Editing Open cluster

{{Short description|Large group of stars less bound than globular clusters}}
{{redirect-distinguish|Galactic cluster|galaxy cluster}}{{Infobox astronomical formation|caption=The Pleiades is one of the most famous open clusters.|image=File:Pleiades.jpg|name=Open cluster|thing=Loose cluster of stars|commonscat=Open clusters|qid=Q11387|density=~ 1.5 stars / cubic {{abbr|ly|light year}}|size=< 30 ly in diameter}}
An '''open cluster''' is a type of [[star cluster]] made of tens to a few thousand [[star]]s that were formed from the same [[giant molecular cloud]] and have roughly the same age. More than 1,100 open clusters have been discovered within the [[Milky Way]] galaxy, and many more are thought to exist.<ref name="seds" /> Each one is loosely bound by mutual [[gravity|gravitational attraction]] and becomes disrupted by close encounters with other clusters and clouds of gas as they orbit the [[Galactic Center]]. This can result in a loss of cluster members through internal close encounters and a dispersion into the main body of the galaxy.<ref name="Karttunen_et_al_03" /> Open clusters generally survive for a few hundred million years, with the most massive ones surviving for a few billion years. In contrast, the more massive [[globular cluster]]s of stars exert a stronger gravitational attraction on their members, and can survive for longer. Open clusters have been found only in [[spiral galaxy|spiral]] and [[irregular galaxy|irregular galaxies]], in which active [[star formation]] is occurring.<ref name="payne-gaposchkin79" />

Young open clusters may be contained within the molecular cloud from which they formed, illuminating it to create an [[H II region]].<ref name=apj62/> Over time, [[radiation pressure]] from the cluster will disperse the molecular cloud. Typically, about 10% of the mass of a gas cloud will coalesce into stars before radiation pressure drives the rest of the gas away.

Open clusters are key objects in the study of [[stellar evolution]]. Because the cluster members are of similar age and [[chemical element|chemical composition]], their properties (such as distance, age, [[metallicity]], [[Extinction (astronomy)|extinction]], and velocity) are more easily determined than they are for isolated stars.<ref name="seds" /> A number of open clusters, such as the [[Pleiades]], the [[Hyades (star cluster)|Hyades]] and the [[Alpha Persei Cluster]], are visible with the naked eye. Some others, such as the [[Double Cluster]], are barely perceptible without instruments, while many more can be seen using [[binoculars]] or [[optical telescope|telescopes]]. The [[Wild Duck Cluster]], M11, is an example.<ref name=neata09/>

==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/>

==Formation==
[[File:Trapezium cluster optical and infrared comparison.jpg|thumb|[[Infrared]] light reveals the dense open cluster forming at the heart of the [[Orion nebula]].]]

The formation of an open cluster begins with the collapse of part of a [[giant molecular cloud]], a cold dense cloud of gas and dust containing up to many thousands of times the [[solar mass|mass of the Sun]]. These clouds have densities that vary from 10<sup>2</sup> to 10<sup>6</sup> molecules of [[neutral hydrogen]] per cm<sup>3</sup>, with star formation occurring in regions with densities above 10<sup>4</sup> molecules per cm<sup>3</sup>. Typically, only 1–10% of the cloud by volume is above the latter density.<ref name=ptrsa368_1913_713/> Prior to collapse, these clouds maintain their mechanical equilibrium through magnetic fields, turbulence and rotation.<ref name=araa25_23/>

Many factors may disrupt the equilibrium of a giant molecular cloud, triggering a collapse and initiating the burst of star formation that can result in an open cluster. These include shock waves from a nearby [[supernova]], collisions with other clouds and gravitational interactions. Even without external triggers, regions of the cloud can reach conditions where they become unstable against collapse.<ref name=araa25_23/> The collapsing cloud region will undergo hierarchical fragmentation into ever smaller clumps, including a particularly dense form known as [[infrared dark cloud]]s, eventually leading to the formation of up to several thousand stars. This star formation begins enshrouded in the collapsing cloud, blocking the protostars from sight but allowing infrared observation.<ref name=ptrsa368_1913_713/> In the Milky Way galaxy, the formation rate of open clusters is estimated to be one every few thousand years.<ref name=mnras249/>

[[File:Eagle nebula pillars.jpg|right|thumb|The so-called "[[Pillars of Creation]]", a region of the [[Eagle Nebula]] where the molecular cloud is being evaporated by young, massive stars]]
The hottest and most massive of the newly formed stars (known as [[OB star]]s) will emit intense [[ultraviolet radiation]], which steadily ionizes the surrounding gas of the giant molecular cloud, forming an [[H II region]]. [[Stellar wind]]s and [[radiation pressure]] from the massive stars begins to drive away the hot ionized gas at a velocity matching the speed of sound in the gas. After a few million years the cluster will experience its first [[core-collapse supernova]]e, which will also expel gas from the vicinity. In most cases these processes will strip the cluster of gas within ten million years, and no further star formation will take place. Still, about half of the resulting protostellar objects will be left surrounded by [[circumstellar disk]]s, many of which form accretion disks.<ref name=ptrsa368_1913_713/>

As only 30 to 40 percent of the gas in the cloud core forms stars, the process of residual gas expulsion is highly damaging to the star formation process. All clusters thus suffer significant infant weight loss, while a large fraction undergo infant mortality. At this point, the formation of an open cluster will depend on whether the newly formed stars are gravitationally bound to each other; otherwise an unbound [[stellar association]] will result. Even when a cluster such as the Pleiades does form, it may hold on to only a third of the original stars, with the remainder becoming unbound once the gas is expelled.<ref name=mnras321_4_699/> The young stars so released from their natal cluster become part of the Galactic field population.

