Star Clusters

Click the icon to view Star Clusters of the Messier Catalog

Globular

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. Spectroscopic study of globular clusters shows that they are much lower in heavy element abundance than stars such as the Sun that form in the disks of galaxies. Thus, globular clusters are believed to be very old and formed from an earlier generation of stars (Population II). More recent estimates yield an age of 12 to 20 billion years; the best value for observation is perhaps 14 to 16 billion (see e.g. the discussion at M92). As their age is crucial as a lower limit for the age of our universe, it was subject to vivid and continuous discussion since decades. The age of globular clusters is determined by investigating their H-R diagrams, as discussed in our globular cluster page.

The disk stars, by contrast, have evolved through many cycles of starbirth and supernovae, which enrich the heavy element concentration in star-forming clouds and may also trigger their collapse.

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 (e.g., for M87) containing several thousands of globulars!

Open

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 (composed of young Population I stars) - therefore, these nebulae are adequately called star-forming nebulae or star-forming regions. Open clusters populate about the same regions of the Milky Way and other galaxies as star-forming 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.

Binary and Multiple Star Systems

Star formation leads to the formation of multiple star systems at least as often as it does single star systems, such as our own Solar System. In fact, if the mass of the planet Jupiter were a few times larger, it would become a star.

One should keep in mind that almost all Messier clusters are members of our Milky Way Galaxy - with two particularities: The globular clusters M54 and M79 are probably recent immigrants from dwarf galaxies which are currently undergoing destruction due to their close encounters with the Milky Way: M54 belongs to, or may be the former nucleus, of the Sagittarius Dwarf Elliptical Galaxy, a closely neighbored dwarf spheroidal galaxy which was discovered in 1994, and M79 may have been captured from the almost-dissolved Canis Major Dwarf discovered in 2003. Other galaxies contain clusters of any type, too, which can be detected with sufficiently sensitive instruments.

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Hartmut Frommert
Christine Kronberg
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Last Modification: August 14, 2007