What is Open star cluster?
[[File:The star cluster NGC 3572 and its dramatic surroundings.jpg|thumb|300px|Star cluster [[NGC 3572]] and its surroundings.<ref>{{cite news|title=Young stars paint spectacular stellar landscape|url=http://www.eso.org/public/news/eso1347/|accessdate=20 November 2013|newspaper=ESO Press Release}}</ref> ]] An '''open star cluster''', also known as '''galactic cluster''', is a [[star cluster|group]] of a few hundred or thousand [[star]]s. They have roughly the same age, and were formed from the same giant [[molecular cloud]]. More than 1,100 open clusters have been discovered within the [[Milky Way Galaxy]], and many more are thought to exist. They are loosely bound together by mutual [[gravity|gravitational attraction]], but they get disrupted by close encounters with other clusters and clouds of gas.<ref>Karttunen, Hannu ''et al'' 2003. ''Fundamental astronomy''. Physics and Astronomy Online Library, 4th ed. Springer. p. 321. {{ISBN|3-540-00179-4}}</ref> Open clusters generally survive for a few hundred million years, with the most massive ones surviving for a few billion years. In contrast, the more massive [[globular cluster]]s of stars exert a stronger gravitational attraction on their members. So, they can survive for longer. Open clusters have been found only in [[spiral galaxy|spiral]] and [[irregular galaxy|irregular galaxies]], in which active [[star formation]] is occurring.<ref>Payne-Gaposchkin C. 1979. ''Stars and clusters''. Cambridge, Mass: Harvard University Press. [http://adsabs.harvard.edu/abs/1979stcl.book.....P]</ref> Young open clusters may still be in the molecular cloud from which they formed. They light it up and create an [[H II region]].<ref>A good example of this is NGC 2244, in the Rosette Nebula. See also Johnson, Harold L. 1962. The Galactic Cluster NGC 2244. ''Astrophysical Journal'' '''136''': 1135.</ref> Over time, [[radiation pressure]] from the cluster will disperse the molecular cloud. Typically, about 10% of the mass of a gas cloud will form stars before radiation pressur