in the extremely early universe. The best current candidates for such a potential achievement are thought to be supersymmetry and string theory.
While grand unified theories attempt to describe three of the four known interactions of nature - the weak, strong, and electromagnetic interactions - in a unified way, the fourth interaction, gravity, is omitted. Theories which attempt to include gravity as well, such as superstrings, are called superunified.
Ordinary electron-proton bremsstrahlung viewed from the rest frame of the electron rather than the proton; in other words, the electron is at rest and the heavy particle (proton) is moving.
A term originally used by P. Merrill to describe stars of two essentially dissimilar kinds which seem to occur together and which seem to "need" each other. In practice, it has come to signify a peculiar group of objects (usually spectral type Me) that display a combination of low-temperature absorption spectra and high-temperature emission lines. These objects undergo semiperiodic nova-like outbursts and display the spectral changes of a slow nova superposed on the features of a late-type star. Their spectra are midway between those of planetary nebulae and true stellar objects. A symbiotic star is now usually taken to be a small, hot, blue star surrounded by an extensive variable envelope. As of 1973 about 30 were known.
A property of a physical system that does not change when the system is transformed in some manner. For instance, a sphere is rotationally symmetrical since its appearance does not change if it is rotated.
State of a system such that it has a significant quantity that remains invariant after a transformation. More generally, an apt or pleasing proportion based upon such a state.
The property of being unchanged after some transformation. A square, for example, has a 4-sided rotational symmetry. It appears the same after it is rotated by 90 degrees.
In cosmology and particle physics, a state in which traces of an earlier symmetry may be discerned. A broken symmetry condition differs from chaos in that its parts can in theory be united in a symmetrical whole, like the pieces of a jigsaw puzzle.
The process by which an intrinsic symmetry of a system is disrupted. For example, a compass, in the absence of any outside magnetic field, has rotational symmetry and is equally likely to point in any direction. The magnetic field of the earth breaks the symmetry and causes the compass to point in a particular direction, toward the earth's north magnetic pole. In some cosmological models, the infant universe was much more symmetric than it is today. As the universe aged and cooled, some of these symmetries were permanently broken.
Also called T Tauri variable, a type of variable star of spectral classification F, G or K (giants above the main sequence on the Hertzsprung-Russell diagram) that loses an appreciable proportion of its mass in its (irregular) more luminous periods, and is thus surrounded by volumes of gas and dust.
Eruptive variable subgiant star associated with interstellar matter and believed to be still in the process of gravitational contraction on their way to the main sequence. They are found only in nebulae or very young clusters. They have low-temperature (G-M) spectra with strong emission lines and broad absorption lines. Their absolute magnitudes are brighter than those of main-sequence stars of similar spectral types. They have a high lithium abundance. T Tau itself is dG5e. (sometimes called RW Aurigae stars)
Luminous variable stars with low effective temperatures and strong emission lines, associated with interstellar gas clouds and found in very young clusters. They are believed to be still in the process of gravitational contraction from their protostellar phase and have not yet arrived at the main sequence and begun to burn hydrogen.
The third mirror to be encountered by the light in a telescope system. A tertiary mirror is required on alt-az telescopes to direct light to the stationary Nasmyth foci.
The area of physics that analyzes the behavior of a system with very many members, such as a gas with many individual molecules. In such a situation, the behavior of the whole system is obtained by averaging over the behavior of individual members.
The stellar population that contains Arcturus and about 4 percent of the other stars near the Sun. It has a scale height of about 3500 light-years and consists of old stars.
The stellar population that contains the Sun and most other nearby stars. Most of its stars have a scale height of 1000 light-years and orbit the Galaxy on fairly circular orbits. The stars of the thin disk range in age from 0 to about 10 billion years. The thin disk breaks into two subpopulations, the young thin disk and the old thin disk. The young thin disk has a smaller scale height than the old thin disk, and the former's stars have more circular orbits.
An experiment that cannot be or is not carried out in practice, but can, given sufficient imagination and rigor, be reasoned through by thought and intuition alone.
A CCD construction in which three overlapping metal electrodes are used to define a pixel and effect the transfer of charge, in either direction along a column, by the charge-coupling method. If only two electrodes are used then the device is two-phase.
A dimension distinguishing past, present, and future. In relativity, time is portrayed as a geometrical dimension, analogous to the dimensions of space.
The hypothesis that light may be degraded in energy, thereby increasing in wavelength and becoming redshifted, during its passage through intergalactic space. This would provide an alternative to the Big Bang model in accounting for the redshifts of distant galaxies. However, there is no evidence for any such tired-light effect.
