Star Clusters

Star clusters are groups of stars which are gravitationally bound. Two types of star cluster can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars, while open clusters generally contain less than a few hundred members, and are often very young

Globular clusters are gravitationally bound concentrations of approximately ten thousand to one million stars. They populate the halo or bulge of the Milky Way and other galaxies with a significant concentration toward the Galactic Center.

Recent estimates of their age ranges from 12 to 20 billion years; the best value for observation is perhaps 14 to 16 billion

Our galaxy has about 200 globular clusters, most in highly eccentric orbits that take them far outside the Milky Way. Most other galaxies have globular cluster systems as well, in some cases containing several thousands of globulars!

Open (or galactic) clusters are physically related groups of stars held together by mutual gravitational attraction. They are believed to originate from large cosmic gas/dust clouds in the Milky Way, and to continue to orbit the galaxy through the disk. In many clouds visible as diffuse nebulae star formation takes still place at this moment, so that we can observe the formation of new young open star clusters.

Open clusters populate about the same regions of the Milky Way and other galaxies as diffuse nebulae, notably spiral arms in disk galaxies, and irregular galaxies, and are thus found along the band of the Milky Way in the sky.

Most open clusters have only a short life as stellar swarms. As they drift along their orbits, some of their members escape the cluster, due to velocity changes in mutual closer encounters, tidal forces in the galactic gravitational field, and encounters with field stars and interstellar clouds crossing their way. An average open cluster has spread most of its member stars along its path after several 100 million years; only few of them have an age counted by billions of years. The escaped individual stars continue to orbit the Galaxy on their own as field stars: All field stars in our and the external galaxies are thought to have their origin in clusters.

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