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globular cluster

A roughly spherical group of old stars in the halo of a galaxy. Globular clusters contain from tens of thousands to millions of stars, and have diameters of 100–300 l.y. At the center of the cluster, where most stars are concentrated, the density may be over 250 stars per cubic light year. About 140 globular clusters are known in our Galaxy, travelling on highly elongated orbits around the galactic center. They are very old, about 1010 years, having formed early in the history of the Galaxy. Stars in globular clusters are members of Population II, with a low content of heavy elements (only a few per cent of the solar value), although a few show values approaching that of the Sun.

On the Hertzsprung-Russell diagram of a globular cluster, the main sequence and the giant branch are smoothly joined via a subgiant branch; also conspicuous is an asymptotic giant branch and a horizontal branch, punctuated by an instability strip containing RR Lyrae variables. Globular clusters are found around all large galaxies, but are most abundant around giant elliptical galaxies. Some galaxies, particularly those that have undergone recent mergers, contain large numbers of young globular clusters.

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