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, suggesting they were formed by a single molecular cloud. They are common in spiral arms. They are gravitationally bound, but loosely so, the stars further apart than in globular clusters. Also unlike globular clusters, they typically have just few thousand stars, all young. They are presumed to be pulled apart by encounters with other dense regions, relaxing into stellar associations, and eventually any remaining stars are dispersed. More than a thousand open clusters have been identified within the Milky Way.
The stars of an open cluster also tend toward similar mass and other characteristics. This makes 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.
Open clusters can have from hundreds to thousands of stars and their mass can total on the order of fifty to fifty thousand solar masses.