Open clusters, also called galactic clusters, are the result of recent star formation that has occurred mostly near the Galactic plane. Compared to their much older and larger cousins, the globular clusters, open clusters are more loosely formed and contain but a fraction of the number of stars. The younger of these clusters contain dark dust and bright nebulosity.
Star clusters play a crucial role in astronomers' understanding of how stars evolve. Since all members of a cluster formed at aboiut the same time, the stars differ only by their masses and chemical compositions. As clusters age, higher mass stars evolve faster into red giants and eventually into the hard-to-see white dwarfs, neutron stars, or black holes. Thus older clusters appear to contain mostly low-mass, reddish stars, like in M67. The brilliant blue supergiants are characteristic of very young clusters, like M45. M6, NGC 6405