The topological name for the shape of a donut. While a donut is a two-dimensional surface in a three-dimensional space, the torus can be generalized to higher numbers of dimensions.
A theory which is known to be too simple to describe reality, but which is nonetheless useful for theorists to study because it incorporates some important features of reality. For example, most of what is known about magnetic monopoles in grand unified theories was discovered first in a toy theory that includes only three Higgs fields, while the simplest realistic grand unified theory includes twenty-four of them.
A stationary support structure for a telescope. Motion is allowed along the meridian from the zenith to the horizon, but stars cannot be tracked east/west. Measurements are only possible when the objects "transit" the meridian due to the Earth's rotation.
The inner satellite of Neptune, discovered by Lassell in 1846. It is larger than the Moon (R ≈ 2900 km), with an almost circular retrograde orbit of 5 days 21 hours.
The period of one complete revolution of the mean longitude of the sun with respect to the dynamical equinox. The tropical year is longer than the Besselian year by 0s.148 T, where T is centuries from B1900.0.
Lowest level of Earth's atmosphere, from zero altitude to about 15 km above the surface. This is the region where most weather occurs. Its temperature decreases from about 290 K to 240 K.
An observed relation between the intrinsic luminosity of a spiral galaxy and the rotational speed of its stars. More luminous galaxies have stars that are moving faster.
A theoretical model for radio galaxies in which a compact source in the galactic nucleus is assumed to emit twin beams of rapidly moving plasma that traverse hundreds of thousands of light-years, eventually splattering to a halt in the ambient intergalactic gas, where the resulting dissipation energizes the radio lobes.
Model of the Universe proposed by Roger Penrose, based on the application of complex numbers (involving (-1)1/2) used in calculations in the microscopic world of atoms and quantum theory to the macroscopic ordinary world of physical laws and relativity. The result is an eight-dimensional concept of reality that although complicated is possibly a more logical understanding of the constitution of the Universe.
Remnant of a Type I supernova (B Cas), 3-5 kpc distant, which Tycho observed and described in 1572. At its peak it was as bright as Venus and was visible in the daytime, reaching a magnitude of about -4. It is an X-ray source (2U 0022+63). (3C 10)
A gigantic stellar explosion in which the star's luminosity suddenly increases by as much as a billion times. Most of the star's substance is blown off, leaving behind, at least in some cases, an extremely dense core which may be a neutron star.
A gigantic stellar explosion in which the star's luminosity suddenly increases by as much as a billion times. Most of the star's substance is blown off, leaving behind, at least in some cases, an extremely dense core which may be a neutron star.
A gigantic stellar explosion in which the star's luminosity suddenly increases by as much as a billion times. Most of the star's substance is blown off, leaving behind, at least in some cases, an extremely dense core which may be a neutron star.
A type of dwarf nova. All U Geminorum stars are binaries containing a white dwarf and a red dwarf with total masses of roughly 1-2 Msun and with periods of less than 12 hours (period of U Gem, 1.5 × 104 seconds). About 150 are known.
The component of a star's motion away from the Galactic center. If a star moves away from the Galactic center, the star's U velocity is positive; if a star moves toward the Galactic center, the U velocity is negative; and if the star moves neither toward nor away from the Galactic center, the U velocity is zero. The Sun has a U velocity of -9 kilometers per second, so the Sun is moving toward the Galactic center at 9 kilometers per second.
Part of the electromagnetic spectrum immediately above visible light (but below gamma-rays and X-rays); it therefore comprises a range of radiation of shorter wavelength and higher frequency than those of visible light.
In particle physics, any theory exposing relationships between seemingly disparate classes of particles. More generally, a theory that gathers a wide range of fundamentally different phenomena under a single precept, as in Maxwell's discovery that light and magnetism are aspects of a single, electromagnetic force.
The hypothesis that the extensive changes in the earth, as evinced in the geological record, have resulted, not from massive catastrophes, but from the slow operation of wind, weather, volcanism, and the like over many millions of years.
The total celestial cosmos. According to Gott et al. the universe seems to be on a large scale isotropic, homogeneous, matter-dominated, and with negligible pressure. The total proper mass content of about 1023Msun (Sandage derives 1056 g from his determination of the deceleration parameter q0) and radius of about 2 × 1028 cm are the order of magnitude that most cosmologists would accept if the universe is bounded. Total mass contributed by luminous matter, about 3 × 1053 g (see mass discrepancy). Age about 18 × 109 yr for a Hubble constant H0 = 55 km s-1 Mpc-1.