Because most if not all stars form in clusters, [[star cluster]]s are to be viewed as the fundamental building blocks of galaxies. The violent gas-expulsion events that shape and destroy many star clusters at birth leave their imprint in the morphological and kinematical structures of galaxies.<ref name=kroupa05/> Most open clusters form with at least 100 stars and a mass of 50 or more solar masses. The largest clusters can have over 10<sup>4</sup> solar masses, with the massive cluster [[Westerlund 1]] being estimated at 5 × 10<sup>4</sup> solar masses and [[R136]] at almost 5 x 10<sup>5</sup>, typical of globular clusters.<ref name=ptrsa368_1913_713/> While open clusters and globular clusters form two fairly distinct groups, there may not be a great deal of intrinsic difference between a very sparse globular cluster such as [[Palomar 12]] and a very rich open cluster. Some astronomers believe the two types of star clusters form via the same basic mechanism, with the difference being that the conditions that allowed the formation of the very rich globular clusters containing hundreds of thousands of stars no longer prevail in the Milky Way.<ref name=apj480/>

It is common for two or more separate open clusters to form out of the same molecular cloud. In the [[Large Magellanic Cloud]], both [[Hodge 301]] and [[R136]] have formed from the gases of the [[Tarantula Nebula]], while in our own galaxy, tracing back the motion through space of the [[Hyades (star cluster)|Hyades]] and [[Praesepe]], two prominent nearby open clusters, suggests that they formed in the same cloud about 600 million years ago.<ref name=mnras120/> Sometimes, two clusters born at the same time will form a binary cluster. The best known example in the Milky Way is the [[Double Cluster]] of NGC 869 and NGC 884 (also known as h and χ Persei), but at least 10 more double clusters are known to exist.<ref name=aaa302/> New research indicates the [[Cepheid]]-hosting [[Messier 25|M25]] may constitute a ternary star cluster together with NGC 6716 and Collinder 394.<ref name=Majaess2024/>  Many more binary clusters are known in the [[Small Magellanic Cloud|Small]] and Large Magellanic Clouds—they are easier to detect in external systems than in our own galaxy because [[projection effect (disambiguation)|projection effect]]s can cause unrelated clusters within the Milky Way to appear close to each other.

==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.

==Numbers and distribution==
[[File:NGC 346 in Small magellanic cloud.jpg|thumb|[[NGC 346]], an open cluster in the [[Small Magellanic Cloud]]]]

There are over 1,100 known open clusters in our galaxy, but the true total may be up to ten times higher than that.<ref name=aaa389/> In [[spiral galaxy|spiral galaxies]], open clusters are largely found in the spiral arms where gas densities are highest and so most star formation occurs, and clusters usually disperse before they have had time to travel beyond their spiral arm. Open clusters are strongly concentrated close to the galactic plane, with a [[scale height]] in our galaxy of about 180&nbsp;light years, compared with a galactic radius of approximately 50,000&nbsp;light years.<ref name=aj108/>

In [[irregular galaxy|irregular galaxies]], open clusters may be found throughout the galaxy, although their concentration is highest where the gas density is highest.<ref name=pasp109/> Open clusters are not seen in [[elliptical galaxy|elliptical galaxies]]: Star formation ceased many millions of years ago in ellipticals, and so the open clusters which were originally present have long since dispersed.<ref name=binney_merrifield98/>

In the Milky Way Galaxy, the distribution of clusters depends on age, with older clusters being preferentially found at greater distances from the [[Galactic Center]], generally at substantial distances above or below the [[galactic plane]].<ref name=Friel/> [[Tidal force]]s are stronger nearer the center of the galaxy, increasing the rate of disruption of clusters, and also the giant molecular clouds which cause the disruption of clusters are concentrated towards the inner regions of the galaxy, so clusters in the inner regions of the galaxy tend to get dispersed at a younger age than their counterparts in the outer regions.<ref name=aaa88/>

==Stellar composition==
[[File:Tarantula nebula detail.jpg|thumb|A cluster of stars a few million years old at the lower right illuminates the [[Tarantula Nebula]] in the [[Large Magellanic Cloud]].]]

Because open clusters tend to be dispersed before most of their stars reach the end of their lives, the light from them tends to be dominated by the young, hot blue stars. These stars are the most massive, and have the shortest lives, a few tens of millions of years. The older open clusters tend to contain more yellow stars.<ref>{{Cite web |author1=Stefanie Waldek |date=2021-10-06 |title=What are star clusters? |url=https://www.space.com/star-clusters |access-date=2024-07-19 |website=Space.com |language=en}}</ref>

The frequency of [[binary star]] systems has been observed to be higher within open clusters than outside open clusters. This is seen as evidence that single stars get ejected from open clusters due to dynamical interactions.<ref>{{cite journal|doi=10.3847/1538-4357/ac1585|title=Long-term Spectroscopic Survey of the Pleiades Cluster: The Binary Population|year=2021|last1=Torres|first1=Guillermo|last2=Latham|first2=David W.|last3=Quinn|first3=Samuel N.|journal=The Astrophysical Journal|volume=921|issue=2|page=117|arxiv=2107.10259|bibcode=2021ApJ...921..117T|s2cid=236171384 |doi-access=free }}</ref>

Some open clusters contain hot blue stars which seem to be much younger than the rest of the cluster. These [[blue straggler]]s are also observed in globular clusters, and in the very dense cores of globulars they are believed to arise when stars collide, forming a much hotter, more massive star. However, the stellar density in open clusters is much lower than that in globular clusters, and stellar collisions cannot explain the numbers of blue stragglers observed. Instead, it is thought that most of them probably originate when dynamical interactions with other stars cause a binary system to coalesce into one star.<ref name=baas35/>

