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

star cluster
subject			fact

star cluster	has definition Gravitationally bound aggregations of stars, smaller and less massive than galaxies.
	is a kind of collection of stars
collection of stars	has number of stars
	has catalog catalog about star systems
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 star cluster :

• globular cluster (48 kinds, 339 facts) - 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.
• open cluster (7 kinds, 52 facts) - 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 comparatively loose grouping (mass range 10²-10³ M)_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.)