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There are two major types of clusters:
- open star clusters. 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, which is an open cluster.
- globular clusters. 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.
Super star clusters
Super star clusters of very young large stars are known. They are thought to be precursors of globular clusters. The short-lived huge blue stars emit lots of UV radiation which ionises the surrounding gas. Examples are Westerlund 1 and 2 in the Milky Way, and R136 in the Larger Magellanic Cloud.
- "star cluster (astronomy) -- Britannica Online Encyclopedia". britannica.com. http://www.britannica.com/EBchecked/topic/563485/star-cluster. Retrieved 7 April 2010.
- Gallagher & Grebel (2002). "Extragalactic Star Clusters: speculations on the future". Extragalactic Star Clusters, IAU Symposium: 207.