Once they have exhausted their supply of [[hydrogen]] through [[nuclear fusion]], medium- to low-mass stars shed their outer layers to form a [[planetary nebula]] and evolve into [[white dwarf]]s. While most clusters become dispersed before a large proportion of their members have reached the white dwarf stage, the number of white dwarfs in open clusters is still generally much lower than would be expected, given the age of the cluster and the expected initial mass distribution of the stars. One possible explanation for the lack of white dwarfs is that when a [[red giant]] expels its outer layers to become a planetary nebula, a slight asymmetry in the loss of material could give the star a 'kick' of a few [[kilometre per second|kilometres per second]], enough to eject it from the cluster.<ref name=apj595/>

Because of their high density, close encounters between stars in an open cluster are common.{{Citation needed|date=April 2018}} For a typical cluster with 1,000 stars with a 0.5&nbsp;parsec half-mass radius, on average a star will have an encounter with another member every 10 million years. The rate is even higher in denser clusters. These encounters can have a significant impact on the extended circumstellar disks of material that surround many young stars. Tidal perturbations of large disks may result in the formation of massive planets and [[brown dwarf]]s, producing companions at distances of 100&nbsp;[[Astronomical Unit|AU]] or more from the host star.<ref name=apj717_1_577/>

==Eventual fate==
[[File:Nursery of New Stars - GPN-2000-000972.jpg|left|thumb|[[NGC 604]] in the [[Triangulum Galaxy]] is a very massive open cluster surrounded by an [[H II region]].]]

Many open clusters are inherently unstable, with a small enough mass that the [[escape velocity]] of the system is lower than the average [[velocity]] of the constituent stars. These clusters will rapidly disperse within a few million years. In many cases, the stripping away of the gas from which the cluster formed by the radiation pressure of the hot young stars reduces the cluster mass enough to allow rapid dispersal.<ref name=apj235_1/>

Clusters that have enough mass to be gravitationally bound once the surrounding nebula has evaporated can remain distinct for many tens of millions of years, but, over time, internal and external processes tend also to disperse them. Internally, close encounters between stars can increase the velocity of a member beyond the escape velocity of the cluster. This results in the gradual 'evaporation' of cluster members.<ref name=pasp110/>

Externally, about every half-billion years or so an open cluster tends to be disturbed by external factors such as passing close to or through a molecular cloud. The gravitational [[tidal force]]s generated by such an encounter tend to disrupt the cluster. Eventually, the cluster becomes a stream of stars, not close enough to be a cluster but all related and moving in similar directions at similar speeds. The timescale over which a cluster disrupts depends on its initial stellar density, with more tightly packed clusters persisting longer. Estimated cluster [[half life|half lives]], after which half the original cluster members will have been lost, range from 150–800 million years, depending on the original density.<ref name=pasp110/>

After a cluster has become gravitationally unbound, many of its constituent stars will still be moving through space on similar trajectories, in what is known as a [[stellar association]], moving cluster, or [[Stellar kinematics|moving group]]. Several of the brightest stars in the '[[Big Dipper|Plough]]' of [[Ursa Major]] are former members of an open cluster which now form such an association, in this case the [[Ursa Major Moving Group]].<ref name=aj105_1/> Eventually their slightly different relative velocities will see them scattered throughout the galaxy. A larger cluster is then known as a stream, if we discover the similar velocities and ages of otherwise well-separated stars.<ref name=aspcs92/><ref name=baas38/>

==Studying stellar evolution==
[[File:Open cluster HR diagram ages.gif|thumb|[[Hertzsprung–Russell diagram]]s for two open clusters. [[NGC 188]] is older, and shows a lower turn-off from the [[main sequence]] than that seen in [[Messier 67|M67]].]]

When a [[Hertzsprung–Russell diagram]] is plotted for an open cluster, most stars lie on the [[main sequence]].<ref name=orsa/> The most massive stars have begun to evolve away from the main sequence and are becoming [[red giant]]s; the position of the turn-off from the main sequence can be used to estimate the age of the cluster.<ref name=BOB4767>{{cite book |last1=Carroll |first1=B. W. |last2=Ostlie |first2=D. A. |title=An Introduction to Modern Astrophysics |edition=2nd |publisher=[[Cambridge University Press]] |date=2017 |isbn=978-1-108-42216-1 |pages=476–477}}</ref>

Because the stars in an open cluster are all at roughly the same distance from [[Earth]], and were born at roughly the same time from the same raw material, the differences in apparent brightness among cluster members are due only to their mass.<ref name=orsa/> This makes open clusters very useful in the study of stellar evolution, because when comparing one star with another, many of the variable parameters are fixed.<ref name=BOB4767/>

The study of the abundances of [[lithium]] and [[beryllium]] in open-cluster stars can give important clues about the evolution of stars and their interior structures. While [[hydrogen]] nuclei cannot fuse to form [[helium]] until the temperature reaches about 10 million&nbsp;[[Kelvin|K]], lithium and beryllium are destroyed at temperatures of 2.5 million&nbsp;K and 3.5 million&nbsp;K respectively. This means that their abundances depend strongly on how much mixing occurs in stellar interiors. Through study of their abundances in open-cluster stars, variables such as age and chemical composition can be fixed.<ref name=pasp116/>

Studies have shown that the abundances of these light elements are much lower than models of stellar evolution predict. While the reason for this underabundance is not yet fully understood, one possibility is that [[convection]] in stellar interiors can 'overshoot' into regions where [[radiant energy|radiation]] is normally the dominant mode of energy transport.<ref name=pasp116/>

==Astronomical distance scale==
[[File:Messier11.jpg|left|thumb|[[Wild Duck Cluster|M11]], also known as 'the Wild Duck Cluster', is a very rich cluster located towards the center of the [[Milky Way]].]]