A series of nuclear reactions, primarily among the iron group of elements, accompanied by a high rate of neutrino formation and postulated as a cause of stellar collapse. Neutrinos carry away energy quickly and invisibly, so this process was named for the Urca casino in Rio de Janeiro, which carried away money the same way.
Late-type dwarf with spectra showing hydrogen emission lines. Faint flare star of very low mass. Like other flare stars, it is a member of a binary system in which both components are of nearly equal brightness. Radio flares have also been observed.
A star's velocity in the direction of Galactic rotation, as measured relative to a nearby star that has a circular orbit. If a star revolves faster than such a star, the V velocity is positive; if it revolves more slowly, the V velocity is negative; and if both revolve at the same rate, the V velocity is zero. The Sun has a V velocity of +12 kilometers per second, so it revolves 12 kilometers per second faster than it would if it had a circular orbit. Since a star on a circular orbit revolves around the Galaxy at 220 kilometers per second, a star with a V velocity of 0 is not stationary; rather, it revolves at 220 kilometers per second. The Sun therefore revolves around the Galaxy at 220 + 12 = 232 kilometers per second.
One of two doughnut-shaped belts in the Earth's magnetosphere, where many energetic charged particles from the solar wind are trapped in Earth's magnetic field. The energy of the particles is highest in the inner belt.
One of two toroidal zones of high radiation in Earth's upper atmosphere, above the equator, caused by the trapping of charged particles in the magnetosphere.
A star that varies in luminosity. The first variable discovered in a given constellation has the letter R preceding the name of the constellation. Then S, . . . , Z. Then RR, RS, . . . , Rz, SS, . . . , Sz, . . . , ZZ. Then AA, . . . , AZ (the letter J is never used), BB, . . . , BZ, . . . , QQ, . . . QZ. The next variable (the 335th) is given the designation V335.
A star whose light varies. Some variables vary simply because they consist of two stars, one of which eclipses the other; Algol is the most famous example. Other variables, however, vary because the stars themselves actually change in brightness; the most famous are the Cepheids, RR Lyraes, and Miras, all of which pulsate.
Star whose luminosity changes over periods of time; there are many reasons and many types. Periods vary widely in length and even regularity. Novae and supernovae are classed as variables. The present brightest variable star is Betelgeuse (Alpha Orionis).
a star whose apparent magnitude varies by at least 0.1 magnitudes in the visible spectrum (a star whose optical brightness variations can be detected by the human eye)
A theory of Hoyle and Narlikar in which the masses of fundamental particles are assumed to vary with time in a manner that precisely accounts for the Hubble redshift law.
A hypothetical elementary particle that acts as intermediary for the weak interaction, carrying its effect from one particle to another as the photon does for electromagnetic interactions and as various mesons do for the strong interactions.
Force-carrying particles of nature. Three vector bosons are responsible for the weak nuclear force. By admitting the photon on an equal footing it is possible to create a unified electroweak theory. As a result of symmetry-breaking processes, however, this photon remains massless while the three other vector bosons pick up mass.
The small theoretical precession of the axis of an orbiting body due to the gravitational influence of its primary. This effect is predicted by general relativity, but so far it has not been observed.
A compact radio source about 400-500 pc distant associated with the Vela supernova remnant (q.v.). It has a nonthermal radio spectrum and is about 20 percent polarized. It is associated with the Gum Nebula, the Vela pulsar, and the X-ray source 2U 0832-45, although the pulsar and the X-ray source are displaced about 0°.7 from the center of the Vela X radio emission. Vela Y and Vela Z are outlying components, also nonthermal, but too weak to exhibit polarization.
A sequence of satellites launched to monitor possible violations of the nuclear test ban treaties. The system consists of four satellites in a circular orbit around the Earth with a radius of 120000 km. The Vela satellites have detected cosmic gamma-ray bursts (q.v.).
A gaseous nebula in the middle of the Gum Nebula, the remnant of a Type II supernova whose light reached Earth about 10000 to 30000 years ago. It consists of bright filaments that form a D-shaped ring in Hα and a rough circle in the ultraviolet. It includes the Vela X, Y, and Z radio complexes and is a strong X-ray source.
The radius of a rotating neutron star at which the rotational velocity of the plasma approaches the velocity of light. (also called velocity-of-light cylinder)
Second planet from the Sun. Has retrograde rotation. Mariner 10 has established that the cloud tops rotate every 4 hours retrograde. Radar experiments have established that the surface is somewhat smoother than the Moon, but there are mountains and there is extensive cratering. Last transit of Sun was in 1882; next one will be 2004. Venus's rotation period is in synchronism with Earth - that is, at inferior conjunction the same side is always toward the Earth.
