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{{dablink|This article is about stars in outer space. For the Hollywood star system, see [[star system (film)]].}}
U SUCK''''
{{dablink|For a system of planets, see [[planetary system]]}}
 
A '''star system''' or '''stellar system''' is a small number of [[star]]s which orbit each other,<ref>"Star system" in ''Modern Dictionary of Astronomy and Space Technology''. A.S. Bhatia, ed. New Delhi: Deep & Deep Publications, 2005. ISBN 81-7629-741-0</ref> bound by [[gravitation|gravitational attraction]]. A large number of stars bound by gravitation is generally called a ''[[star cluster]]'' or ''[[galaxy]]'', although, broadly speaking, they too are star systems.
__NOTOC__
[[Image:Procyonorbitillustration.JPG|thumb|right|248px|The [[binary star|binary]] [[orbit]] of Procyon B around [[Procyon A]].]]
==Binary star systems==
{{main|Binary star}}
 
A stellar system of two stars is known as a ''[[binary star]]'', ''binary star system'' or ''physical [[double star]]''. If there are no [[Tidal force|tidal]] effects, no perturbation from other forces, and no transfer of [[mass]] from one star to the other, such a system is stable, and both stars will trace out an [[ellipse|elliptical]] [[orbit]] around the [[center of mass]] of the system indefinitely. See ''[[Two-body problem]]''.
 
Examples of binary systems are [[Sirius]], [[Procyon]] and [[Cygnus X-1]], the last of which probably consists of a star and a [[black hole]].
 
== Multiple star systems ==
'''Multiple star systems''' or '''physical [[multiple star]]s''' are systems of more than two stars. Multiple star systems are called ''triple'', ''trinary'' or ''ternary'' if they contain three stars; ''quadruple'' or ''quaternary'' if they contain four stars; ''quintuple'' with five stars; ''sextuple'' with six stars; ''septuple'' with seven stars; and so on.
===Dynamics===
Theoretically, modelling a multiple star system is more difficult than modelling a binary star, as the [[dynamical system]] involved, the ''[[n-body problem]]'', may exhibit [[chaos theory|chaotic]] behavior.
Many configurations of small groups of stars are found to be unstable, as eventually one star will approach another closely and be accelerated so much that it will escape from the system.<ref>Multiple Stellar Systems: Types and Stability, Peter J. T. Leonard, in ''Encyclopedia of Astronomy and Astrophysics'', P. Murdin, ed., online edition at the [http://eaa.iop.org/ Institute of Physics], orig. ed. published by Nature Publishing Group, 2001.</ref> This instability can be avoided if the system is what Evans<ref>[http://adsabs.harvard.edu/abs/1968QJRAS...9..388E Stars of Higher Multiplicity], David S. Evans, ''Quarterly Journal of the Royal Astronomical Society'' '''9''' (1968), 388&ndash;400.</ref> has called ''hierarchical''. In a hierarchical system, the stars in the system can be divided into two smaller groups, each of which traverses a larger orbit around the system's [[center of mass]]. Each of these smaller groups must also be hierarchical, which means that they must be divided into smaller subgroups which themselves are hierarchical, and so on. In this case, the stars' motion will continue to approximate stable [[Kepler's laws of planetary motion|Keplerian]] orbits around the system's center of mass<ref>[http://www.ctio.noao.edu/~atokovin/papers/dynamics.pdf Dynamics of multiple stars: observations], A. Tokovinin, in "Massive Stars in Interacting Binaries", August 16&ndash;20, 2004, Quebec (ASP Conf. Ser., in print).</ref>, unlike the more complex [[dynamics (mechanics)|dynamics]] of the large number of [[stars]] in [[star cluster]]s and [[galaxy|galaxies]].
 
[[Image:Triple star system.jpg|right|thumb|250px|Artist's impression of the orbits of [[HD 188753]], a triple star system]]
===Observation===
Most multiple star systems known are triple; for higher multiplicities, the number of known systems with a given multiplicity decreases exponentially with multiplicity.<ref name="tok1">[http://www.aip.de/IAU200/proc/tokovinin.ps.gz Statistics of multiple stars: some clues to formation mechanisms], A. Tokovinin, in the proceedings of IAU Symposium 200, The Formation of Binary Stars, Potsdam, Germany, April 10&ndash;15, 2000. Bibcode [http://adsabs.harvard.edu/abs/2001IAUS..200...84T 2001IAUS..200...84T].</ref> For example, in the 1999 revision of
Tokovinin's catalog<ref>[http://adsabs.harvard.edu/abs/1997A&AS..124...75T MSC&mdash;a catalogue of physical multiple stars], A. A. Tokovinin, ''Astronomy and Astrophysics Supplement Series'' '''124''' (1997), 75&ndash;84; online versions at [http://cdsweb.u-strasbg.fr/viz-bin/VizieR?-source=J/A%2bAS/124/75 VizieR] and the [http://www.ctio.noao.edu/~atokovin/stars/ Multiple Star Catalog].</ref> of physical multiple stars,
551 out of the 728 systems described are triple. However, because of [[selection effect]]s, our knowledge of these statistics is very incomplete.<ref>[http://adsabs.harvard.edu/abs/2004RMxAC..21....7T Statistics of multiple stars], A. Tokovinin, in The Environment and Evolution of Double and Multiple Stars, Proceedings of IAU Colloquium 191, held 3-7 February, 2002 in Merida, Yucatan, Mexico, edited by Christine Allen and Colin Scarfe, ''Revista Mexicana de Astronomía y Astrofísica (Serie de Conferencias)'' '''21''' (August 2004), pp. 7&ndash;14.</ref><sup>,&nbsp;&sect;2.</sup>
 