Determining the distances to astronomical objects is crucial to understanding them, but the vast majority of objects are too far away for their distances to be directly determined. Calibration of the [[cosmic distance ladder|astronomical distance scale]] relies on a sequence of indirect and sometimes uncertain measurements relating the closest objects, for which distances can be directly measured, to increasingly distant objects.<ref name=keel09/> Open clusters are a crucial step in this sequence.

The closest open clusters can have their distance measured directly by one of two methods. First, the [[parallax]] (the small change in apparent position over the course of a year caused by the Earth moving from one side of its orbit around the Sun to the other) of stars in close open clusters can be measured, like other individual stars. Clusters such as the Pleiades, Hyades and a few others within about 500&nbsp;light years are close enough for this method to be viable, and results from the [[Hipparcos]] position-measuring satellite yielded accurate distances for several clusters.<ref name=rmaa11/><ref name=aaa400/>

The other direct method is the so-called [[moving cluster method]]. This relies on the fact that the stars of a cluster share a common motion through space. Measuring the proper motions of cluster members and plotting their apparent motions across the sky will reveal that they converge on a [[vanishing point]]. The radial velocity of cluster members can be determined from [[Doppler shift]] measurements of their [[electromagnetic spectrum|spectra]], and once the radial velocity, proper motion and angular distance from the cluster to its vanishing point are known, simple [[trigonometry]] will reveal the distance to the cluster. The [[Hyades (star cluster)|Hyades]] are the best-known application of this method, which reveals their distance to be 46.3&nbsp;[[parsec]]s.<ref name=aj80/>

Once the distances to nearby clusters have been established, further techniques can extend the distance scale to more distant clusters. By matching the [[main sequence]] on the Hertzsprung–Russell diagram for a cluster at a known distance with that of a more distant cluster, the distance to the more distant cluster can be estimated. The nearest open cluster is the Hyades: The stellar association consisting of most of the [[Ursa Major Moving Group|Plough stars]] is at about half the distance of the Hyades, but is a stellar association rather than an open cluster as the stars are not gravitationally bound to each other. The most distant known open cluster in our galaxy is [[Berkeley 29]], at a distance of about 15,000&nbsp;parsecs.<ref name=aaa429/> Open clusters, especially [[super star clusters]], are also easily detected in many of the galaxies of the [[Local Group]] and nearby: e.g., [[NGC 346]] and the SSCs [[R136]] and [[NGC 1569|NGC 1569 A and B]].

Accurate knowledge of open cluster distances is vital for calibrating the period–luminosity relationship shown by [[variable star]]s such as [[Cepheid]] stars, which allows them to be used as [[standard candle]]s. These luminous stars can be detected at great distances, and are then used to extend the distance scale to nearby galaxies in the Local Group.<ref name=ssr48_1_2/> Indeed, the open cluster designated NGC 7790 hosts three [[classical Cepheids]].<ref name=sa1958>Sandage, Allan (1958). [http://adsabs.harvard.edu/abs/1958ApJ...128..150S ''Cepheids in Galactic Clusters. I. CF Cass in NGC 7790.''], AJ, 128</ref><ref name=ma2013>Majaess, D.; Carraro, G.; Moni Bidin, C.; Bonatto, C.; Berdnikov, L.; Balam, D.; Moyano, M.; Gallo, L.; Turner, D.; Lane, D.; Gieren, W.; Borissova, J.; Kovtyukh, V.; Beletsky, Y. (2013). [http://adsabs.harvard.edu/abs/2013A%26A...560A..22M ''Anchors for the cosmic distance scale: the Cepheids U Sagittarii, CF Cassiopeiae, and CEab Cassiopeiae''], A&A, 260</ref>  [[RR Lyrae variables]] are too old to be associated with open clusters, and are instead found in [[globular clusters]].

==Planets==
The stars in open clusters can host exoplanets, just like stars outside open clusters. For example, the open cluster [[NGC 6811]] contains two known planetary systems, [[Kepler-66]] and [[Kepler-67]]. Additionally, several [[hot Jupiter]]s are known to exist in the [[Beehive Cluster]].<ref>{{cite journal | title =Two 'b's in the Beehive: The Discovery of the First Hot Jupiters in an Open Cluster | journal =The Astrophysical Journal | volume =756 | issue =2 | pages =L33 | publisher =[[The American Astronomical Society]] | date = 2012-08-22  | doi =10.1088/2041-8205/756/2/L33 | last1 =Quinn | first1 =Samuel N. | last2 =White | first2 =Russel J. | last3 =Latham | first3 =David W. | last4 =Buchhave | first4 =Lars A. | last5 =Cantrell | first5 =Justin R. | last6 =Dahm | first6 =Scott E. | last7 =Fűrész | first7 =Gabor | last8 =Szentgyorgyi | first8 =Andrew H. | last9 =Geary | first9 =John C. | last10 =Torres | first10 =Guillermo | last11 =Bieryla | first11 =Allyson | last12 =Berlind | first12 =Perry | last13 =Calkins | first13 =Michael C. | last14 =Esquerdo | first14 =Gilbert A. | last15 =Stefanik | first15 =Robert P. | arxiv =1207.0818 | bibcode =2012ApJ...756L..33Q | s2cid =118825401 }}</ref>

==See also==
* [[List of open clusters]]
* [[Stellar kinematics|Moving groups]]
* [[Open cluster family]]
* [[Open cluster remnant]]

==References==
{{Reflist|30em|refs=
<ref name=Friel>{{cite journal | author=Friel, Eileen D.|author-link=Eileen Friel | title=The Old Open Clusters Of The Milky Way | journal=[[Annual Review of Astronomy and Astrophysics]] | date=1995 | volume=33 | pages=381–414 | bibcode=1995ARA&A..33..381F | doi=10.1146/annurev.aa.33.090195.002121 }}</ref>

