Editing Open cluster (section)

==Morphology and classification==
[[File:The colourful star cluster NGC 2367.jpg|thumb|[[NGC 2367]] is an infant stellar grouping that lies at the center of an immense and ancient structure on the margins of the [[Milky Way]].<ref>{{cite web|title=Buried in the Heart of a Giant|url=http://www.eso.org/public/news/eso1526/|access-date=1 July 2015}}</ref>]]
Open clusters range from very sparse clusters with only a few members to large [[wikt:agglomeration|agglomeration]]s containing thousands of stars. They usually consist of quite a distinct dense core, surrounded by a more diffuse 'corona' of cluster members. The core is typically about 3–4&nbsp;[[light year]]s across, with the corona extending to about 20&nbsp;light years from the cluster center. Typical star densities in the center of a cluster are about 1.5 stars per [[cubic light year]]. For comparison, stellar density near the Sun is about 0.003 stars per cubic light year.<ref name=aaa383/>

Open clusters are often classified according to a scheme developed by [[Robert Trumpler]] in 1930. The Trumpler scheme gives a cluster a three-part designation, with a [[Roman numeral]] from I-IV for little to very disparate, an [[Arabic numeral]] from 1 to 3 for the range in brightness of members (from small to large range), and ''p'', ''m'' or ''r'' to indication whether the cluster is poor, medium or rich in stars. An 'n' is appended if the cluster lies within [[nebula|nebulosity]].<ref name=lob420/>

Under the Trumpler scheme, the Pleiades are classified as I3rn, and the nearby Hyades are classified as II3m.