open cluster

(galactic cluster) A loose cluster of stars that contains at most a few thousand stars and sometimes fewer than twenty. Examples visible to the naked eye are the Hyades and the Pleiades. About 1200 open clusters are known. They are population I systems and occur in or close to the plane of the Galaxy. The brightest stars in an open cluster can be either red or blue giants, depending upon its age. Stars in the older clusters, such as M67 in Cancer, are similar in appearance to those in globular clusters, although with some subtle differences due to the higher metal content of the material from which they were formed. Open clusters are more loosely bound systems than globular clusters and they tend to be gradually dispersed by the combined effects of the differential rotation of the Galaxy and perturbations due to close encounters with interstellar clouds. Calculations suggest that many will not survive more than one or two circuits of the Galaxy. Hence most open clusters are comparatively young systems. Some, such as NGC 2264, are less than 10 million years old and in these clusters star-formation is probably still taking place (see OB cluster). See also Hertzsprung–Russell diagram; moving cluster; turnoff point.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006

open cluster

[′ō·pən ′kləs·tər]

(astronomy)

One of the groupings of stars that are concentrated along the central plane of the Milky Way; most have an asymmetrical appearance and are loosely assembled, and the stars are concentrated in their central region; they may contain from a dozen to many hundreds of stars. Also known as galactic cluster.