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physical object > natural object > celestial body > collection of stars

collection of stars
subject			fact

collection of stars	has number of stars
	has catalog catalog about star systems
	has definition Two or more stars forming a gravitationally bound system
	is a kind of celestial body
celestial body	has spectra
physical object	has location or center of gravity
	has angular momentum
	has mass
	has velocity
	has momentum
	has temperature
	has volume
	has extent
	has material

Kinds of collection of stars :

• association (7 kinds, 29 facts) - A sparsely populated grouping (mass range 10²-10³ M_sun) 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.
• collection of galaxies (24 kinds, 134 facts)
• galaxy (104 kinds, 448 facts) - 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., 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 collection of matter which usually manifests itself by the production of stars., 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., A large (10⁸-10¹³ M_sun), gravitationally bound aggregate of stars and interstellar matter. Galaxy formation is currently believed to have occurred around z ≈ 3-4., 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.
• star cluster (57 kinds, 393 facts) - Gravitationally bound aggregations of stars, smaller and less massive than galaxies.
• star system (61 kinds, 364 facts) - A few stars that orbit each other.