Changes

Open clusters have been known since [[antiquity]], although before the [[telescope]] the individual stars could not be resolved and they appeared as dim patches of light.  One exception to this is the most well-known open cluster, the [[Pleiades]], which was easily observable as a group of similar stars, another known cluster was the [[Hyades]], the closest open cluster to us at only 150 [[light year|light years]].

Open clusters have been known since [[antiquity]], although before the [[telescope]] the individual stars could not be resolved and they appeared as dim patches of light.  One exception to this is the most well-known open cluster, the [[Pleiades]], which was easily observable as a group of similar stars, another known cluster was the [[Hyades]], the closest open cluster to us at only 150 [[light year|light years]].

Although there are various recording of the related stars of open clusters as far back as [[Ptolemy]] in 138 A.D, it wasn't until 1767 that the stars were recognized as such when [[Reverend]] [[John Michell]] first postulated that the stars of an open clusters had to be related because the chance of finding even one such cluster if they just happen to be a collection of unrelated stars were too astronomical.<ref name="am">http://www.maa.clell.de/Messier/open.html</ref>  It wasn't until 1869 that the relation of stars in an open cluster was conclusively shown.  That year Richard Proctor in his study of the Pleiades and the [[Ursa Major|Ursa Major Moving Cluster]], demonstrated using spectroscopy that the stars of the clusters have the same [[radical velocity]], indicating they are bound to each other by gravity.  Proctor also showed that the stars of such clusters were all about the same distance from the [[Earth]], further showing they were moving together and were related.<ref>http://adsabs.harvard.edu/cgi-bin/bib_query?1869RSPS...18..169P</ref>

Although there are various recording of the related stars of open clusters as far back as [[Ptolemy]] in 138 A.D, it wasn't until 1767 that the stars were recognized as such when [[Reverend]] [[John Michell]] first postulated that the stars of an open clusters had to be related because the chance of finding even one such cluster if they just happen to be a collection of unrelated stars were too astronomical.<ref name="am">http://www.maa.clell.de/Messier/open.html</ref>  It wasn't until 1869 that the relation of stars in an open cluster was conclusively shown.  That year Richard Proctor in his study of the Pleiades and the [[Ursa Major|Ursa Major Moving Cluster]], demonstrated using spectroscopy that the stars of the clusters have the same [[radial velocity]], indicating they are bound to each other by gravity.  Proctor also showed that the stars of such clusters were all about the same distance from the [[Earth]], further showing they were moving together and were related.<ref>http://adsabs.harvard.edu/cgi-bin/bib_query?1869RSPS...18..169P</ref>

==Classification==

==Classification==