The ascending node of the ecliptic on the celestial equator; also the time at which the apparent longitude (see apparent place; longitude, celestial) of the Sun is 0°.
The point of intersection between the ecliptic and the celestial equator, where the Sun crosses from south to north. It is sometimes called the First Point of Aries because several thousand years ago it was in Aries. Because of precession it has now slid west into Pisces and in 200-300 years it will edge into Aquarius. By definition, the vernal equinox is at α = 0°, δ = 0°.
An asteroid in diameter. It is the brightest of all minor planets, at times approaching naked-eye visibility. Its spectrum can also be interpreted to mean a rotation period of 10h40m58s.84.
A type of galaxy differentiated only recently. Violent galaxies include QSOs and exploding galaxies like M82. About 1 percent of the galaxies are classified as violent. Violent galaxies release on the average 1058 ergs of energy, compared with a supernova release of 1049 ergs. Nearest violent galaxy is Cen A.
The observed gravitational motion of nearby galaxies toward the Virgo cluster of galaxies, about 50 million light years away. The Virgo cluster represents a strong concentration of mass, a strong departure from a uniform distribution of matter, and it therefore causes galaxies in its vicinity to deviate from the Hubble flow.
Particle and antiparticle that exist for an extremely short time, often as the intermediate stage of a nuclear transition. According to Dirac's theory, the vacuum can be visualized as consisting of a sea of virtual electron-positron pairs that can only be released or separated when sufficient energy is made available.
Particles that erupt from the vacuum momentarily; they exist on borrowed energy, consistent with the uncertainty principle, and rapidly annihilate, thereby repaying the energy loan.
Quantum uncertainties in energy make it possible for virtual particles to be constantly created and annihilated during elementary particle interactions. Elementary particles are able to make use of these virtual particles within their interactions.
A type of CCD in which only one electrode is physically outside the silicon and is such as to obscure only half of the pixel. A specially doped layer under the transparent part acts as another or virtual electrode.
A composite spectrum star. One observes a spectrum of a K or M supergiant, showing emissions of hydrogen and [FeII] plus the spectrum of the secondary, which is generally of type B.
Eclipsing binaries with M supergiant primaries and blue (usually B) supergiant or giant secondaries. They have a rich emission spectrum. Sandage (1974) suggests Mv = - 7.3 for the M2p component of VV Cep.
Particle that transmits the unified electromagnetic and weak nuclear forces. These particles were predicted by the Weinberg-Salam theory of the 1960s and later discovered in the 1980s.
A large class of double-lined eclipsing binaries with very short periods (a few hours) whose spectra indicate mass transfer. They are distinguished by the fact that their primary and secondary minima are equal. They are all F or G binaries on or near the main sequence. They may be the progenitors of dwarf novae.
A star's velocity perpendicular to the Galactic plane. If a star is moving up, its W velocity is positive; if a star is moving down, its W velocity is negative; and if a star does neither, its W velocity is zero. The Sun has a W velocity of +7 kilometers per second, so it is moving up at 7 kilometers per second. In general, the greater a star's W velocity when it crosses the Galactic plane, the farther above and below the plane the star will travel.
An arbitrary period of days, usually seven days; approximately equal to the number of days counted between the four phases of the Moon. (See lunar phases.)
Radio waves generated by a flash of lightning, which travel along Earth's magnetic field out beyond the ionosphere and back to Earth. They arrive back with a descending pitch because the high-frequency end of the wave train arrives first (see dispersion).
A small, faint, dense, dying star that has used up its nuclear fuel and is slowly fading from view. A typical white dwarf has 60 percent of the Sun's mass but is little larger than the Earth. White dwarfs are common, accounting for 10 percent of all stars in the Galaxy; the nearest is Sirius B, just 8.6 light-years away. But no white dwarf is visible to the naked eye.
Star of high surface temperature, low luminosity, and high density (105-108 g cm-3), with roughly the mass of the Sun and the radius of the Earth, that has exhausted most or all of its nuclear fuel, believed to be a star near its final stage of evolution. When the Sun becomes a white dwarf, its radius will be about 0.01 of its present radius. DA white dwarfs are hydrogen-rich; DB white dwarfs are helium-rich; DC are carbon rich; DF are calcium-rich; DP are magnetic stars. White dwarfs have relatively low rotational velocities.
The time-reversal of a black hole. A white hole is a singularity from which matter emerges unpredictably, but into which matter cannot enter. The initial singularity of the standard big bang theory is an example of a white hole. It can be shown that the creation of a new universe from a false vacuum bubble in the context of classical general relativity would require a white hole singularity, which means essentially that it cannot be done, even in principle. However, a false vacuum bubble could conceivably grow to become a new universe through a process of quantum tunneling.
