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Stars do not occur in space at completely arbitrary places. Some, such as the Sun, are single (field star), but others are members of pairs or form multiple-star systems. Still others form clusters of various types, and size. All of them are condensed from clouds of gas and dust. Two main types of star clusters occur: the small and sparse open (galactic) clusters containing tens to thousands of young stars, and the large and dense globular clusters containing up to several million stars. Large-scale groupings of some stellar types are called associations. | An OB Association is a group of stars of spectral types O and B that are close together, because they were all formed from a single cloud of gas. Since O and B stars are very bright their lives are short (up to about 20 million years), they do not have time to wander far away from their place of origin. Stars lighter than those of spectral types O and B are also formed from the same cloud of gas, and reside in the same region of space. When the O and B stars have all reached the ends of their lives, only the lesser stars are left and form an open cluster. T associations are groups of new-born stars before the ignition of nuclear burning. |