Changes

An '''open cluster''', sometimes referred to as an '''open star cluster''' or a '''galactic cluster''', is a group of physically related [[star]]s that have similar chemical composition held together by mutual [[gravity]]. In contrast to a [[globular cluster]]s, the stars of an open cluster are only loosely gravitationally bound to each other.  Open clusters have only been observed in galaxies with active star formation, such as [[spiral galaxy|spiral]] and [[irregular galaxy|irregular galaxies]], for spiral galaxies, they are found within the spiral arms.<ref>http://adsabs.harvard.edu/abs/1979stcl.book.....P</ref>  These clusters are made up of [[metallicity|population I stars]].  Some clusters may still be contained in interstellar clouds, illuminating the clouds.<ref>http://adsabs.harvard.edu/abs/1962ApJ...136.1135J</ref>  At current count, there are over 1,100 known open clusters in the [[Milky Way]] alone, although this may be only a small percentage of the total.<ref name="am">http://www.maa.clell.de/Messier/open.html</ref>

An '''open cluster''', sometimes referred to as an '''open star cluster''' or a '''galactic cluster''', is a group of physically related [[star]]s that have similar chemical composition held together by mutual [[gravity]]. In contrast to a [[globular cluster]]s, the stars of an open cluster are only loosely gravitationally bound to each other.  Open clusters have only been observed in galaxies with active star formation, such as [[spiral galaxy|spiral]] and [[irregular galaxy|irregular galaxies]], for spiral galaxies, they are found within the spiral arms.<ref>Payne-Gaposchkin, C. (1979). Stars and Clusters. ''Astronomy Quarterly'', 3(10), p.109. [http://adsabs.harvard.edu/abs/1979stcl.book.....P Online]</ref>  These clusters are made up of [[metallicity|population I stars]].  Some clusters may still be contained in interstellar clouds, illuminating the clouds.<ref>Johnson, H. (1962). The Galactic Cluster, NGC 2244. ''The Astrophysical Journal'', 136, p.1135. [http://adsabs.harvard.edu/abs/1962ApJ...136.1135J Online]</ref>  At current count, there are over 1,100 known open clusters in the [[Milky Way]] alone, although this may be only a small percentage of the total.<ref name="am">[http://www.maa.clell.de/Messier/open.html Open Star Clusters] from maa.clell.de</ref>

The clusters themselves range from large conglomerates of thousands of stars to sparse clusters of only a dozen or less.  The core or center of these clusters are a few light years across with an average density of a star per 1.5 cubic light years, about 500 times as dense as our solar neighborhood.

The clusters themselves range from large conglomerates of thousands of stars to sparse clusters of only a dozen or less.  The core or center of these clusters are a few light years across with an average density of a star per 1.5 cubic light years, about 500 times as dense as our solar neighborhood.

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]]s.

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]]s.

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"/>  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>

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"/>  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>Proctor, R. (1869). Preliminary Paper on Certain Drifting Motions of the Stars. ''Proceedings of the Royal Society of London'', 18(114-122), pp.169-171. [http://adsabs.harvard.edu/cgi-bin/bib_query?1869RSPS...18..169P Online]</ref>

==Classification==

==Classification==

In 1930, Swiss-American astronomer R.J Trumpler introduced a classification system for open clusters, later known as the ''Trumpler classification'', based on three defining attributes.<ref>http://books.google.com/books?id=40JzBYGREL0C&pg=PA74&dq=%22Trumpler+classification%22&ei=bMLDSLHnD5TqjgHkqeTrDQ&sig=ACfU3U0fxdckqWt_w6eawA44vGIJmi2yJg#v=onepage&q=%22Trumpler%20classification%22&f=false</ref> The first attribute is according to how concentrated the cluster is towards it's center, ranging from I-IV:

In 1930, Swiss-American astronomer R.J Trumpler introduced a classification system for open clusters, later known as the ''Trumpler classification'', based on three defining attributes.<ref>Garfinkle, R. (1999). ''Star-hopping: Your Visa to Viewing the Universe''. Cambridge (England): Cambridge University Press. [http://books.google.com/books?id=40JzBYGREL0C&pg=PA74&dq=%22Trumpler+classification%22&ei=bMLDSLHnD5TqjgHkqeTrDQ&sig=ACfU3U0fxdckqWt_w6eawA44vGIJmi2yJg#v=onepage&q=%22Trumpler%20classification%22&f=false Online]</ref> The first attribute is according to how concentrated the cluster is towards it's center, ranging from I-IV:

*I - Detached from the surrounding stars with strong concentration towards the cluster center

*I - Detached from the surrounding stars with strong concentration towards the cluster center

*r - Cluster has over 100 stars, rich

*r - Cluster has over 100 stars, rich

If the cluster lies within a [[nebula|nebulous structure]] it's classification is appended with an 'n'.  Under this classification method for example, the [[Pleiades]] is classified as '''I3rn''' (Strong concentration towards the center, richly populated with stars that have a wide range in brightness, with nebulous material present).<ref>http://adsabs.harvard.edu/cgi-bin/bib_query?1930LicOB.420..154T</ref>

If the cluster lies within a [[nebula|nebulous structure]] it's classification is appended with an 'n'.  Under this classification method for example, the [[Pleiades]] is classified as '''I3rn''' (Strong concentration towards the center, richly populated with stars that have a wide range in brightness, with nebulous material present).<ref>Trumpler, R. (1930). Preliminary results on the distances, dimensions and space distribution of open star clusters. ''Lick Observatory Bulletins'', 14, pp.154-188. [http://adsabs.harvard.edu/cgi-bin/bib_query?1930LicOB.420..154T Online]</ref>

 

==References==

==References==

{{reflist}}

{{reflist}}