Hot stars characterized by wide emission lines of highly ionized elements, standing out distinctly from the continuous spectrum. There exist three varieties: WN, WC and WO
One of a class of very luminous, very hot (as high as 50000 K) stars whose spectra have broad emission lines (mainly He I and He II), which are presumed to originate from material ejected from the star at very high (~ 2000 km s-1 ) velocities. Some W-R spectra show emission lines due to carbon (WC stars); others show emission lines due to nitrogen (WN stars). (Hiltner and Schild classification: WN-A, narrow lines; WN-B, broad lines.)
An intriguing solution to the equations of general relativity which describes a neck that can connect two completely separate universes. Wormholes arise in the discussion of the creation of a universe in the laboratory, because the new universe disappears through a wormhole and completely detaches itself from the parent universe.
A recurrent DAe old nova (1913 and 1946) with the shortest known orbital period (about 80 minutes). It is almost certainly a close binary system in which mass is being transferred onto a white-dwarf primary.
The unknown nucleosynthetic process that Burbidge, Burbidge, Fowler, and Hoyle said had formed the light nuclei deuterium, lithium, beryllium, and boron.
A large band of electromagnetic radiation with wavelengths smaller than extreme ultraviolet light. A typical X-ray photon has over one thousand times as much energy as a photon of visible light.
Detection of stellar and interstellar X-ray emission. Because X-rays are almost entirely filtered out by the Earth's upper atmosphere, the use of balloon- and rocket-borne equipment is essential.
Pulsar (q.v.) that radiates in the X-ray region of the spectrum. Best verified examples are Her X-1 and Cen X-3. They are thought to be rotating, strongly magnetic neutron stars of about 1 Msun in a grazing orbit around a more massive star from which they are accreting matter.
A class of celestial objects whose dominant mechanism of energy dissipation is through X-ray emission. Galactic X-ray sources appear optically as starlike objects, peculiar in their ultraviolet intensity, variability (on time scales ranging from milliseconds to weeks), and spectral features. All known compact X-ray sources are members of close binary systems; a current popular model is mass accretion onto a compact object from a massive companion. (Four X-ray sources - all variable - are known to be associated with globular clusters.) The 21 known extended X-ray sources associated with clusters of galaxies seem to be clouds of hot gas trapped in the cluster's gravitational field.
Primordial state of matter - neutrons and their decay products (protons and electrons) - before the Big Bang. The term was taken from Aristotle and used for the α-β-γ theory.
The word used by Gamow and his collaborators for the primordial material of the Big Bang. In most of his work Gamow assumed that the ylem consisted entirely of neutrons. In inflationary cosmology, the role of the ylem is played by the false vacuum.
A subPopulation In the thin disk whose stars range in age from 0 to 1 billion years old. The stars of the young thin disk have a scale height of 350 light-years and have very circular orbits around the Galaxy.
Very young, late-type, low-mass stars in the gravitationally contracting stage in which the star is still accreting matter from the protostellar cloud.
A class of dwarf nova with standstills in their light curves. Z Cam itself is a semidetached binary (period 7h21m) consisting of a dG1 star and a hot white dwarf or a hot blue subdwarf which is probably degenerate. Mean time between eruptions, 20 days. Peak-to-peak amplitude, about 0.5 mag.
Particle that transmits the unified electromagnetic and weak nuclear forces. These particles were predicted by the Weinberg-Salam theory of the 1960s and later discovered in the 1980s.
Angular distance on the celestial sphere measured along the great circle from the zenith to the celestial object. Zenith distance is 90° minus altitude.
The energy of the lowest state of a quantum system. Amount of vibrational energy allowed by quantum mechanics to be associated with atomic particles at 0 K, whereas classical mechanics requires this to be zero. Also, the energy of an electron in its ground state.
A reddened O star (a runaway star from the Sco-Cen association) with a high rotational velocity. It is well known for its strong interstellar absorption lines in the visible part of the spectrum.
A faint glow that extends away from the Sun in the ecliptic plane of the sky, visible to the naked eye in the western sky shortly after sunset or in the eastern sky shortly before sunrise. Its spectrum indicates it to be sunlight scattered by interplanetary dust. (Pioneer 10 has determined that its brightness varies inversely as the square of the distance out to 2.25 AU and then decreases more rapidly.) The zodiacal light contributes about a third of the total light in the sky on a moonless night.
An irregular zone near the plane of the Milky Way where the absorption due to interstellar dust is so great that no external galaxies can be seen through it.
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