Carney, Bruce W. Department of Physics and Astronomy, University of North Carolina, Chapel Hill, North Carolina.
- Open clusters
- Stellar associations
- Globular clusters
- Clusters in other galaxies
- Related Primary Literature
- Additional Reading
Groups of stars held together by mutual gravitational attraction. There are two basic morphological types: open clusters and globular clusters. Typical densities of field stars near the Sun are 0.1 star per cubic parsec. (One parsec equals 1.9 × 1013 mi or 3.1 × 1013 km.) Open clusters, with dozens to thousands of stars, have central densities of 0.3 to 10 stars per cubic parsec, and are often elongated or amorphous in shape (Fig. 1). Globular clusters, with a thousand to several million stars, and central densities of a few hundred to over 100,000 stars per cubic parsec, are generally spherical (Fig. 2). Associations are even looser assemblages, with a few hundred stars and central densities lower than the field. They are often recognized because of unusually large numbers of special types of stars. OB associations are dominated by hot, luminous, young O and B stars. T associations are dominated by young T Tauri variable stars. See also: Spectral type; T Tauri star
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