An Open Cluster or Galactic Cluster 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 only gravitationally bound in a limited way, i.e., stars can be thrown out of the cluster through close encounters. They are typically of a few thousand stars and are all young, presumed to disperse in time. In contrast, Globular Clusters are more tightly bound and are often much older. 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, that might be the results of 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.