star cluster

a grouping of stars that are bound to each other by forces of mutual attraction and that have a common origin and nearly the same age and chemical composition. Star clusters usually have a dense central concentration, the core, surrounded by a significantly less dense coronal region. The diameters of star clusters range from several parsecs to 150 parsecs, and the radii of the coronal regions exceed the radii of the cores by several (sometimes dozens of) times. Star clusters are divided in open (sometimes called galactic) clusters and globular clusters. The distinction is basically determined by the mass and age of the formations. As a rule, open clusters have dozens or hundreds, rarely thousands, of stars, while globular clusters have tens or hundreds of thousands of stars. The Pleiades, Praesepe, and the Hyades are open clusters; examples of globular clusters are the cluster M3 in the constellation Canes Venatici and the cluster M13 in Hercules.

In our galaxy open clusters are concentrated in the plane of symmetry of the Milky Way (the galactic plane) and possess a small velocity relative to the sun (20 km/sec on the average). Among these it is possible to distinguish clusters that have emerged comparatively recently (less than 100 million years ago) and are associated with the spiral arms and clusters of intermediate age, or disk clusters, which exhibit no link with the spiral arms and are more weakly concentrated toward the galactic plane. All open clusters have the normal metal content inherent in Population I stars. Globular clusters in our galaxy are distributed in a spheroidal volume the center of which coincides with the center of the galaxy. They are strongly concentrated toward this center and are characterized by large velocities relative to the sun (170 km/sec on the average). They are usually poor in metals, but the objects observable in the regions around the center of the galaxy are richer in metals than those observed on the periphery of our stellar system. The study of the Hertzsprung-Russell diagram or magnitude-color diagram yields important information about the evolution of star clusters. Diagrams of the magnitude-color relation of stars in typical open and globular clusters of our galaxy are essentially different. The interpretation of these diagrams from the viewpoint of the modern theory of stellar evolution enables one to conclude that stars in typical globular clusters are 100–1,000 times older than stars in open clusters.

The kinematic characteristics and spatial distribution of globular clusters in our galaxy reflect the distinctive features of the initial distribution of matter, out of which these formations arose at an early stage of the galaxy’s existence. The magnitude-color diagrams of the stars in the globular clusters of that epoch should bring to mind corresponding diagrams in contemporary open clusters. Similar young globular clusters are observed in neighboring galaxies (for example, NGC 1866 in the Large Magellanic Cloud). At present, star clusters in our galaxy originate only near the galactic plane in the regions of the gas and dust spiral arms.

The dynamic evolution of a star cluster occurs simultaneously with the change in the physical characteristics of its members. Close encounters between stars in the cores of star clusters lead to a mutual exchange of the energy of their motion. As a result, certain star cluster members receive excess energy and proceed into the region of the corona or escape from the cluster entirely. During this process the core contracts. The dissipation of the core occurs particularly rapidly in clusters with a small number of members—that is, open clusters. Thus, out of the old clusters in our galaxy only the most massive ones—the globular clusters—have been preserved. Nonperiodic variables and flare variable stars are usually observed among the weak members of young open clusters. Certain globular clusters contain RR Lyrae and W Virginis variables, and cepheids are sometimes encountered in open clusters. Star clusters very close to the sun (for example, the Hyades), in whose members’ proper motion the phenomena of perspective (the directions of their proper motions upon their continuation on the celestial sphere intersect at one point) are observed, are called moving clusters. These clusters play a special role in the problem of determining stellar distances, since the distances to them can be reliably determined by simple geometric methods.