Editing Open cluster (section)

==Historical observations==
[[File:VISTA Finds Star Clusters Galore.jpg|thumb|Mosaic of 30 open clusters discovered from [[VISTA (telescope)|VISTA]]'s data. The open clusters were hidden by the dust in the Milky Way.<ref>{{cite news|title=VISTA Finds 96 Star Clusters Hidden Behind Dust|url=http://www.eso.org/public/news/eso1128/|access-date=3 August 2011|newspaper=ESO Science Release}}</ref> Credit [[ESO]].]]

The prominent open cluster the [[Pleiades]], in the constellation Taurus, has been recognized as a group of stars since antiquity, while the Hyades (which also form part of [[Taurus (constellation)|Taurus]]) is one of the oldest open clusters. Other open clusters were noted by early astronomers as unresolved fuzzy patches of light. In his ''[[Almagest]]'', the Roman astronomer [[Ptolemy]] mentions the [[Beehive Cluster|Praesepe]] cluster, the [[Double Cluster]] in [[Perseus (constellation)|Perseus]], the [[Coma Star Cluster]] and the [[Ptolemy Cluster]], while the Persian astronomer [[Al-Sufi]] wrote of the [[IC 2391|Omicron Velorum cluster]].<ref name=moore_rees2011/> However, it would require the invention of the [[telescope]] to resolve these "nebulae" into their constituent stars.<ref name=jones91/> Indeed, in 1603 [[Johann Bayer]] gave three of these clusters [[Bayer designation|designations]] as if they were single stars.<ref name=Kaler06/>

[[File:NGC 3590 open cluster Eso1416a.jpg|left|thumb|The star cluster [[List of NGC objects (3001–4000)|NGC 3590]]<ref>{{cite news|title=A Star Cluster in the Wake of Carina|url=http://www.eso.org/public/news/eso1416/|access-date=27 May 2014|newspaper=ESO Press Release}}</ref>]]

The first person to use a telescope to observe the night sky and record his observations was the Italian scientist [[Galileo Galilei]] in 1609. When he turned the telescope toward some of the nebulous patches recorded by Ptolemy, he found they were not a single star, but groupings of many stars. For Praesepe, he found more than 40 stars. Where previously observers had noted only 6–7 stars in the Pleiades, he found almost 50.<ref name=maran_marshall2009/> In his 1610 treatise ''[[Sidereus Nuncius]]'', Galileo Galilei wrote, "the galaxy is nothing else but a mass of innumerable stars planted together in clusters."<ref name=donofrio_burigana09/> Influenced by Galileo's work, the Sicilian astronomer [[Giovanni Hodierna]] became possibly the first astronomer to use a telescope to find previously undiscovered open clusters.<ref name=jha16_1_1/> In 1654, he identified the objects now designated [[Messier 41]], [[Messier 47]], [[NGC 2362]] and [[NGC 2451]].<ref name=jha17_50/>

It was realized as early as 1767 that the stars in a cluster were physically related,<ref name=rasqj30_4_399/> when the English naturalist the Reverend [[John Michell]] calculated that the probability of even just one group of stars like the Pleiades being the result of a chance alignment as seen from Earth was just 1 in 496,000.<ref name=pt57/> Between 1774 and 1781, French astronomer [[Charles Messier]] published a catalogue of celestial objects that had a nebulous appearance similar to [[comet]]s. This catalogue included 26 open clusters.<ref name=Kaler06/> In the 1790s, English astronomer [[William Herschel]] began an extensive study of nebulous celestial objects. He discovered that many of these features could be resolved into groupings of individual stars. Herschel conceived the idea that stars were initially scattered across space, but later became clustered together as star systems because of gravitational attraction.<ref name=jha10/> He divided the nebulae into eight classes, with classes VI through VIII being used to classify clusters of stars.<ref name=jha18_1/>

[[File:NGC265.jpg|right|thumb|[[NGC 265]], an open [[star cluster]] in the [[Small Magellanic Cloud]]]]

The number of clusters known continued to increase under the efforts of astronomers. Hundreds of open clusters were listed in the ''[[New General Catalogue]]'', first published in 1888 by the Danish–Irish astronomer [[John Louis Emil Dreyer|J. L. E. Dreyer]], and the two supplemental [[Index Catalogue]]s, published in 1896 and 1905.<ref name=Kaler06/> Telescopic observations revealed two distinct types of clusters, one of which contained thousands of stars in a regular spherical distribution and was found all across the sky but preferentially towards the center of the [[Milky Way]].<ref name=bok81/> The other type consisted of a generally sparser population of stars in a more irregular shape. These were generally found in or near the [[galactic plane]] of the Milky Way.<ref name=binney_merrifield1998/><ref name=basu03/> Astronomers dubbed the former [[globular cluster]]s, and the latter open clusters. Because of their location, open clusters are occasionally referred to as ''galactic clusters'', a term that was introduced in 1925 by the Swiss-American astronomer [[Robert Julius Trumpler]].<ref name=pasp37_220/>

Micrometer measurements of the positions of stars in clusters were made as early as 1877 by the German astronomer [[Eduard Schönfeld|E. Schönfeld]] and further pursued by the American astronomer [[Edward Emerson Barnard|E. E. Barnard]] prior to his death in 1923. No indication of stellar motion was detected by these efforts.<ref name=pyo6_1/> However, in 1918 the Dutch–American astronomer [[Adriaan van Maanen]] was able to measure the proper motion of stars in part of the Pleiades cluster by comparing photographic plates taken at different times.<ref name=cmwo167_1/> As [[astrometry]] became more accurate, cluster stars were found to share a common [[proper motion]] through space. By comparing the photographic plates of the Pleiades cluster taken in 1918 with images taken in 1943, van Maanen was able to identify those stars that had a [[proper motion]] similar to the mean motion of the cluster, and were therefore more likely to be members.<ref name=aj102_26/> [[Astronomical spectroscopy|Spectroscopic]] measurements revealed common [[radial velocity|radial velocities]], thus showing that the clusters consist of stars bound together as a group.<ref name=seds/>

The first [[color–magnitude diagram]]s of open clusters were published by [[Ejnar Hertzsprung]] in 1911, giving the plot for the Pleiades and [[Hyades (star cluster)|Hyades]] [[star cluster]]s. He continued this work on open clusters for the next twenty years. From spectroscopic data, he was able to determine the upper limit of internal motions for open clusters, and could estimate that the total mass of these objects did not exceed several hundred times the mass of the Sun. He demonstrated a relationship between the star colors and their magnitudes, and in 1929 noticed that the Hyades and [[Beehive Cluster|Praesepe]] clusters had different stellar populations than the Pleiades. This would subsequently be interpreted as a difference in ages of the three clusters.<ref name=strand1977/>