Editing Star cluster

{{short description|Group of stars}}
[[File:M44 47x300s-10°C O30 G0 PM RGB 03032022.jpg|thumb|upright=1.4|[[Messier 44]], a cluster in the [[constellation]] of [[Cancer (constellation)|Cancer]]]]

A '''star cluster''' is a group of [[star]]s held together by [[self-gravitation]]. Two main types of star clusters can be distinguished: [[globular cluster]]s, tight groups of ten thousand to millions of old stars which are [[gravitation]]ally bound; and [[open cluster]]s, less tight groups of stars, generally containing fewer than a few hundred members.

As they move through the [[galaxy]], over time, open clusters become disrupted by the gravitational influence of [[giant molecular cloud]]s, so that the clusters we observe are often young. Even though they are no longer gravitationally bound, they will continue to move in broadly the same direction through space and are then known as [[stellar association]]s, sometimes referred to as ''moving groups''.  Globular clusters, with more members and more mass, remain intact for far longer and the globular clusters we observe are usually billions of years old.

Star clusters visible to the naked eye include the [[Pleiades]] and [[Hyades (star cluster)|Hyades]] open clusters, and the globular cluster [[47 Tucanae]].

==Open cluster==
[[File:Pleiades large.jpg|thumb|The [[Pleiades]], an [[open cluster]] dominated by hot [[OB star|blue stars]] surrounded by [[reflection nebulosity]]]]
{{Main|Open cluster}}
Open clusters are very different from globular clusters. Unlike the spherically distributed globulars, they are confined to the [[galactic plane]], and are almost always found within [[spiral arm]]s. They are generally young objects, up to a few tens of millions of years old, with a few rare exceptions as old as a few billion years, such as [[Messier 67]] (the closest and most observed old open cluster) for example.<ref name="ArchinalHynes2003">{{cite book | author1 = Brent A. Archinal | author2 = Steven J. Hynes | date = 2003 | title = Star Clusters | publisher = Willmann-Bell | pages = | isbn = 978-0-943396-80-4 | url = https://books.google.com/books?id=2WJ3AAAACAAJ}}</ref> They form in [[H II region]]s such as the [[Orion Nebula]].

Open clusters typically have a few hundred members and are located in an area up to 30 light-years across. Being much less densely populated than globular clusters, they are much less tightly gravitationally bound, and over time, are disrupted by the gravity of [[giant molecular cloud]]s and other clusters. Close encounters between cluster members can also result in the ejection of stars, a process known as "evaporation".

The most prominent open clusters are the [[Pleiades]] and [[Hyades (star cluster)|Hyades]] in [[Taurus (constellation)|Taurus]]. The [[Double Cluster]] of [[NGC 869|h]]+[[NGC 884|Chi Persei]] can also be prominent under dark skies. Open clusters are often dominated by hot young blue stars, because although such stars are short-lived in stellar terms, only lasting a few tens of millions of years, open clusters tend to have dispersed before these stars die.

A subset of open clusters constitute a binary or aggregate cluster.<ref name=PeickaPaunzen2021>{{citation
 | title=Aggregates of clusters in the Gaia data
 | last1=Piecka | first1=M. | last2=Paunzen | first2=E.
 | display-authors=1
 | journal=Astronomy & Astrophysics
 | volume=649 | issue=A54 | pages=12 | date=May 2021
 | doi=10.1051/0004-6361/202040139 | doi-access=free | postscript=. | arxiv=2106.08920 | bibcode=2021A&A...649A..54P }}</ref>  New research indicates [[Messier 25]] may constitute a ternary star cluster together with NGC 6716 and Collinder 394.<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 | bibcode=2024RNAAS...8..205M | postscript=. }}</ref>

