STAR CLUSTER

either of two general types of stellar assemblages held together by the mutual gravitational attraction of its members, which are physically related through common origin. The two types are open (formerly called galactic) clusters and globular clusters. See also stellar association. Open and globular clusters differ markedly in their physical properties and in their distribution. They occur in distinct regions of the Milky Way Galaxy, the vast stellar system in which the Sun and its planets are located. A large number of the star clusters have also been found in the external galaxies, such as those of the Local Group and the Virgo cluster of galaxies. More than 1,000 open clusters have so far been discovered in the Milky Way Galaxy, most of them in or near its spiral arms, which are embedded in the galactic disk. Their distribution is similar in other spiral galaxies. Open clusters are so called because their stars are much more dispersed than those in the compact globular variety. All known open clusters in the Galaxy contain from about 10 to many hundreds of stars, virtually all of which are Population I objects (i.e., younger stars). Well-known examples of open clusters include the Pleiades and Hyades in the constellation Taurus, Praesepe in Cancer, and the Coma Cluster in Coma Berenices. Astronomers have identified about 100 globular clusters in the Milky Way. The majority of them occur above and below the galactic disk in the region known as the halo. Globular clusters contain many more stars than do open clusters, each having about 10,000 to 1 million members. The component stars are closely packed, particularly near the centre of a cluster, in a symmetrical, nearly spherical form. In contrast to the members of open clusters, these stellar bodies are Population II objects (i.e., old stars). As globular clusters are so distant from the solar system (in some cases as far away as 60,000 light-years), most of them are not visible to the unaided eye. Omega Centauri, M13 in the constellation Hercules, and a few others can be seen without a telescope as hazy patches of light. Since the mid-1970s astronomers, making use of telescopes on board Earth-orbiting satellites, have detected bursts of X-ray pulses from various globular clusters. This has led to the speculation that a massive black hole (invisible object with an intense gravitational field) exists at the centre of such stellar assemblages. either of two general types of stellar assemblages held together by the mutual gravitational attraction of its members, which are physically related through common origin. The two types are open (formerly called galactic) clusters and globular clusters. Additional reading Harlow Shapley, Star Clusters (1930), a dated but classic work; Helen Sawyer Hogg, Star Clusters, in Handbuch der Physik, vol. 53 (1959), pp. 129207; G. Alter, J. Ruprecht, and V. Vansek, Catalogue of Star Clusters and Associations, trans. from Czech, 2nd enl. ed. (1970), and its Supplement 1, 3 vol., edited by J. Ruprecht, B. Balzs, and R.E. White (1981); and Cecilia H. Payne Gaposchkin, Stars and Clusters (1979), an account of modern theory and observational evidence regarding the structure and evolution of stars. Publications of symposia held by the International Astronomical Union are technical but informative; two of particular interest are James E. Hesser (ed.), Star Clusters (1980); and Jeremy Goodman and Piet Hut (eds.), Dynamics of Star Clusters (1985). Helen Sawyer Hogg-Priestly Eric J. Chaisson