OB clusterAn open cluster that contains main-sequence O and B stars of high luminosity. These bright and very massive stars evolve more rapidly than less massive stars; their presence in a cluster suggests that it is comparatively young (10 million years or less) and that it may contain stars still in the process of formation. Examples are NGC 2264 and h and Chi Persei. The young low-mass T Tauri stars are often observed in these clusters. See also association.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006