open cluster

Any group of Population I (see Populations I and II) stars with a common origin, held together by mutual gravitation (not to be confused with a cluster of galaxies). Stars in open clusters are much more scattered than those in globular clusters. All known open clusters contain from about 10 to 1,000 or more stars (about half contain fewer than 100) and have diameters of 5–75 light-years. More than 1,000 have been discovered in the Milky Way Galaxy; well-known examples include the Pleiades and the Hyades.