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. 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. 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. Other galaxies contain clusters of any type, too, which can be detected with sufficiently sensitive instruments.