A cluster of galaxies is a large collection of galaxies in a single gravitational field. Rich clusters of galaxies usually contain at least a dozen large galaxies as massive as the Milky Way, along with hundreds of smaller galaxies. At the center of large clusters of galaxies there is usually a group of elliptical galaxies called “cD” galaxies. Clusters of galaxies are usually about ten million light-years across. The Milky Way galaxy is near—but not in—the Virgo cluster, which itself is near the center of the Virgo supercluster.