Stellar association

A stellar association is a very loose star cluster, looser than both open clusters and globular clusters. Stellar associations will normally contain from 10 to 100 or more stars. The stars share a common origin, but have become gravitationally unbound and are still moving together through space. Associations are primarily identified by their common movement vectors and ages. Identification by chemical composition is also used to factor in association memberships.

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.

Stellar associations were first discovered by the Armenian astronomer Viktor Ambartsumian in 1947. He categorized them into two groups, OB and T, based on the properties of their stars.

Types

OB associations

Young associations will contain 10–100 massive stars of spectral class O and 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.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
accessdate = 2006-06-08 ] It is believed that the majority of all stars in the Milky Way were formed in OB associations.

O class stars are short-lived, and will expire as supernovae after roughly a million years. 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 within10 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 parsecs of the Sun. [cite journal
author=de Zeeuw, P. T.; Hoogerwerf, R.; de Bruijne, J. H. J.; Brown, A. G. A.; Blaauw, A.
title=A HIPPARCOS Census of the Nearby OB Associations
journal=The Astronomical Journal | year=1999
volume=117 | issue=1 | pages=354–399
url=http://adsabs.harvard.edu/abs/1999AJ....117..354D
doi=10.1086/300682 ] The nearest OB association is the Scorpius-Centaurus Association, located about 400 light years from the Sun. [cite journal
last = Maíz-Apellániz | first = Jesús
title=The Origin of the Local Bubble
journal=The Astrophysical Journal | year=2001
volume=560 | pages=L83–L86
url=http://www.journals.uchicago.edu/doi/full/10.1086/324016 | doi = 10.1086/324016 ]

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 years in diameter. [cite journal
author = Elmegreen, B.; 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/template/AssetDetail/assetid/15714/page/2
accessdate=2006-08-23 | doi=10.1511/1998.3.264 ]

T associations

Young stellar groups can contain a number of infant 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 parsecs from the Sun. [cite journal
author=Frink, S.; Roeser, S.; Neuhaeuser, R.; 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 ] 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.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
url=http://adsabs.harvard.edu/abs/1975AJ.....80..212H
doi=10.1086/111734 ] These young stellar groupings contain main-sequence stars that are not sufficiently massive to disperse the interstellar clouds in which they formed.cite journal
author=Herbst, W.; Racine, R.
title=R associations. V. MON R2.
journal=Astronomical Journal | year=1976 | volume=81
pages=840
url=http://adsabs.harvard.edu/abs/1976AJ.....81..840H
doi=10.1086/111963] 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. [cite journal
first=W. | last=Herbst
title=R-associations III. Local optical spiral structure
journal=Astronomical Journal | year=1975 | volume=80
pages=503
url=http://adsabs.harvard.edu/abs/1975AJ.....80..503H
doi=10.1086/111771 ] An example of an R-association is Monoceros R2, located 830 ± 50 parsecs from the Sun.

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. Some groups also display properties of both OB and T associations, so the categorization is not always clear-cut.

Known associations

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

Other young moving groups include:
*Local Association (Pleiades moving group)
*Hyades supercluster
*IC 2391 supercluster
*Beta Pictoris moving group
*Castor moving group
*AB Doradus moving group
*Zeta Herculis moving group
*Alpha Persei moving cluster

ee also

* OB star
* Moving groups
* Open clusters

References

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