Editing Stellar association (section)

==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" />