During the gravitational collapse of large clumps of interstellar material, the material may often fragment into smaller pieces. From these smaller pieces of interstellar matter, multiple star systems or star clusters may form whose mutual gravitation makes them stable against dispersal for reasonably long periods of time. Being of the same age, stars of different masses within the clusters will be found in different evolutionary stages, providing a major test of evolutionary theory. Young clusters allow astronomers to test theories on how stellar birth occurs, and old clusters with white dwarfs enable them to test theories of the last stages of stellar evolution. Star clusters are also important because they enable astronomers to place stars of different types in their proper positions within the HR diagram. Moreover, because star clusters are relatively compact objects that are easily recognized against the more uniform distribution of background or foreground stars, they are easy to discover. Consequently, star clusters have played decisive roles in mapping the overall structure and size of the Galaxy, identifying regions of the most recent star formation, establishing the age of the Galaxy, and providing information about its evolution.
The two types of clusters, open clusters and globular clusters, have distinctly different characteristics. Astronomers also recognize a third type of stellar assemblage, associations, whose stars are coeval in formation, but that over time have dispersed over a much wider region in space.
Open clusters are the smaller clusters of stars found in the Galaxy, both in terms of number of member stars (typically 10–500 stars) and size (3–10 pc in diameter). These clusters are located in the plane (or disk) of the Milky Way where they have the same motion as the Sun around the center of the Galaxy. As a result, their observed velocities are small with respect to the Sun. Such a small grouping of stars is obvious against the general background (or foreground) of stars, hence they are easily identified; some 4,000–5,000 are listed in modern catalogs. Open cluster ages are predominantly young, and they are often associated with interstellar material by virtue of this youth. The youngest of these clusters have bright main sequence stars, blue supergiants, and sometimes a few variables, either young Cepheids or even younger T Tauri stars. Open clusters are not tightly bound, as their few stars give them a density of 0.1–10 stars/pc 3 and a gravitational self‐attraction that is relatively weak. Few open clusters therefore survive to old age; the oldest identified clusters are about 8 × 10 9 years old. Chemically, their spectra show compositions similar to that of the Sun (that is, a solar abundance of about 98 percent hydrogen and helium, and 2 percent heavy elements). All these properties represent those characteristic of Population I objects in the Galaxy.
Associations are loose groupings of stars. About 80 associations are known, and they are related to open clusters in that they are young objects located in the galactic disk and they have a common formation. Unlike open clusters, however, associations are not gravitationally bound, and the stars in them are slowly moving away from each other. Associations are identified on the basis of the similarity of their motions through space and other properties, although their few dozen to few hundred stars may be spread over several hundred parsecs of space (representative stellar density of <0.01 pc ‐3). They are typed as O, B, A, or T Associations on the basis of the types of stars in the group. Associations older than about 10 million years are not known because their stars have become too dispersed to be recognizable on the basis of their common properties. One prominent example of an association includes five of the seven stars in the Big Dipper, the bright star Sirius, and several others.
In all their properties, the globular clusters (there are about 150 associated with the Milky Way Galaxy) are distinct from the open clusters. They are old clusters, from 12 to possibly 18 billion years old, and are certainly among the first objects to have formed in the Galaxy. Globular clusters are populous, with 10 4 to a few times 10 6 stars with diameters of up to a hundred or more parsecs and their central densities as tightly packed as 10–1000 stars/pc 3. Containing no young stars, they have many red giants and old variable stars such as RR Lyraes. In the Milky Way, these clusters are found distributed in a more or less spherical halo surrounding the Galaxy, with their greatest concentration toward the center of the Galaxy (in the direction of the constellation Sagittarius). Overall, the system of globular clusters shows little evidence of rotation, with orbital motions in randomly oriented and highly elliptical paths (predominantly radial motions). Relative to the Sun, these clusters have heavy element abundances much smaller than the Sun, hence are termed metal‐poor. In sum, the properties (age, chemistry, spatial distribution, motions) illustrated by the globular clusters are characteristic of Population II stars in the Galaxy.