<ref name=Karttunen_et_al_03>{{cite book | author=Karttunen, Hannu |display-authors=etal | title=Fundamental astronomy | page=321 | series=Physics and Astronomy Online Library | edition=4th | publisher=Springer | date=2003 | isbn=3-540-00179-4 }}</ref>

<ref name=payne-gaposchkin79>{{cite book | author=Payne-Gaposchkin, C. | title=Stars and clusters | location=Cambridge, Mass. | publisher=Harvard University Press | date=1979 | isbn=0-674-83440-2 | bibcode=1979stcl.book.....P | url-access=registration | url=https://archive.org/details/starsclusters00gapo }}</ref>

<ref name=apj62>A good example of this is [[NGC 2244]], in the [[Rosette Nebula]]. See also {{cite journal | author=Johnson, Harold L. | title=The Galactic Cluster, NGC 2244 | journal=Astrophysical Journal |date=November 1962 | volume=136 | page=1135 | bibcode=1962ApJ...136.1135J | doi=10.1086/147466 | doi-access= }}</ref>

<ref name=basu03>{{cite book | first=Baidyanath | last=Basu | date=2003 | title=An Introduction to Astrophysics | page=218 | publisher=PHI Learning Pvt. Ltd. | isbn=81-203-1121-3 }}</ref>

<ref name=seds>{{cite web | author=Frommert, Hartmut | author2=Kronberg, Christine | date=August 27, 2007 | title=Open Star Clusters | work=SEDS | publisher=University of Arizona, Lunar and Planetary Lab | url=http://www.seds.org/messier/open.html | access-date=2009-01-02 | url-status=dead | archive-url=https://web.archive.org/web/20081222064251/http://seds.org/messier/open.html | archive-date=December 22, 2008 }}</ref>

<ref name=bok81>{{cite book | author=Bok, Bart J. | author2=Bok, Priscilla F. | date=1981 | title=The Milky Way | url=https://archive.org/details/milkyway0000bokb | url-access=registration | edition=5th | page=[https://archive.org/details/milkyway0000bokb/page/136 136] | series=Harvard books on astronomy | publisher=Harvard University Press | isbn=0-674-57503-2 }}</ref>

<ref name=pt57>{{cite journal | last=Michell | first=J. | date=1767 | title=An Inquiry into the probable Parallax, and Magnitude, of the Fixed Stars, from the Quantity of Light which they afford us, and the particular Circumstances of their Situation | journal=Philosophical Transactions | volume=57 | pages=234–264 | bibcode=1767RSPT...57..234M | doi=10.1098/rstl.1767.0028 | doi-access=free }}</ref>

<ref name=jones91>{{cite book | first=Kenneth Glyn | last=Jones | date=1991 | title=Messier's nebulae and star clusters | pages=6–7 | issue=2 | series=Practical astronomy handbook | edition=2nd | publisher=Cambridge University Press | isbn=0-521-37079-5 }}</ref>

<ref name=neata09>{{cite web | first=Emil | last=Neata | title=Open Star Clusters: Information and Observations | work=Night Sky Info | url=http://www.nightskyinfo.com/open_clusters/ | access-date=2009-01-02 }}</ref>

<ref name=apj480>{{cite journal | author=Elmegreen, Bruce G. | author2=Efremov, Yuri N. | title=A Universal Formation Mechanism for Open and Globular Clusters in Turbulent Gas | journal=The Astrophysical Journal | volume=480 | issue=1 | pages=235–245 | date=1997 | doi=10.1086/303966| bibcode=1997ApJ...480..235E| doi-access=free }}</ref>

<ref name=mnras249>{{cite journal | date=1991 | title=Formation and evolutionary properties of the Galactic open cluster system | journal=[[Monthly Notices of the Royal Astronomical Society]] | volume=249 | pages=76–83 | bibcode= 1991MNRAS.249...76B | author=Battinelli, P. | author2=Capuzzo-Dolcetta, R. | doi=10.1093/mnras/249.1.76| doi-access=free }}</ref>

<ref name=kroupa05>{{cite conference | last=Kroupa | first=P. | publication-date=2005 | title=The Fundamental Building Blocks of Galaxies | book-title=Proceedings of the Gaia Symposium "The Three-Dimensional Universe with Gaia (ESA SP-576) | location=Observatoire de Paris-Meudon | date=October 4–7, 2004 | editor=C. Turon | editor2 = K.S. O'Flaherty | editor3 = M.A.C. Perryman | page=629 | arxiv=astro-ph/0412069 |bibcode = 2005ESASP.576..629K }}</ref>

<ref name=mnras120>{{cite journal | date=1960 | title=Stellar groups, VII. The structure of the Hyades group | journal=Monthly Notices of the Royal Astronomical Society | volume=120 | issue=6 | pages=540–562 | bibcode=1960MNRAS.120..540E | last=Eggen | first=O. J. | doi = 10.1093/mnras/120.6.540 | doi-access=free }}</ref>

<ref name=aaa302>{{cite journal | date=1995 | title=Probable binary open star clusters in the Galaxy | journal=Astronomy and Astrophysics |volume=302 | pages=86–89 | bibcode=1995A&A...302...86S | author=Subramaniam, A. | author2=Gorti, U. | author3=Sagar, R. | author4=Bhatt, H. C. | url=http://repository.ias.ac.in/65959/ }}</ref>

<ref name=aaa383>{{cite journal | author=Nilakshi, S.R. | author2=Pandey, A.K. | author3=Mohan, V. | date=2002 | title=A study of spatial structure of galactic open star clusters | journal=Astronomy and Astrophysics | volume=383 | issue=1 | pages=153–162 | doi=10.1051/0004-6361:20011719 | bibcode=2002A&A...383..153N | doi-access=free }}</ref>

