A type of star cluster containing several hundred to several thousand stars distributed in a region a few light years across. The member stars are much more spaced out than in globular clusters. Open clusters are relatively young, typically containing many hot, highly luminous stars. They are located within the disc of the Galaxy and so appear to lie within the Milky Way. Well-known open clusters include the Pleiades, the Hyades and the Jewel Box. See also: Trumpler classification.