Establishing precise distances to open clusters enables the calibration of the period-luminosity relationship shown by [[Cepheids]] [[variable stars]], which are then used as [[standard candle]]s. Cepheids are luminous and can be used to establish both the distances to remote galaxies and the expansion rate of the Universe ([[Hubble constant]]). Indeed, the open cluster NGC 7790 hosts three [[classical Cepheids]] which are critical for such efforts.<ref name=sa1958>{{cite journal |bibcode=1958ApJ...128..150S |doi=10.1086/146532 |title=Cepheids in Galactic Clusters. I. CF Cass in NGC 7790 |year=1958 |last1=Sandage |first1=Allan |journal=The Astrophysical Journal |volume=128 |page=150 |doi-access=free }}</ref><ref name=ma2013>{{cite journal |bibcode=2013A&A...560A..22M |doi=10.1051/0004-6361/201322670 |title=Anchors for the cosmic distance scale: The Cepheids U Sagittarii, CF Cassiopeiae, and CEab Cassiopeiae |year=2013 |last1=Majaess |first1=D. |last2=Carraro |first2=G. |last3=Moni Bidin |first3=C. |last4=Bonatto |first4=C. |last5=Berdnikov |first5=L. |last6=Balam |first6=D. |last7=Moyano |first7=M. |last8=Gallo |first8=L. |last9=Turner |first9=D. |last10=Lane |first10=D. |last11=Gieren |first11=W. |last12=Borissova |first12=J. |last13=Kovtyukh |first13=V. |last14=Beletsky |first14=Y. |journal=Astronomy & Astrophysics |volume=560 |pages=A22 |arxiv=1311.0865 |s2cid=55934597 }}</ref>

===Embedded cluster===
[[File:Chandra_X-ray_View_of_Orion.jpg|thumb|The [[embedded cluster|embedded]] Trapezium cluster seen in [[X-ray astronomy|X-rays]] which penetrate the surrounding clouds]]
{{main|Embedded cluster}}

Embedded clusters are groups of very young stars that are partially or fully encased in [[Cosmic dust|interstellar dust or gas]] which is often impervious to optical observations. Embedded clusters form in [[molecular clouds]], when the clouds begin to collapse and [[star formation|form stars]]. There is often ongoing star formation in these clusters, so embedded clusters may be home to various types of [[young stellar object]]s including [[protostar]]s and [[pre-main-sequence star]]s. An example of an embedded cluster is the [[Trapezium Cluster]] in the [[Orion Nebula]]. In [[Rho Ophiuchi cloud complex|ρ Ophiuchi cloud]] (L1688) core region there is an embedded cluster.<ref>{{cite journal|bibcode=1995ApJ...450..233G|title=An Infrared Spectroscopic Survey of the rho Ophiuchi Young Stellar Cluster: Masses and Ages from the H-R Diagram|journal=Astrophysical Journal |volume=450|pages=233|last1=Greene|first1=Thomas P|last2=Meyer|first2=Michael R|year=1995|doi=10.1086/176134}}</ref>

The embedded cluster phase may last for several million years, after which gas in the cloud is depleted by star formation or dispersed through [[radiation pressure]], [[stellar wind]]s and [[astrophysical jet|outflows]], or [[supernova explosion]]s. In general less than 30% of cloud mass is converted to stars before the cloud is dispersed, but this fraction may be higher in particularly dense parts of the cloud. With the loss of mass in the cloud, the energy of the system is altered, often leading to the disruption of a star cluster. Most young embedded clusters disperse shortly after the end of star formation.<ref name="Lada Lada 2003 pp. 57–115">{{cite journal | last1=Lada | first1=Charles J. | last2=Lada | first2=Elizabeth A. | title=Embedded Clusters in Molecular Clouds | journal=Annual Review of Astronomy and Astrophysics | volume=41 | issue=1 | year=2003 | issn=0066-4146 | doi=10.1146/annurev.astro.41.011802.094844 | pages=57–115| bibcode=2003ARA&A..41...57L | arxiv=astro-ph/0301540 | s2cid=16752089 }}</ref>

