Star Clusters, as the name implies, are clusters of stars that are gravitationally bounded. They are divided into two categories: Globular Clusters and Open Clusters.

Globular Clusters contain usually very old, tightly grouped stars, with numbers reaching the millions, spanned in a spherical region of 10 to 30 light years across. They usually contain Population II stars, which are relatively poor in metal content. Our galaxy has approximately 1500 Globular Clusters which are distributed spherically in the galactic halo.

Open Clusters, on the other hand, are made up of very young stars, up to a few hundred members, distributed within regions of about 30 light years across. Since they are not tightly bounded gravitationally, in time, they are disrupted by gravity of other clusters or giant molecular clouds.