<ref name=lob420>{{cite journal | last=Trumpler | first=R.J. | date=1930 | title=Preliminary results on the distances, dimensions and space distribution of open star clusters | journal=Lick Observatory Bulletin | issue=420 | location=Berkeley | publisher=University of California Press | pages=154–188 | bibcode=1930LicOB..14..154T | volume=14 |doi = 10.5479/ADS/bib/1930LicOB.14.154T | doi-access=free }}</ref>

<ref name=aaa389>{{cite journal | author=Dias, W.S. | author2=Alessi, B.S. | author3=Moitinho, A. | author4=Lépine, J.R.D. | date=2002 | title=New catalogue of optically visible open clusters and candidates | journal=Astronomy and Astrophysics | volume=389 | issue=3 | pages=871–873 | doi=10.1051/0004-6361:20020668 | bibcode=2002A&A...389..871D |arxiv = astro-ph/0203351 | s2cid=18502004 }}</ref>

<ref name=aj108>{{cite journal | author=Janes, K.A. | author2=Phelps, R.L. | date=1980 | title=The galactic system of old star clusters: The development of the galactic disk | journal=The Astronomical Journal | volume=108 | pages=1773–1785 | doi=10.1086/117192 | bibcode=1994AJ....108.1773J | doi-access=free }}</ref>

<ref name=pasp109>{{cite journal | author=Hunter, D. | title=Star Formation in Irregular Galaxies: A Review of Several Key Questions | bibcode=1997PASP..109..937H | journal=Publications of the Astronomical Society of the Pacific | date=1997 | volume=109 | pages=937–950 | doi=10.1086/133965 | doi-access=free }}</ref>

<ref name=aaa88>{{cite journal |author=van den Bergh, S. |author2=McClure, R.D. | date=1980 | title=Galactic distribution of the oldest open clusters | bibcode=1980A&A....88..360V | journal=Astronomy & Astrophysics | volume=88 | page=360 }}</ref>

<ref name=binney_merrifield98>{{cite book |last=Binney | first=J. |author2=Merrifield, M. | title=Galactic Astronomy | date=1998 | publisher=Princeton University Press | location=Princeton | isbn=978-0-691-02565-0 | oclc=39108765}}</ref>

<ref name=baas35>{{cite journal | author=Andronov, N. | author2=Pinsonneault, M. | author3=Terndrup, D. | date=2003 | title=Formation of Blue Stragglers in Open Clusters | journal=Bulletin of the American Astronomical Society | volume=35 | page=1343 | bibcode=2003AAS...203.8504A }}</ref>

<ref name=apj595>{{cite journal | author=Fellhauer, M. | author2=Lin, D.N.C. | author3=Bolte, M. | author4=Aarseth, S.J. | author5=Williams K.A. | date=2003 | journal=The Astrophysical Journal | volume=595 | issue=1 | pages=L53–L56 | doi=10.1086/379005 | bibcode=2003ApJ...595L..53F | title=The White Dwarf Deficit in Open Clusters: Dynamical Processes |arxiv = astro-ph/0308261 | s2cid=15439614 }}</ref>

<ref name=apj235_1>{{cite journal | author=Hills, J. G. | title=The effect of mass loss on the dynamical evolution of a stellar system – Analytic approximations | bibcode=1980ApJ...235..986H | journal=Astrophysical Journal | volume=235 | issue=1 | pages=986–991 | date=February 1, 1980 | doi=10.1086/157703 }}</ref>

<ref name=pasp110>{{cite journal | author=de La Fuente, M.R. | date=1998 | title=Dynamical Evolution of Open Star Clusters | journal=Publications of the Astronomical Society of the Pacific | volume=110 | issue=751 | pages=1117 | doi=10.1086/316220 | bibcode=1998PASP..110.1117D| doi-access=free }}</ref>

<ref name=aj105_1>{{cite journal | author=Soderblom, David R. | author2=Mayor, Michel | title=Stellar kinematic groups. I – The Ursa Major group | bibcode=1993AJ....105..226S | journal=Astronomical Journal | date=1993 | volume=105 | issue=1 | pages=226–249 | doi=10.1086/116422 | issn=0004-6256 | doi-access=free }}</ref>

<ref name=aspcs92>{{cite journal | author=Majewski, S. R. | author2=Hawley, S. L. | author3=Munn, J. A. | title=Moving Groups, Stellar Streams and Phase Space Substructure in the Galactic Halo | journal=ASP Conference Series | date=1996 | volume=92 | page=119 | bibcode=1996ASPC...92..119M }}</ref>

<ref name=baas38>{{cite journal | author=Sick, Jonathan | author2=de Jong, R. S. | title=A New Method for Detecting Stellar Streams in the Halos of Galaxies | bibcode=2006AAS...20921105S|journal=Bulletin of the American Astronomical Society | date=2006 | volume=38 | page=1191 }}</ref>

<ref name=orsa>{{cite web | title=Diagrammi degli ammassi ed evoluzione stellare | publisher=O.R.S.A. – Organizzazione Ricerche e Studi di Astronomia | url=http://www.orsapa.it/evstellare/Ammassi.htm | language=it | access-date=2009-01-06 }}</ref>

<ref name=pasp116>{{cite journal | author=VandenBerg, D.A. | author2=Stetson, P.B. | date=2004 | title=On the Old Open Clusters M67 and NGC 188: Convective Core Overshooting, Color–Temperature Relations, Distances, and Ages | journal=Publications of the Astronomical Society of the Pacific | volume=116 | issue=825 | pages=997–1011 | doi=10.1086/426340 | bibcode=2004PASP..116..997V | doi-access=free }}</ref>

<ref name=rmaa11>{{cite journal | author=Brown, A.G.A. | date=2001 | title=Open clusters and OB associations: a review | journal=Revista Mexicana de Astronomía y Astrofísica | volume=11 | pages=89–96 | bibcode=2001RMxAC..11...89B }}</ref>

