The open cluster M39 in Cygnus. It lies about 800 light-years away and contains stars that are only about 300 million years old. Image: WIYN telescope, Kitt Peak.
An open cluster is a loose aggregation of dozens or hundreds of young stars, in a volume of space typically less than 50 light-years across. Open clusters are only weakly held together by gravity and are fated to disperse over a period of several hundred million years.
The stars in open clusters have formed together within the same interstellar cloud; indeed, in many diffuse nebulae, the birth of new open clusters can be seen taking place. As open clusters drift along, some of their members escape due to velocity changes in mutual closer encounters, tidal forces in the galactic gravitational field, and encounters with field stars and interstellar clouds passing their way.
Well known examples of open clusters include:
Wild Duck Cluster
Compare with globular clusters.