Open cluster

An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. They are loosely bound to each other by mutual gravitational attraction and become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic center resulting in being located within the main body of the galaxy, as well as losing cluster members through internal close encounters.[1] Open clusters generally survive for a few hundred million years. In contrast, the more massive globular clusters of stars exert a stronger gravitational attraction on their members, and can survive for many billions of years. Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring.[2]
Young open clusters may still be contained within the molecular cloud from which they formed, illuminating it to create an H II region.[3] Over time, radiation pressure from the cluster will disperse the molecular cloud. Typically, about 10% of the mass of a gas cloud will coalesce into stars before radiation pressure drives the rest of the gas away.

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