A star cluster is a group of stars that are attracted to each other by their own gravity. There are two main types of star clusters: globular clusters and open (or galactic) clusters.

Globular clusters are dense clusters, they can contain from hundreds, thousands or even millions of old stars, while open clusters usually contain hundreds or thousands of young stars. One of the best known globular clusters is the one that decorates this page, it is M13 The Great Cluster of Hercules.