Open clusters are physically related groups of stars held together by mutual gravitational attraction. In many clouds visible as bright diffuse nebulae, star formation takes still place at this moment, so that we can observe the formation of new young star clusters.
Most open clusters have only a short life as stellar swarms.
As they drift along their orbits, some of their members escape the cluster, due to velocity changes in mutual closer encounters, tidal forces in the galactic gravitational field, and encounters with field stars and interstellar clouds crossing their way. First thought to be nebulae, it was Galileo who in 1609 discovered that they are composed of stars, when observing M44 . Moreover, the Milky Way starcloud M24 contains the open star cluster NGC 6603. A Site Devoted to Stellar Open Clusters , by Jean-Claude Mermilliod.