Open clusters are physically related groups of stars held together by mutual gravitational attraction. They are believed to originate from large cosmic gas and dust clouds (
diffuse nebulae) in the
Milky Way (or other parent galaxy), and to continue to orbit the galaxy within or near their parent galaxy's disk. In many clouds visible as bright diffuse nebulae, star formation still takes place at this moment, so that we can observe the formation of new young star clusters. The process of formation takes only a considerably short time compared to the lifetime of the cluster, so that all member stars are of similar age. Also, as all the stars in a cluster formed from the same diffuse nebula, they are all of similar initial chemical composition.
Over 1100 open clusters are known in our
Milky Way Galaxy, and this is probably only a small percentage of the total population which is probably some factor higher; estimates of as many as about 100,000 Milky Way open clusters have been given.
