A star cluster is a gravitationally bound collection of stars that formed from the same gas cloud. There are two main types. Globular clusters are found in the haloes of galaxies and contain from tens of thousands to millions of ancient stars crowded into a more or less spherical volume of space. Open clusters inhabit the disks of galaxies and contain up to a few hundred relatively young stars in a looser arrangement from which stars may occasionally escape. Looser still are associations.