The open clusters found in the Galaxy are former embedded clusters that were able to survive early cluster evolution. However, nearly all freely floating stars, including the [[Sun]],<ref name="Gounelle Meynet p=A4">{{cite journal | last1=Gounelle | first1=M. | last2=Meynet | first2=G. | title=Solar system genealogy revealed by extinct short-lived radionuclides in meteorites | journal=Astronomy & Astrophysics | publisher=EDP Sciences | volume=545 | date=2012-08-27 | issn=0004-6361 | doi=10.1051/0004-6361/201219031 | page=A4| bibcode=2012A&A...545A...4G | arxiv=1208.5879 | s2cid=54970631 }}</ref> were originally born into embedded clusters that disintegrated.<ref name="Lada Lada 2003 pp. 57–115" />

==Globular cluster==
[[File:New_Hubble_image_of_star_cluster_Messier_15.jpg|thumb|The [[globular cluster]] [[Messier 15]] photographed by [[Hubble Space Telescope|HST]]]]
{{Main|Globular cluster}}

Globular clusters are roughly spherical groupings of from 10&nbsp;thousand to several million stars packed into regions of from 10 to 30&nbsp;[[light-year]]s across. They commonly consist of very old [[stellar populations|Population&nbsp;II]] stars – just a few hundred million years younger than the universe itself – which are mostly yellow and red, with masses less than two [[solar mass]]es.<ref name=snp>{{cite book |first1=Robert |last1=Dinwiddie |first2=Will |last2=Gater |first3=Giles |last3=Sparrow |first4=Carole |last4=Stott |year=2012 |series=Nature Guide |title=Stars and Planets |publisher=DK |isbn=978-0-7566-9040-3 |pages=14, 134–137}}</ref> Such stars predominate within clusters because hotter and more massive stars have exploded as [[supernova]]e, or evolved through [[planetary nebula]] phases to end as [[white dwarf]]s. Yet a few rare blue stars exist in globulars, thought to be formed by stellar mergers in their dense inner regions; these stars are known as [[blue straggler]]s.

In the Milky Way galaxy, globular clusters are distributed roughly spherically in the [[galactic halo]], around the [[Galactic Center]], orbiting the center in highly elliptical [[orbit]]s. In 1917, the astronomer [[Harlow Shapley]] made the first respectable estimate of the Sun's distance from the Galactic Center, based on the distribution of globular clusters.

Until the mid-1990s, globular clusters were the cause of a great mystery in astronomy, as theories of [[stellar evolution]] gave ages for the oldest members of globular clusters that were greater than the estimated age of the universe. However, greatly improved distance measurements to globular clusters using the [[Hipparcos]] satellite and increasingly accurate measurements of the [[Hubble constant]] resolved the paradox, giving an age for the universe of about 13&nbsp;billion years and an age for the oldest stars of a few hundred million years less.

Our Galaxy has about 150&nbsp;globular clusters,<ref name=snp/> some of which may have been captured cores of small galaxies stripped of stars previously in their outer margins by the tides of the [[Milky Way]], as seems to be the case for the globular cluster [[Messier 79|M79]]. Some galaxies are much richer in globulars than the Milky Way: The giant [[elliptical galaxy]] [[Messier 87|M87]] contains over a thousand.

A few of the brightest globular clusters are visible to the [[naked eye]]; the brightest, [[Omega Centauri]], was observed in antiquity and catalogued as a star, before the telescopic age. The brightest globular cluster in the northern hemisphere is [[Messier 13|M13]] in the constellation of [[Hercules (constellation)|Hercules]].

==Super star cluster==
{{Main|Super star cluster}}
[[Super star cluster]]s are very large regions of recent star formation, and are thought to be the precursors of globular clusters. Examples include [[Westerlund 1]] in the Milky Way.<ref>{{cite news|date=2005-03-22|publisher=ESO|title=Young and Exotic Stellar Zoo: ESO's Telescopes Uncover Super Star Cluster in the Milky Way|url=https://www.eso.org/public/unitedkingdom/news/eso0510/|access-date=2017-11-27|url-status=live|archive-url=https://web.archive.org/web/20171201042434/https://www.eso.org/public/unitedkingdom/news/eso0510/|archive-date=2017-12-01}}</ref>

