A star cluster is a group of stars, that share an origin, formed at roughly the same time and held together by self-gravitation. Two main types of star clusters can be distinguished: globular clusters, tight groups of ten thousand to millions of old stars which are gravitationally bound; and open clusters, less tight groups of stars, generally containing fewer than a few hundred members. If a star cluster is obscured in visible light due to extinction, but visible in infrared light, it is classified as an infrared cluster.