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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.

Nearby stellar associations and moving groups. The green cross in the middle shows the position of the sun.
Close up on the Orion Arm, with major stellar associations (yellow), nebulae (red) and dark nebulae (grey) around the Local Bubble.
Main associations of the galactic plane in the night sky

Stellar associations were first discovered by the Soviet Armenian astronomer Victor Ambartsumian in 1947. 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.

Contents

Victor Ambartsumian first categorized stellar associations into two groups, OB and T, based on the properties of their stars. A third category, R, was later suggested by Sidney van den Bergh for associations that illuminate reflection nebulae.

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.

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. 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 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 parsecs of the Sun. The nearest OB association is the Scorpius–Centaurus Association, located about 400 light-years from the Sun.

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.

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. 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. These young stellar groupings contain main sequence stars that are not sufficiently massive to disperse the interstellar clouds in which they formed. 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. An example of an R-association is Monoceros R2, located 830 ± 50 parsecs from the Sun.

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:

  1. Lankford, John, ed. (2011) [1997]. "Ambartsumian, Viktor Amazaspovich (b. 1908)". History of Astronomy: An Encyclopedia. Routledge. p. 10. ISBN 9781136508349.
  2. Israelian, Garik (1997). "Obituary: Victor Amazaspovich Ambartsumian, 1912 [i.e. 1908] -1996". Bulletin of the American Astronomical Society. 29 (4): 1466–1467. Bibcode:1997BAAS...29.1466I.
  3. Saxon, Wolfgang (15 August 1996). "Viktor A. Ambartsumyan, 87, Expert on Formation of Stars". The New York Times. p. 22.
  4. Herbst, W. (1976). "R associations. I - UBV photometry and MK spectroscopy of stars in southern reflection nebulae". Astronomical Journal. 80: 212–226. Bibcode:1975AJ.....80..212H. doi:10.1086/111734.
  5. Herbst, W.; Racine, R. (1976). "R associations. V. MON R2". Astronomical Journal. 81: 840. Bibcode:1976AJ.....81..840H. doi:10.1086/111963.
  6. "OB Associations". The GAIA Study Report: Executive Summary and Science Section. 2000-04-06. Retrieved2006-06-08.
  7. de Zeeuw, P. T.; Hoogerwerf, R.; de Bruijne, J. H. J.; Brown, A. G. A.; Blaauw, A. (1999). "A HIPPARCOS Census of the Nearby OB Associations". The Astronomical Journal. 117 (1): 354–399. arXiv:astro-ph/9809227. Bibcode:1999AJ....117..354D. doi:10.1086/300682. S2CID 16098861.
  8. Maíz-Apellániz, Jesús (2001). "The Origin of the Local Bubble". The Astrophysical Journal. 560 (1): L83–L86. arXiv:astro-ph/0108472. Bibcode:2001ApJ...560L..83M. doi:10.1086/324016. S2CID 119338135.
  9. Elmegreen, B.; Efremov, Y. N. (1999). "The Formation of Star Clusters". American Scientist. 86 (3): 264. Bibcode:1998AmSci..86..264E. doi:10.1511/1998.3.264. Retrieved2006-08-23.
  10. Frink, S.; Roeser, S.; Neuhaeuser, R.; Sterzik, M. K. (1999). "New proper motions of pre-main sequence stars in Taurus-Auriga". Astronomy and Astrophysics. 325: 613–622. arXiv:astro-ph/9704281. Bibcode:1997A&A...325..613F.
  11. Herbst, W. (1975). "R-associations III. Local optical spiral structure". Astronomical Journal. 80: 503. Bibcode:1975AJ.....80..503H. doi:10.1086/111771.

