Editing Stellar association

{{Short description|Loose star cluster}}
[[File:Sky_Map_7_-_Paper_29.png|thumb|350px|Main associations of the [[galactic plane]] in the [[night sky]]]]
A '''stellar association''' is a very loose [[star cluster]], looser than both [[open cluster]]s and [[globular cluster]]s. Stellar associations will normally contain from 10 to 100 or more visible stars. An association is primarily identified by commonalities in its member stars' movement vectors, ages, and chemical compositions. These shared features indicate that the members share a common origin. Nevertheless, they have become gravitationally unbound, unlike star clusters, and the member stars will drift apart over millions of years, becoming a [[moving group]] as they scatter throughout their neighborhood within the galaxy.<ref name="nao-cas"/>

Stellar associations were discovered by [[Victor Ambartsumian]] in 1947.<ref>{{cite book |editor1-last=Lankford |editor1-first=John |title=History of Astronomy: An Encyclopedia |date=2011|orig-year=1997 |publisher=[[Routledge]] |isbn=9781136508349 |page=[https://books.google.com/books?id=9jyExgmZxBoC&dq=ambartsumian&pg=PA10 10] |chapter=Ambartsumian, Viktor Amazaspovich (b. 1908)}}</ref><ref name=baas29 /><ref>{{cite news |last1=Saxon |first1=Wolfgang |title=Viktor A. Ambartsumyan, 87, Expert on Formation of Stars |url=https://www.nytimes.com/1996/08/15/world/viktor-a-ambartsumyan-87-expert-on-formation-of-stars.html |work=[[The New York Times]] |date=15 August 1996 |page=22}}</ref> The conventional name for an association uses the names or abbreviations of the [[constellation]] (or constellations) in which they are located; the association type, and, sometimes, a numerical identifier.

==Types==
Victor Ambartsumian first categorized stellar associations into two groups, OB and T, based on the properties of their stars.<ref name=baas29>{{cite journal
 | last=Israelian | first=Garik | title=Obituary: Victor Amazaspovich Ambartsumian, 1912 [i.e. 1908] -1996
 | journal=Bulletin of the American Astronomical Society
 | year=1997 | volume=29 | issue=4 | pages=1466–1467
 | bibcode=1997BAAS...29.1466I
 }}</ref> A third category, R, was later suggested by [[Sidney van den Bergh]] for associations that illuminate [[reflection nebula]]e.<ref name="ra1">{{cite journal
 | first=W. | last=Herbst
 | title=R associations. I - UBV photometry and MK spectroscopy of stars in southern reflection nebulae
 | journal=Astronomical Journal | year=1976 | volume=80
 | pages=212–226
 | doi=10.1086/111734 | bibcode=1975AJ.....80..212H| doi-access=free}}</ref>

The OB, T, and R associations form a continuum of young stellar groupings. But it is currently uncertain whether they are an evolutionary sequence, or represent some other factor at work.<ref name="Mon R2" /> Some groups also display properties of both OB and T associations, so the categorization is not always clear-cut.

===OB associations===
Young associations will contain 10–100 massive stars of [[Stellar classification|spectral class]] [[Stellar classification#Class O|O]] and [[Stellar classification#Class B|B]], and are known as ''OB associations''. These are believed to form within the same small volume inside a [[giant molecular cloud]]. Once the surrounding dust and gas is blown away, the remaining stars become unbound and begin to drift apart.<ref name="GAIA">{{cite web
 | date =2000-04-06
 | url = http://www.rssd.esa.int/SA-general/Projects/GAIA_files/LATEX2HTML/node27.html
 | title = OB Associations
 | publisher = The GAIA Study Report: Executive Summary and Science Section
 | access-date = 2006-06-08 }}</ref> It is believed that the majority of all stars in the Milky Way were formed in OB associations.<ref name="GAIA" />

[[Stellar classification#Class O|O class]] stars are short-lived, and will expire as [[supernova]]e after roughly one to fifteen million years, depending on the mass of the star. As a result, OB associations are generally only a few million years in age or less. The O-B stars in the association will have burned all their fuel within
10 million years. (Compare this to the current age of the [[Sun]] at about 5 billion years.)

The [[Hipparcos]] satellite provided measurements that located a dozen OB associations within 650 [[parsec]]s of the Sun.<ref>{{cite journal
 |author1=de Zeeuw, P. T. |author2=Hoogerwerf, R. |author3=de Bruijne, J. H. J. |author4=Brown, A. G. A. |author5=Blaauw, A.
 | title=A HIPPARCOS Census of the Nearby OB Associations
 | journal=The Astronomical Journal | year=1999
 | volume=117 | issue=1 | pages=354–399
 | doi=10.1086/300682 | arxiv=astro-ph/9809227| bibcode=1999AJ....117..354D|s2cid=16098861 }}</ref> The nearest OB association is the [[Scorpius–Centaurus association]], located about 400 [[light-year]]s from the [[Sun]].<ref>{{cite journal
 | last = Maíz-Apellániz  | first = Jesús
 | title=The Origin of the Local Bubble
 | journal=The Astrophysical Journal | year=2001
 | volume=560 | issue = 1
 | pages=L83–L86
 | doi = 10.1086/324016 | arxiv=astro-ph/0108472| bibcode=2001ApJ...560L..83M| s2cid = 119338135
 }}</ref>

