star cluster, a group of stars near each other in space and resembling each other in certain characteristics that suggest a common origin for the group. Stars in the same cluster move at the same rate and in the same direction. Two types of clusters can be distinguished—open clusters, also called galactic clusters because of their wide distribution in our galaxy (the Milky Way), and globular clusters. More than one thousand open clusters have been cataloged in the Milky Way, most of which are found in the spiral arms of the galaxy. Typically, an open cluster contains from a few dozen to a thousand loosely scattered stars and exists in a region rich in gas and dust. Among those which can be detected with the unaided eye are the Hyades cluster in the constellation Taurus, the Coma Berenices cluster, the Pleiades cluster, and the Praesepe cluster. Globular clusters are spherical aggregates of from thousands to hundreds of thousands of densely concentrated stars. Rather than lying on the galactic plane, these clusters are members of the outer halo, moving around the nucleus of our galaxy in highly inclined orbits. Because of their distribution around the galaxy, they provide an outline of its shape. About 150 globular clusters are known in the Milky Way galaxy, and others have been found in nearby galaxies. Visible to the unaided eye are Omega Centauri and 47 Tucanae, both in the southern skies, and M13 in the northern sky. Star clusters are cosmologically important as a first step to understanding the distance scale of the universe (see Hyades); and theoretical astronomers use observations of globular clusters to investigate the evolution and life span of stars. Because all the stars in a particular cluster are coeval (the same age), astronomers can infer that massive stars change more rapidly over time than less massive ones. X-ray sources have been detected recently in some globular clusters. Millisecond pulsars have also been found.