<ref name=keel09>{{cite web | author=Keel, Bill | title=The Extragalactic Distance Scale | url=http://www.astr.ua.edu/keel/galaxies/distance.html | publisher=Department of Physics and Astronomy – University of Alabama | access-date=2009-01-09 }}</ref>

<ref name=aaa400>{{cite journal | author=Percival, S. M. | author2=Salaris, M. | author3=Kilkenny, D. | title=The open cluster distance scale – A new empirical approach | journal=Astronomy & Astrophysics | volume=400 | issue=2 | pages=541–552 | date=2003 | doi=10.1051/0004-6361:20030092 | bibcode=2003A&A...400..541P |arxiv = astro-ph/0301219 | s2cid=10544370 }}</ref>

<ref name=aj80>{{cite journal | last=Hanson | first=R.B. | date=1975 | title=A study of the motion, membership, and distance of the Hyades cluster | journal=Astronomical Journal | volume=80 | pages=379–401 | doi=10.1086/111753 | bibcode=1975AJ.....80..379H }}</ref>

<ref name=aaa429>{{cite journal | author=Bragaglia, A. | author2=Held, E.V. | author3=Tosi M. | date=2005 | title=Radial velocities and membership of stars in the old, distant open cluster Berkeley 29 | journal=Astronomy and Astrophysics | volume=429 | issue=3 | pages=881–886 | doi=10.1051/0004-6361:20041049 | bibcode=2005A&A...429..881B |arxiv = astro-ph/0409046 | s2cid=16669438 }}</ref>

<ref name=ssr48_1_2>{{cite journal | author=Rowan-Robinson, Michael | title=The extragalactic distance scale | journal=Space Science Reviews |date=March 1988 | volume=48 | issue=1–2 | pages=1–71 | doi=10.1007/BF00183129 | bibcode=1988SSRv...48....1R | s2cid=121736592 | issn=0038-6308 }}</ref>

<ref name=jha10>{{cite journal | last=Hoskin | first=M. | date=1979 | title=Herschel, William's Early Investigations of Nebulae – a Reassessment | journal=Journal for the History of Astronomy | volume=10 | pages=165–176 | bibcode=1979JHA....10..165H | doi=10.1177/002182867901000302 | s2cid=125219390 | doi-access= }}</ref>

<ref name=jha18_1>{{cite journal | last=Hoskin | first=M. | title=Herschel's Cosmology | journal=Journal for the History of Astronomy | volume=18 | issue=1 |date=February 1987 | pages=1–34, 20 | bibcode=1987JHA....18....1H | doi=10.1177/002182868701800101 | s2cid=125888787 }}</ref>

<ref name=donofrio_burigana09>{{cite book | author=D'Onofrio, Mauro | author2=Burigana, Carlo | title=Questions of Modern Cosmology: Galileo's Legacy | page=1 | editor=Mauro D'Onofrio | editor2=Carlo Burigana | publisher=Springer, 2009 | isbn=978-3-642-00791-0 | chapter=Introduction | date=17 July 2009 }}</ref>

<ref name=jha17_50>{{cite journal | last=Jones | first=K. G. | title=Some Notes on Hodierna's Nebulae | journal=Journal for the History of Astronomy | volume=17 | issue=50 |date=August 1986 | pages=187–188 | bibcode=1986JHA....17..187J | doi=10.1177/002182868601700303 | s2cid=117590918 }}</ref>

<ref name=Kaler06>{{cite book | last=Kaler | first=James B. | title=Cambridge Encyclopedia of Stars | date=2006 | publisher=Cambridge University Press | isbn=0-521-81803-6 | page=167 }}</ref>

<ref name=pasp37_220>{{cite journal | title=Spectral Types in Open Clusters | author=Trumpler, R. J. | journal=Publications of the Astronomical Society of the Pacific | volume=37 | issue=220 | page=307 |date=December 1925 | doi=10.1086/123509 | bibcode=1925PASP...37..307T | s2cid=122892308 | doi-access= }}</ref>

<ref name=rasqj30_4_399>{{citation | title=William Herschel and the Measurement of Space | last1=Chapman | first1=A. | journal=Quarterly Journal of the Royal Astronomical Society | volume=30 | issue=4 | pages=399–418 |date=December 1989 | bibcode=1989QJRAS..30..399C }}</ref>

<ref name=binney_merrifield1998>{{citation | first1=James | last1=Binney | first2=Michael | last2=Merrifield | date=1998 | title=Galactic astronomy | page=377 | series=Princeton series in astrophysics | publisher=Princeton University Press | isbn=0-691-02565-7 | url=https://books.google.com/books?id=arYYRoYjKacC&pg=PA377 }}</ref>

<ref name=pyo6_1>{{citation | title=Micrometric measures of star clusters | last1=Barnard | first1=E. E. | journal=Publications of the Yerkes Observatory | volume=6 | pages=1–106 | bibcode=1931PYerO...6....1B | year=1931 }}</ref>

<ref name=aj102_26>{{citation | title=Investigations on Proper Motion. XXIV. Further Measures in the Pleiades Cluster | last1=van Maanen | first1=Adriaan | journal=Astrophysical Journal | volume=102 | pages=26–31 |date=July 1945 | doi=10.1086/144736 | bibcode=1945ApJ...102...26V }}</ref>

<ref name=cmwo167_1>{{citation | last1=van Maanen | first1=Adriaan | title=No. 167. Investigations on proper motion. Furst paper: The motions of 85 stars in the neighborhood of Atlas and Pleione | journal=Contributions from the Mount Wilson Observatory | publisher=Carnegie Institution of Washington | volume=167 | pages=1–15 | date=1919 | bibcode=1919CMWCI.167....1V }}</ref>