OB associations have also been found in the [[Large Magellanic Cloud]] and the [[Andromeda Galaxy]]. These associations can be quite sparse, spanning 1,500 [[light-year]]s in diameter.<ref>{{cite journal
 |author1=Elmegreen, B. |author2=Efremov, Y. N.
 | title=The Formation of Star Clusters
 | journal=American Scientist | year=1999 | volume=86
 | issue=3 | pages=264
 | url=http://www.americanscientist.org/issues/feature/the-formation-of-star-clusters/2
 | access-date=2006-08-23 | doi=10.1511/1998.3.264 | bibcode=1998AmSci..86..264E|s2cid=262334560
 | url-access=subscription}}</ref>

===T associations===
Young stellar groups can contain a number of infant [[T Tauri star|T Tauri]] stars that are still in the process of entering the [[main sequence]]. These sparse populations of up to a thousand T Tauri stars are known as ''T associations''. The nearest example is the [[Taurus-Auriga T association]] (Tau-Aur T association), located at a distance of 140 [[parsec]]s from the Sun.<ref>{{cite journal
 |author1=Frink, S. |author2=Roeser, S. |author3=Neuhaeuser, R. |author4=Sterzik, M. K.
 | title=New proper motions of pre-main sequence stars in Taurus-Auriga
 | journal=Astronomy and Astrophysics | year=1999
 | volume=325 | pages=613–622
 | url=http://www.ari.uni-heidelberg.de/publikationen/pap1997/1997021/1997021.htm |bibcode=1997A&A...325..613F |arxiv=astro-ph/9704281 }}</ref> Other examples of T associations include the [[R Corona Australis T association]], the [[Lupus T association]],  the [[Chamaeleon T association]] and the [[Velorum T association]]. T associations are often found in the vicinity of the molecular cloud from which they formed. Some, but not all, include O-B class stars. To summarize the characteristics of Moving groups members: they have the same age and origin, the same chemical composition and they have the same amplitude and direction in their vector of velocity.

===R associations===
Associations of stars that illuminate reflection nebulae are called ''R associations'', a name suggested by Sidney van den Bergh after he discovered that the stars in these nebulae had a non-uniform distribution.<ref name="ra1" /> These young stellar groupings contain main sequence stars that are not sufficiently massive to disperse the interstellar clouds in which they formed.<ref name="Mon R2">{{cite journal
 |author1=Herbst, W. |author2=Racine, R.
 | title=R associations. V. MON R2.
 | journal=Astronomical Journal | year=1976 | volume=81
 | pages=840
 | doi=10.1086/111963| bibcode=1976AJ.....81..840H| doi-access=free}}</ref> This allows the properties of the surrounding dark cloud to be examined by astronomers. Because R-associations are more plentiful than OB associations, they can be used to trace out the structure of the galactic spiral arms.<ref>{{cite journal
 | first=W. | last=Herbst
 | title=R-associations III. Local optical spiral structure
 | journal=Astronomical Journal | year=1975 | volume=80
 | pages=503
 | doi=10.1086/111771 | bibcode=1975AJ.....80..503H}}</ref> An example of an R-association is [[Monoceros R2]], located 830 ± 50 [[parsec]]s from the Sun.<ref name="Mon R2" />

== Known associations ==
The [[Ursa Major Moving Group]] is one example of a stellar association. (Except for [[Alpha Ursae Majoris|α Ursae Majoris]] and [[Eta Ursae Majoris|η Ursae Majoris]], all the stars in the [[Big Dipper|Plough/Big Dipper]] are part of that group.)

Other young moving groups include:
* Local Association ([[Pleiades (star cluster)|Pleiades]] moving group)
* [[Hyades Stream]]
* [[IC 2391]] supercluster
* [[Beta Pictoris moving group]]
* [[Castor Moving Group|Castor moving group]]
* [[AB Doradus moving group]]
* [[Zeta Herculis]] moving group
* [[Alpha Persei moving cluster]]
* [[Cam OB1 association|Cameleopardis OB1 association]]<ref>{{cite journal
 | title=The Stars in Camelopardalis OB1: Their Distance and Evolutionary History
 | last=Lyder | first=David A.
 | journal=The Astronomical Journal
 | volume=122 | issue=5 | pages=2634–2643 | date=November 2001
 | doi=10.1086/323705 | bibcode=2001AJ....122.2634L | s2cid=120758592 | doi-access=free }}</ref>

==See also==
* [[OB star]]
* [[Moving groups]]
* [[Open clusters]]
* [[List of nearby stellar associations and moving groups]]
* [[Stellar_kinematics]]

==References==
{{reflist|refs=

<ref name="nao-cas">{{cite web
 |title = Discovery: New Moving Group in the Local Arm of the Milky Way
 |url = https://english.nao.cas.cn/ne2015/rn2015/202005/t20200513_237000.html
 |work = National Astronomical Observatories of China
 |publisher = Chinese Academy of Sciences
 |date = 13 May 2020
 |accessdate = 13 July 2024}}</ref>

}}

== External links ==
* [http://www.ucm.es/info/Astrof/invest/actividad/skg/skg.html Stellar kinematic groups, Superclusters, Moving Groups] - D. Montes, UCM
* [http://www.ucm.es/info/Astrof/invest/actividad/new_associations_ys.html New associations of young stars] - D. Montes, UCM

{{Stellar system}}
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[[Category:Star clusters]]
[[Category:Stellar associations| ]]