04-07-2017, 10:23 AM
An open cluster is a group of up to thousands of stars that formed from the same giant molecular cloud and are about the same age. More than 1,100 open clusters have been discovered in the Milky Way galaxy, and many more are believed to exist. They are freely bound by mutual gravitational attraction and are interrupted by close encounters with other clusters and gas clouds as they orbit the galactic center. This can result in a migration to the main body of the galaxy and a loss of group members through close internal encounters. Open groups generally survive a few hundred million years, and the most massive ones survive for a few billion years. In contrast, the more massive globular groups of stars exert a stronger gravitational pull on their members, and can survive longer. Open clusters have been found only in spiral and irregular galaxies, in which active formation of the star is occurring.
Open young groups may not be contained within the molecular cloud from which they were formed, illuminating it to create an H II region. Over time, the radiation pressure of the cluster will disperse the molecular cloud. Typically, about 10% of the mass of a gas cloud will join in stars before the radiation pressure pushes the rest of the gas.
Open clusters are key objects in the study of stellar evolution. Because group members have a similar age and chemical composition, their properties (such as distance, age, metallicity, and extinction) are easier to determine than for isolated stars. A number of open clusters, such as Pleiades, Hyades or Persei Alpha Cluster are visible to the naked eye. Some others, such as the Double Cluster, are barely perceptible without instruments, while many more can be seen using binoculars or telescopes. The group of wild ducks, M11, is an example.