An
open cluster, also known as
galactic 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. More than 1,100 open clusters have been discovered within the
Milky Way Galaxy, and many more are thought to exist. They are loosely bound 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 a migration to the main body of the galaxy as well as a loss of cluster members through internal close encounters. Open clusters generally survive for a few hundred million years, with the most massive ones surviving for a few billion years. In contrast, the more massive
globular clusters of stars exert a stronger gravitational attraction on their members, and can survive for longer. Open clusters have been found only in
spiral and
irregular galaxies, in which active
star formation is occurring.