==Intermediate forms==
[[File:A Ten Billion Year Stellar Dance.jpg|thumb|[[Messier 68]], a loose [[globular cluster]] whose constituent stars span a volume of space more than a hundred light-years across]]

In 2005, astronomers discovered a new type of star cluster in the Andromeda Galaxy, which is, in several ways, very similar to globular clusters although less dense. No such clusters (which also known as ''extended globular clusters'') are known in the Milky Way. The three discovered in [[Andromeda Galaxy]] are [[M31WFS C1]]<ref>{{cite web|url=http://simbad.u-strasbg.fr/simbad/sim-id?Ident=@1592523&M31WFS+C1|title=@1592523|website=u-strasbg.fr|access-date=28 April 2018}}</ref> [[M31WFS C2]], and [[M31WFS C3]].

These new-found star clusters contain hundreds of thousands of stars, a similar number to globular clusters. The clusters also share other characteristics with globular clusters, ''e.g.'' the stellar populations and metallicity. What distinguishes them from the globular clusters is that they are much larger – several hundred light-years across – and hundreds of times less dense. The distances between the stars are thus much greater. The clusters have properties intermediate between globular clusters and [[dwarf spheroidal galaxy|dwarf spheroidal galaxies]].<ref name="extended">{{cite journal|author=A. P. Huxor|author2=N. R. Tanvir|author3=M.J. Irwin|author4= R. Ibata|title=A new population of extended, luminous, star clusters in the halo of M31|journal=[[Monthly Notices of the Royal Astronomical Society]]|date=2005|volume= 360|issue=3|pages=993–1006|arxiv=astro-ph/0412223|doi=10.1111/j.1365-2966.2005.09086.x|doi-access=free |bibcode=2005MNRAS.360.1007H|s2cid=6215035}}</ref>

How these clusters are formed is not yet known, but their formation might well be related to that of globular clusters. Why M31 has such clusters, while the Milky Way has not, is not yet known. It is also unknown if any other galaxy contains this kind of clusters, but it would be very unlikely that M31 is the sole galaxy with extended clusters.<ref name="extended"/>

Another type of cluster are ''faint fuzzies'' which so far have only been found in [[lenticular galaxy|lenticular galaxies]] like [[NGC 1023]] and [[NGC 3384]]. They are characterized by their large size compared to globular clusters and a ringlike distribution around the centres of their host galaxies. As the latter they seem to be old objects.<ref name="faintfuzzies">{{cite journal|author=A. Burkert|author2=J. Brodie |author3=S. Larsen 3|title=Faint Fuzzies and the Formation of Lenticular Galaxies|journal=The Astrophysical Journal|date=2005|volume= 628|issue=1|pages=231–235|arxiv=astro-ph/0504064|doi=10.1086/430698|bibcode=2005ApJ...628..231B|s2cid=11466131 }}</ref>

==Astronomical significance==
[[File:Artist's impression of an exoplanet orbiting a star in the cluster Messier 67.jpg|thumb|Artist's impression of an exoplanet orbiting a star in the cluster [[Messier 67]]<ref>{{cite news|title=First Planet Found Around Solar Twin in Star Cluster|url=https://www.eso.org/public/unitedkingdom/news/eso1402/|access-date=2017-11-27|newspaper=ESO Press Release|url-status=live|archive-url=https://web.archive.org/web/20171201035459/https://www.eso.org/public/unitedkingdom/news/eso1402/|archive-date=2017-12-01}}</ref> ]]

