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An open cluster or galactic cluster (sometimes abbreviated or symbolized by OC) is a group of stars within a galaxy that are close together and of similar age and metallicity, suggesting they were formed by a single molecular cloud. They are common in spiral arms. They are gravitationally bound, but loosely so, with the stars further apart than in globular clusters. They generally have fewer stars (in the hundreds-to-thousands range) than globular clusters, all of which are relatively young, and have a total mass of on the order of fifty to fifty thousand solar masses. It is presumed that an open cluster will eventually be pulled apart by encounters with other dense regions, relaxing into a stellar association, and eventually any remaining stars are dispersed. More than a thousand open clusters have been identified within the Milky Way. Examples:
Their uniform characteristics make them useful for studies involving many stars because once a cluster's characteristics are determined, it offers a multitude of stars for observation where some idea of their characteristics (distance, mass, age) is known. Some include stars that appear too young (blue) called blue stragglers (BSS or BS) whose origin is a mystery, one theory being that they result from stellar mergers.