Globular star clusters collections of thousands to millions of stars and are among the oldest objects in the Universe. These star clusters are almost spherical in shape and are held together by gravity. There are more than 120 such clusters known in the Milky Way, yet the nearby Andromeda galaxy, M31, has more than 400, and some giant elliptical galaxies contain well over a thousand. The oldest globular clusters trace the formation histories of the halos of galaxies. New globular clusters are thought to form when galaxies merge.