What is Star cluster?
[[File:Messier_2_Hubble_WikiSky.jpg|Globular cluster [[Messier catalogue|Messier]] 2 (or M2, or NGC 7089) is a [[globular cluster]] in the constellation [[Aquarius (constellation)|Aquarius]] |thumb]] A '''star cluster''' is a group of [[star]]s. They are held together by mutual [[gravity|gravitation]]. They are related by common origin, and have similar or identical ages.<ref>{{Cite encyclopedia|url= http://www.britannica.com/EBchecked/topic/563485/star-cluster|title=star cluster (astronomy) -- Britannica Online Encyclopedia|encyclopedia=britannica.com|accessdate=7 April 2010}}</ref> In contrast, [[constellations]] and [[asterism]]s are just line-of-sight visual groups as seen from the Earth. There are two major types of clusters: #'''[[open star cluster]]s'''. Open clusters are loose, and they have less than a few hundred stars, which are often very young. A famous star cluster is the [[Pleiades cluster|Pleiades]], which is an open cluster. #'''[[globular cluster]]s'''. Globular clusters are tight groups of hundreds and thousands of stars. Every large galaxy has hundreds or thousands of globulars revolving round it at great distances, usually outside the disk of the galaxy. Star clusters visible to the naked eye include the [[Pleiades]], the [[Hyades]], and [[47 Tucanae]]. == Super star clusters == Super star clusters of very young large stars are known. They are thought to be precursors of [[globular cluster]]s.<ref name=gallagher02>{{cite journal| author = Gallagher & Grebel | year = 2002| title = Extragalactic star clusters: speculations on the future| url = https://archive.org/details/arxiv-astro-ph0109052 | journal = Extragalactic Star Clusters, IAU Symposium| volume = 207| pages = 207| bibcode = 2002IAUS..207..745G |arxiv = astro-ph/0109052 }}</ref> The short-lived huge blue stars emit lots of [[UV]] [[radiation]] which [[ionisation|ionises]] the surrounding gas. In our galaxy, examples are [[Westerlund 1]] and [[Westerlund 2]] in the [[Milky Way]]. In Andromeda there is an even bigger one: [[Mayall