Globular clusters are nearly spherical distributions of stars, usually a few dozen to a few hundred light-years across. They contain anywhere from several thousand to several million stars, and they are packed relatively close together. The stars are held together by their mutual gravity and are most heavily concentrated at the center of the cluster. In at least one instance—the cluster G1 that orbits the Andromeda galaxy—there appears to be a black hole at the center of a globular cluster.