Definition: Lots of galaxies have clusters of stars surrounding them. These clusters were formed when clouds of hydrogen (H2) gas exploded long ago. Each explosion created groups of between 10,000 to 100,000 stars. The mass that was created means that gravitational force strongly pulls the stars together into spherical clusters. These Globular Clusters contain very old stars, which have been held together for over a billion years. Unlike the stars in Open Clusters, the ancient stars in Globular Clusters do not move apart but remain stuck together, held fast by the force of gravity. Open Clusters contain fewer stars than Globular Clusters and have less mass so the force of gravity is not strong enough to hold them together and they gradually move apart.