Most stars are formed in groups or clusters of various sizes. The largest clusters may contain more than a million stars, while the smallest ones have no more than a few hundred. In our galaxy, the largest clusters are also the oldest ones, and are called globular clusters because of their shape. Like the Sun, these clusters all orbit the center of the galaxy, but while the Sun and nearly all other stars are located in a flat disk, the globular clusters show no preference for that disk. Indeed, as a group, the clusters don't even rotate around the galaxy like the disk does. There are more than 120 such clusters known in the Milky Way, yet the nearby Andromeda galaxy, M31, has more than 400, and some galaxies in the more distant Virgo cluster of galaxies contain well over a thousand,

HST, LGS and DSS images of Clusters and Stars in the new M31 catalog

HST, LGS and DSS images of just Star Clusters in M31

HST, LGS and DSS images of the Star Clusters in M31, ordered by estimated mass

HST, LGS and DSS images of Stars in the M31 catalog

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Pauline Barmby, Nelson Caldwell, John Huchra, Anil Seth