Star clusters are important in many areas of astronomy. The reason behind this is that almost all the stars in old clusters were born at roughly the same time.<ref>{{Citation |chapter=The Food of the Stars |date=2013-10-01 |pages=34 |chapter-url=https://www.degruyter.com/document/doi/10.4159/harvard.9780674423770.c3/html |access-date=2024-06-18 |publisher=Harvard University Press |language=en |doi=10.4159/harvard.9780674423770.c3 |isbn=978-0-674-42377-0 |title=Stars and Clusters }}</ref> Various properties of all the stars in a cluster are a function only of mass, and so stellar evolution theories rely on observations of open and globular clusters. This is primarily true for old globular clusters. In the case of young (age < 1Gyr) and intermediate-age (1 < age < 5 Gyr), factors such as age, mass, chemical compositions may also play vital roles.<ref>{{Citation |title=All the World's a Stage–The Galaxy |date=2013-10-01 |pages=6–7 |url=https://www.degruyter.com/document/doi/10.4159/harvard.9780674423770.c1/html |access-date=2024-06-18 |publisher=Harvard University Press |language=en |doi=10.4159/harvard.9780674423770.c1 |isbn=978-0-674-42377-0|url-access=subscription }}</ref> Based on their ages, star clusters can reveal a lot of information about their host galaxies. For example, star clusters residing in the Magellanic Clouds can provide essential information about the formation of the Magellanic Clouds dwarf galaxies. This, in turn, can help us understand many astrophysical processes happening in our own Milky Way Galaxy. These clusters, especially the young ones can explain the star formation process that might have happened in our Milky Way Galaxy.

Clusters are also a crucial step in determining the [[Distance ladder|distance scale of the universe]]. A few of the nearest clusters are close enough for their distances to be measured using [[parallax]]. A [[Hertzsprung–Russell diagram]] can be plotted for these clusters which has absolute values known on the [[luminosity]] axis. Then, when similar diagram is plotted for a cluster whose distance is not known, the position of the [[main sequence]] can be compared to that of the first cluster and the distance estimated. This process is known as main-sequence fitting. [[Extinction (astronomy)|Reddening]] and [[stellar population]]s must be accounted for when using this method.

Nearly all stars in the Galactic field, including the Sun, were initially born in regions with embedded clusters that disintegrated. This means that properties of stars and planetary systems may have been affected by early clustered environments.<ref>{{Cite journal |last1=Winter |first1=Andrew J. |last2=Kruijssen |first2=J. M. Diederik |last3=Longmore |first3=Steven N. |last4=Chevance |first4=Mélanie |date=2020-10-22 |title=Stellar clustering shapes the architecture of planetary systems |journal=Nature |language=en |volume=586 |issue=7830 |pages=528–532 |doi=10.1038/s41586-020-2800-0 |issn=0028-0836 |pmc=7116760 |pmid=33087913|arxiv=2010.10531 |bibcode=2020Natur.586..528W }}</ref> This appears to be the case for our own [[Solar System]], in which chemical abundances point to the effects of a supernova from a nearby star early in our Solar System's history.

==Star cloud==
[[File:Scutum Star Cloud.jpg|thumb|Scutum Star Cloud with [[open cluster]] [[Messier 11]] at lower left]]
Technically not star clusters, star clouds are large groups of many stars within a [[galaxy]], spread over very many light-years of space. Often they contain star clusters within them. The stars appear closely packed, but are not usually part of any structure.<ref>{{cite book|title=The Observer's Year: 366 Nights in the Universe|date=2005|author=Patrick Moore|publisher=Springer|isbn=1-85233-884-9|page=199}}</ref> Within the [[Milky Way]], star clouds show through gaps between dust clouds of the [[Great Rift (astronomy)|Great Rift]], allowing deeper views along our particular line of sight.<ref>{{Cite web|title=Paddle the Milky Way's Dark River |author=Bob King |date=2016-07-13 |url=https://skyandtelescope.org/observing/a-trip-down-the-great-rift/|website=skyandtelescope.org|access-date=2020-09-29}}</ref> Star clouds have also been identified in other nearby galaxies.<ref>{{Cite web|title=Resolving Andromeda - How to See Stars 2.5 Million Light-Years Away |author=Bob King |date=2016-10-05 |url=https://skyandtelescope.org/astronomy-blogs/explore-night-bob-king/see-stars-andromeda-galaxy/|website=skyandtelescope.org|access-date=2020-09-20}}</ref> Examples of star clouds include the [[Large Sagittarius Star Cloud]], [[Small Sagittarius Star Cloud]], Scutum Star Cloud, Cygnus Star Cloud, Norma Star Cloud, and [[NGC 206]] in the [[Andromeda Galaxy]].

