For questions concerning clusters of stars such as open clusters or globular clusters. Do not use for questions about galaxies, which belong to a distinct classification.
Star clusters are groups of gravitationally interacting stars on a scale smaller than galaxies. While a galaxy is often defined as a collection of stars, gas, dark matter and/or a central super-massive black hole, the only component of a typical cluster is its stars. Most star clusters are associated with (part of) a galaxy.
Star clusters come in two main classifications, open and globular clusters. Open clusters typically contain young stars which were formed in the same giant molecular cloud. Such clusters are relatively short-lived, up to a few tens of millions of years, after which interactions with other clusters or gas clouds in the host galaxy disrupt the cluster and its stars simply become part of the larger galactic stellar distribution. Typical open clusters contain stars numbering in the thousands, are usually morphologically irregular. They are particularly useful in astronomy as they offer samples of stellar populations (rather than single stars) formed at nearly the same time, including short-lived young stars.
Globular clusters, on the other hand, are typically composed of much older stars, typical ages being on the order of billions of years. They are more massive than open clusters, containing typically between 10 000 and 10 000 000 stars, and they are invariably nearly spherical in shape. The detailed formation mechanism for globular clusters is as yet unknown. Their long lifetimes, nearly co-eval stellar populations and relatively short dynamical times, and other properties make them highly interesting systems for the study of many topics in astronomy.