Stellar association
Stellar association Language Watch Edit 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 Nearby stellar associations and moving groups The green cross in the middle shows the position of the sun Close up on the Orion Arm with major stellar associations yellow nebulae red and dark nebulae grey around the Local Bubble Main associations of the galactic plane in the night sky Stellar associations were first discovered by the Soviet Armenian astronomer Victor Ambartsumian in 1947 1 2 3 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 Contents 1 Types 1 1 OB associations 1 2 T associations 1 3 R associations 2 Known associations 3 See also 4 References 5 External linksTypes EditVictor Ambartsumian first categorized stellar associations into two groups OB and T based on the properties of their stars 2 A third category R was later suggested by Sidney van den Bergh for associations that illuminate reflection nebulae 4 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 5 Some groups also display properties of both OB and T associations so the categorization is not always clear cut OB associations Edit 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 6 It is believed that the majority of all stars in the Milky Way were formed in OB associations 6 O class stars are short lived and will expire as supernovae 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 parsecs of the Sun 7 The nearest OB association is the Scorpius Centaurus Association located about 400 light years from the Sun 8 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 9 T associations Edit 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 10 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 Edit 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 4 These young stellar groupings contain main sequence stars that are not sufficiently massive to disperse the interstellar clouds in which they formed 5 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 11 An example of an R association is Monoceros R2 located 830 50 parsecs from the Sun 5 Known associations EditThe Ursa Major Moving Group is one example of a stellar association Except for a Ursae Majoris and h 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 Stream IC 2391 supercluster Beta Pictoris moving group Castor moving group AB Doradus moving group Zeta Herculis moving group Alpha Persei moving clusterSee also EditOB star Moving groups Open clusters List of nearby stellar associations and moving groupsReferences Edit Lankford John ed 2011 1997 Ambartsumian Viktor Amazaspovich b 1908 History of Astronomy An Encyclopedia Routledge p 10 ISBN 9781136508349 a b Israelian Garik 1997 Obituary Victor Amazaspovich Ambartsumian 1912 i e 1908 1996 Bulletin of the American Astronomical Society 29 4 1466 1467 Bibcode 1997BAAS 29 1466I Saxon Wolfgang 15 August 1996 Viktor A Ambartsumyan 87 Expert on Formation of Stars The New York Times p 22 a b Herbst W 1976 R associations I UBV photometry and MK spectroscopy of stars in southern reflection nebulae Astronomical Journal 80 212 226 Bibcode 1975AJ 80 212H doi 10 1086 111734 a b c Herbst W Racine R 1976 R associations V MON R2 Astronomical Journal 81 840 Bibcode 1976AJ 81 840H doi 10 1086 111963 a b OB Associations The GAIA Study Report Executive Summary and Science Section 2000 04 06 Retrieved 2006 06 08 de Zeeuw P T Hoogerwerf R de Bruijne J H J Brown A G A Blaauw A 1999 A HIPPARCOS Census of the Nearby OB Associations The Astronomical Journal 117 1 354 399 arXiv astro ph 9809227 Bibcode 1999AJ 117 354D doi 10 1086 300682 S2CID 16098861 Maiz Apellaniz Jesus 2001 The Origin of the Local Bubble The Astrophysical Journal 560 1 L83 L86 arXiv astro ph 0108472 Bibcode 2001ApJ 560L 83M doi 10 1086 324016 S2CID 119338135 Elmegreen B Efremov Y N 1999 The Formation of Star Clusters American Scientist 86 3 264 Bibcode 1998AmSci 86 264E doi 10 1511 1998 3 264 Retrieved 2006 08 23 Frink S Roeser S Neuhaeuser R Sterzik M K 1999 New proper motions of pre main sequence stars in Taurus Auriga Astronomy and Astrophysics 325 613 622 arXiv astro ph 9704281 Bibcode 1997A amp A 325 613F Herbst W 1975 R associations III Local optical spiral structure Astronomical Journal 80 503 Bibcode 1975AJ 80 503H doi 10 1086 111771 External links EditStellar kinematic groups Superclusters Moving Groups D Montes UCM New associations of young stars D Montes UCM Retrieved from https en 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