<ref name=strand1977>{{citation | last1=Strand | first1=K. Aa. | contribution=Hertzsprung's Contributions to the HR Diagram | title=The HR Diagram, In Memory of Henry Norris Russell, IAU Symposium No. 80, held November 2, 1977 | volume=80 | issue=80 | place=National Academy of Sciences, Washington, DC | editor1-first=A. G. Davis | editor1-last=Philip | editor2-first=David H. | editor2-last=DeVorkin | pages=55–59 |date=December 1977 | bibcode=1977IAUS...80S..55S }}</ref>

<ref name=moore_rees2011>{{citation | first1=Patrick | last1=Moore | first2=Robin | last2=Rees | title=Patrick Moore's Data Book of Astronomy | edition=2nd | publisher=Cambridge University Press | date=2011 | isbn=978-0-521-89935-2 | page=339 | url=https://books.google.com/books?id=2FNfjWKBZx8C&pg=PA339 }}</ref>

<ref name=maran_marshall2009>{{citation | first1=Stephen P. | last1=Maran | first2=Laurence A. | last2=Marschall | title=Galileo's new universe: the revolution in our understanding of the cosmos | publisher=BenBella Books | date=2009 | isbn=978-1-933771-59-5 | page=128 | url=https://books.google.com/books?id=Cs41G1EZRm0C&pg=PA128 }}</ref>

<ref name=jha16_1_1>{{citation | last1=Fodera-Serio | first1=G. | last2=Indorato | first2=L. | last3=Nastasi | first3=P. | title=Hodierna's Observations of Nebulae and his Cosmology | journal=Journal for the History of Astronomy | volume=16 | issue=1 | page=1 |date=February 1985 | bibcode=1985JHA....16....1F | doi=10.1177/002182868501600101 | s2cid=118328541 | doi-access= }}</ref>

<ref name=mnras321_4_699>{{citation | last1=Kroupa | first1=Pavel | last2=Aarseth | first2=Sverre | last3=Hurley | first3=Jarrod | title=The formation of a bound star cluster: from the Orion nebula cluster to the Pleiades | journal=Monthly Notices of the Royal Astronomical Society | volume=321 | issue=4 | pages=699–712 |date=March 2001 | doi=10.1046/j.1365-8711.2001.04050.x | doi-access=free | bibcode=2001MNRAS.321..699K |arxiv = astro-ph/0009470 | s2cid=11660522 }}</ref>

<ref name=ptrsa368_1913_713>{{citation | last1=Lada | first1=C. J. | title=The physics and modes of star cluster formation: observations | journal=Philosophical Transactions of the Royal Society A | volume=368 | issue=1913 | pages=713–731 |date=January 2010 | doi=10.1098/rsta.2009.0264 | pmid=20083503 | bibcode=2010RSPTA.368..713L |arxiv = 0911.0779 | s2cid=20180097 }}</ref>

<ref name=araa25_23>{{citation | last1=Shu | first1=Frank H. | last2=Adams | first2=Fred C. | last3=Lizano | first3=Susana | title=Star formation in molecular clouds – Observation and theory | journal=Annual Review of Astronomy and Astrophysics | volume=25 | pages=23–81 | date=1987 | doi=10.1146/annurev.aa.25.090187.000323 | bibcode=1987ARA&A..25...23S }}</ref>

<ref name=apj717_1_577>{{citation | last1=Thies | first1=Ingo | last2=Kroupa | first2=Pavel | last3=Goodwin | first3=Simon P. | last4=Stamatellos | first4=Dimitrios | last5=Whitworth | first5=Anthony P. | title=Tidally Induced Brown Dwarf and Planet Formation in Circumstellar Disks | journal=The Astrophysical Journal | volume=717 | issue=1 | pages=577–585 |date=July 2010 | doi=10.1088/0004-637X/717/1/577 | bibcode=2010ApJ...717..577T |arxiv = 1005.3017 | s2cid=3438729 }}</ref>

<ref name=Majaess2024>{{citation
 | title=A Rare Cepheid-hosting Open Cluster Triad in Sagittarius
 | last1=Majaess | first1=D. | last2=Turner | first2=D.
 | last3=Usenko | first3=I. | display-authors=1
 | journal=Research Notes of the AAS
 | volume=8 | issue=8 | pages=205 | date=August 2024
 | doi=10.3847/2515-5172/ad7139 | doi-access=free | postscript=. }}</ref>

}}

==Further reading==
*{{cite book|author=Kaufmann, W. J.|title=Universe | publisher=W H Freeman | date=1994 | isbn=0-7167-2379-4 }}
* {{cite book | author=Smith, E.V.P.|author-link=Elske Smith | author2=Jacobs, K.C. | author3=Zeilik, M. | author4=Gregory, S.A. | title=Introductory Astronomy and Astrophysics | publisher=Thomson Learning | date=1997 | isbn=0-03-006228-4 }}

==External links==
{{Commons category|Open clusters}}
{{Spoken Wikipedia|En-Open cluster-article.ogg|date=2006-07-16}}
* [http://www.sky-map.org/?ra=12.895&de=-60.3667&zoom=10&img_source=IMG_all The Jewel Box (also known as NGC 4755 or Kappa Crucis Cluster) – open cluster in the Crux constellation @ SKY-MAP.ORG]
* [http://messier.seds.org/open.html Open Star Clusters @ SEDS Messier pages]
* [https://web.archive.org/web/20050218133220/http://www.peripatus.gen.nz/astronomy/OpeClu.html A general overview of open clusters]
* [https://web.archive.org/web/20050408171117/http://zebu.uoregon.edu/~soper/Stars/movingcluster.html The moving cluster method]
* [http://www.nightskyinfo.com/open_clusters Open Clusters – Information and amateur observations]

{{Stellar system}}
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{{DEFAULTSORT:Open Cluster}}
[[Category:Open clusters]]
[[Category:Star clusters]]