In addition to individual, multiple and binary stars in the galaxy, there are entire clusters of stars which are classified into two basic types. The "open" clusters are relatively young, having formed a few million to a few billion years ago, while "globular" clusters are truly ancient, being perhaps 13 to 15 billion years old.

Globular clusters contain anywhere from tens of thousands of stars to as many as a million. They are nearly spherical star systems with the greatest concentration near their centers. There are so many stars at the center that individual members cannot be distinguished; only a continuous glow is visible. (Still, there is plenty of space between these stars.) In one of the closer globulars, M13 in the constellation Hercules, over 30,000 starshave been counted. There may be many more stars that are too faint to be seen. Over one hundred globular clusters have been...