==Nomenclature==
In 1979, the [[International Astronomical Union]]'s 17th general assembly recommended that newly discovered star clusters, open or globular, within the Galaxy have designations following the convention "Chhmm±ddd", always beginning with the prefix ''C'', where ''h'', ''m'', and ''d'' represent the approximate coordinates of the cluster centre in hours and minutes of [[right ascension]], and degrees of [[declination]], respectively, with leading zeros. The designation, once assigned, is not to change, even if subsequent measurements improve on the location of the cluster centre.<ref name="IAU 17GA 1979">{{cite conference
 |url         = https://www.iau.org/static/resolutions/IAU1979_French.pdf
 |title       = XVIIth General Assembly
 |date        = Summer 1979
 |type        = 14–23 August 1979
 |publisher   = [[International Astronomical Union]]
 |page        = 13
 |location    = Montreal, Canada
 |access-date  = 18 December 2014
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20150118133621/http://www.iau.org/static/resolutions/IAU1979_French.pdf
 |archive-date = 18 January 2015
}}</ref> The first of such designations were assigned by [[Gosta Lynga]] in 1982.<ref name="Lynga">{{cite journal
| last       = Lynga
| first      = G.
| date       = October 1982
| title      = IAU numbers for some recently discovered clusters
| bibcode        = 1982BICDS..23...89L
| journal    = Bulletin d'Information du Centre de Données Stellaires
| volume     = 23
| page       = 89
}}<!--| access-date = 18 December 2014
--></ref><ref name="simbad">{{cite web |url=http://cds.u-strasbg.fr/cgi-bin/Dic-Simbad?C |title=Dictionary of Nomenclature of Celestial Objects |date=1 December 2014<!-- last update as of 21 December 2014 --> |website=Simbad |publisher=Centre de données astronomiques de Strasbourg |access-date=21 December 2014 |url-status=live |archive-url=https://web.archive.org/web/20141008193320/http://cds.u-strasbg.fr/cgi-bin/Dic-Simbad?C |archive-date=8 October 2014 }}</ref>

==See also==
{{Commons and category|Star cluster|Star clusters}}
*[[Hypercompact stellar system]]
*[[Robust associations of massive baryonic objects|Robust associations of massive baryonic objects (RAMBOs)]]
*[[Stellar flyby]]

==References==
{{Reflist}}

==External links==
*[https://webda.physics.muni.cz/webda.html WEBDA open cluster database]
*[http://www.sky-map.org/?ra=7.6357&de=38.882&zoom=11&img_source=IMG_all NGC 2419] -Globular Cluster on SKY-MAP.ORG
*[http://messier.seds.org/cluster.html Star Clusters], SEDS Messier pages
*[http://www2.cfa.harvard.edu/rg/star_and_planet_formation/embedded_clusters.html RG Research: Embedded Clusters] {{Webarchive|url=https://web.archive.org/web/20200806163957/https://www.cfa.harvard.edu/rg/star_and_planet_formation/embedded_clusters.html |date=2020-08-06 }}
*[http://www.britannica.com/eb/article-9110473/star-cluster Star cluster - full article] ''Encyclopædia Britannica'',
*[http://www.universetoday.com/am/publish/super_star_cluster.html Super Star Cluster Discovered in Our Own Milky Way]
*[http://www.arxiv.org/abs/astro-ph/0506507 Probing the Birth of Super Star Clusters: Implications for Massive Star Formation], Kelsey E. Johnson, 2005
*[https://arxiv.org/abs/astro-ph/0412223 A new population of extended, luminous star clusters in the halo of M31], A.P. Huxor et al., 2004
*[https://arxiv.org/abs/astro-ph/0306377 HST/NICMOS Observations of the Embedded Cluster in NGC 2024: Constraints on the IMF and Binary Fraction], Wilson M. Liu et al., 2003
*[https://arxiv.org/abs/astro-ph/0002530 The Discovery of an Embedded Cluster of High-Mass Stars Near SGR 1900+14], Frederick J. Vrba et al., 2000

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