An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud, and are still loosely gravitationally bound to each other.
In contrast, globular clusters are very tightly bound by gravity.
Open clusters are found only in spiral and irregular galaxies, in which active star formation is occurring.
They are usually less than a few hundred million years old: they become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic centre, as well as losing cluster members through internal close encounters.
Young open clusters may still be contained within the molecular cloud from which they formed, illuminating it to create an H II region.
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 away.
Open clusters are very important objects in the study of stellar evolution.
Because the stars are all of very similar age and chemical composition, the effects of other more subtle variables on the properties of stars are much more easily studied than they are for isolated stars.
The most prominent open clusters such as the Pleiades have been known and recognised as groups of stars since antiquity.