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. More recent estimates yield an age of 12 to 20 billion years; the best value for observation is perhaps 14 to 16 billion. As their age is crucial as a lower limit for the age of our universe, it was subject to vivid and continuous discussion for decades. The age of globular clusters is determined by investigating their H-R diagrams.
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,
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