Because of the dynamical instabilities mentioned earlier, triple systems are generally hierarchical: they contain a close [[binary star|binary]] pair which has a more distant companion. Systems with higher multiplicities are also generally hierarchical.<ref name="tok1" /> Systems with up to six stars are known; for example, [[Castor (star)|Castor]] (Alpha Geminorum), which consists of a binary pair in a distant orbit of two closer binary pairs.<ref name="castor">[http://adsabs.harvard.edu/abs/2003A%26A...402..719S Castor A and Castor B resolved in a simultaneous Chandra and XMM-Newton observation], B. Stelzer and V. Burwitz, ''Astronomy and Astrophysics'' '''402''' (May 2003), pp. 719&ndash;728.</ref> Another system known with six stars is [[ADS 9731]], which consists of a pair of two triple systems, each of which is a [[spectroscopic binary]] in [[orbit]] together with a single star.<ref name="ads9731">[http://adsabs.harvard.edu/abs/1998AstL...24..795T ADS 9731: A new sextuple system], A. A. Tokovinin, N. I. Shatskii, and A. K. Magnitskii, ''Astronomy Letters'', '''24''', #6 (November 1998), pp. 795&ndash;801.</ref>
 
==Examples==
===Binary===
* [[Sirius]], a [[binary star|binary]] consisting of a [[main-sequence]] type A star and a [[white dwarf]].
* [[Epsilon Aurigae]], an eclipsing [[binary star|binary]].
 
===Triple===
* [[Polaris]], the north star, is a triple star system in which the closer companion star is extremely close to the main star&mdash;so close that it was only known from its gravitational tug on Polaris A until it was photographed by the [[Hubble Space Telescope]] in 2006.
* [[Alpha Centauri]] is a triple star composed of a main binary [[yellow dwarf]] pair ([[Alpha Centauri A]] and [[Alpha Centauri B]]), and an outlying [[red dwarf]], [[Proxima Centauri]]. A and B are a physical [[binary star]], with an eccentric [[orbit]] in which A and B can be as close as 11 [[astronomical unit|AU]] or as far away as 36 AU. Proxima is much further away (~15,000 AU) from A and B than they are to each other. Although this distance is still small compared to other interstellar distances, it is debatable whether Proxima is gravitationally bound to A and B.<ref>[http://adsabs.harvard.edu/abs/2006AJ....132.1995W Are Proxima and α Centauri Gravitationally Bound?], Jeremy G. Wertheimer, Gregory Laughlin, ''Astronomical Journal'' '''132''', #5 (November 2006), pp. 1995&ndash;1997.</ref>
* [[HD 188753]] is a triple star system located approximately 149 [[light year]]s away from [[Earth]] in the [[constellation]] [[Cygnus]]. The system is composed of HD 188753A, a [[yellow dwarf]]; HD 188753B, an [[orange dwarf]]; and HD 188753C, a [[red dwarf]]. B and C orbit each other every 156 days, and, as a group, orbit A every 25.7 years.
 
===Quadruple===
* [[4 Centauri]]<ref>[http://www.ctio.noao.edu/~atokovin/stars/index.php?cat=HD&number=120955 4 Centauri], entry in the [http://www.ctio.noao.edu/~atokovin/stars/ Multiple Star Catalog].</ref>
* [[Mizar]] is often said to have been the first [[binary star]] discovered when it was observed in [[1650]] by [[Giovanni Battista Riccioli]]<ref>''The Binary Stars'', R. G. Aitken, New York: Semi-Centennial Publications of the University of California, 1918.</ref><sup>,&nbsp;p.&nbsp;1;&nbsp;</sup><ref>[http://leo.astronomy.cz/mizar/riccioli.htm Vol. 1, part 1, p. 422, ''Almagestum Novum''], Giovanni Battista Riccioli, Bononiae: Ex typographia haeredis Victorij Benatij, 1651.</ref>, but it was probably observed earlier, by [[Benedetto Castelli]] and [[Galileo]]. Later, [[spectroscopy]] of its components Mizar A and B revealed that they were both binary stars themselves.<ref name="mizar">[http://leo.astronomy.cz/mizar/article.htm A New View of Mizar], Leos Ondra, accessed on line [[May 26]], [[2007]].</ref>
 
===Quintuple===
* [[Nu Scorpii]]<ref>[http://www.ctio.noao.edu/~atokovin/stars/index.php?cat=HD&number=144501 Nu Scorpii], entry in the [http://www.ctio.noao.edu/~atokovin/stars/ Multiple Star Catalog].</ref>
===Sextuple===
* [[Castor (star)|Castor]]<ref name="castor" />
* [[ADS 9731]]<ref name="ads9731" />
 
== See also ==
 
* [[Solar System]]
* [[Binary star]]
* [[Double star]]
* [[Multiple star]]
* [[Planetary system]]
* [[Extrasolar planet]]
 
==References==
{{reflist}}
 
==External links==
*[http://apod.nasa.gov/apod/ap020911.html Triple star system, APOD]
*[http://apod.nasa.gov/apod/ap030323.html Alpha Centauri system, APOD]
*[http://apod.oa.uj.edu.pl/apod/ap020425.html Alpa Centauri, APOD, 2002 April 25]
 
[[Category:Star systems| ]]
 
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