Of the stars in the Milky Way Galaxy, about 1 in every 100,000 is in a dense grouping called a star cluster. All the stars in a given cluster have very nearly the same velocity and thus are moving together through space; therefore they must have originated together. The age of a cluster can be determined by obtaining its color magnitude, or its Hertzsprung-Russell diagram, and comparing it with theoretical calculations.
For the most part, star clusters can be divided into two types, galactic and globular. The distinction is based primarily on the location of a cluster and its orbital motion in the Galaxy. Galactic, or open, clusters are found in the relatively thin disk of the Milky Way orbiting about its center in nearly circular orbits, whereas globular clusters inhabit a spherical halo around the Galaxy and have highly elliptical orbits.
Globular clusters usually consist of several hundred thousand to more than a million stars and have a full, round appearance. The clusters are concentrated toward the center of the Galaxy but extend outward at least 50,000 light-years from the center. About 10 to 15 billion years old, they are among the oldest objects in the Galaxy. The brightest visible stars are the huge, evolving red giants, which have masses approximately 1.5 times that of the Sun. Globular clusters usually contain a few hundred red giants, but most of the stars are much fainter and of lower mass and are still in the long-lived state of equilibrium represented by the main sequence.
Most globular clusters contain variable stars, usually RR Lyrae stars, whose well-known true luminosities are useful in determining the astronomical distances to the clusters. Despite minor differences, all globular clusters have remarkably similar properties.
Galactic clusters are younger (almost 10 billion years for the oldest, but most are much younger) than globular clusters. Thus they are made up of stars built from material enriched in heavy elements by generations of stellar evolution; they contain up to 100 times the amount of heavy elements found in globular clusters. Galactic clusters are small, averaging only 15 light-years in radius, and most have between 100 and 1,000 stellar members.
The Hyades is one of the nearest and most important galactic clusters. Its distance of 160 light-years can be measured by a variety of means and is one of the fundamental yardsticks in the Galaxy. It is easily visible in the night sky in the constellation Taurus. Nearby in the sky is another important cluster, the Pleiades, 424 light-years distant and made up of between 250 and 500 stars. Recent measurements have cast some doubt on these distances, but astronomers suggest that such differences will eventually be resolved.
Clusters in Other Galaxies
Populations of globular and galactic clusters also can be detected in other spiral galaxies and in irregular galaxies as well (see extragalactic systems). Toward the center of some spirals another kind of clustering can be observed, where giant bursts of star formation appear to be taking place. Certain very active radio galaxies, such as Perseus A and M82, have giant "blue globular" clusters, where hundreds of thousands of massive young stars are being formed. These clusters were discovered using the Hubble Space Telescope.