Alpha_Centauri

:''See [[Alpha Centauri (disambiguation)]] for other uses.''
{{Starbox begin
| name = [http://simbad.u-strasbg.fr/sim-id.pl?protocol=html&Ident=alpha+centauri Alpha Centauri A/B/C]
}}
{{Starbox image
| image = [[Image:Position_Alpha_Cen.png|275px]]
| caption = The position of Alpha Centauri.
}}
{{Starbox observe
| epoch = J2000
| ra = {{RA|14|39|36.5/35.1}}
| ra_info = [http://nstars.nau.edu/index.cfm?fuseaction=singleSearch.astrometry&SystemID=1439m6050]
| dec = {{DEC|-60|50|02.3/13.8}}
| appmag_v = -0.01/+1.34/+11.05
| constell = [[Centaurus]]
}}
{{Starbox character
| class = G2 V/K1 V/M5.5 Ve
| b-v = 0.65/0.85/1.97
| u-b = 0.24/0.64/1.54
| variable = None
}}
{{Starbox astrometry
| radial_v = -21.6
| prop_mo_ra = -3678.19
| prop_mo_dec = 481.84
| parallax = 747.23
| p_error = 1.17
| absmag_v = 4.38/5.71/15.49
}}
{{Starbox detail
| age = 5-6 × 10<sup>9</sup>
| metal = 130-230% Sun
| mass = 1.100/0.907/0.1
| radius = 1.227/0.865/0.2
| gravity=4.30/4.37<ref name=aaa449">{{cite journal
| author=Gilli, G.; Israelian, G.; Ecuvillon, A.; Santos, N. C.; Mayor, M.
| title=Abundances of Refractory Elements in the Atmospheres of Stars with Extrasolar Planets
| journal=Astronomy and Astrophysics | year=2006 | volume=449
| issue=2 | pages=723-736
| url=http://adsabs.harvard.edu/abs/2005astro.ph.12219G
| accessdate=2007-06-01 }}</ref>
| rotation = ?
| luminosity = 1.519/0.500/0.00006
| temperature = 5,800/5,300/2700
}}
{{Starbox visbin
| name = Alpha Centauri B
| period = 79.24
| axis = 17.59
| eccentricity = 0.516
| inclination = 79.24
| node = 204.87
| periastron = 1955.56
}}
{{Starbox reference
| Simbad=alpha+centauri
| ARICNS=01151 }}
{{Starbox catalog
| names = Rigil Kentaurus, Rigil Kent, Toliman, Bungula, [[Fifth Fundamental Catalogue|FK5]] 538, [[Cape Photographic Durchmusterung|CP(D)]]−60°5483, GC 19728, CCDM J14396-6050

'''α Cen A'''

[[Gliese Catalogue of Nearby Stars|Gl]] 559 A, [[Harvard Revised catalogue|HR]] 5459, [[Henry Draper catalogue|HD]] 128620, [[General Catalogue of Trigonometric Parallaxes|GCTP]] 3309.00, [[Luyten Half-Second catalogue|LHS]] 50, [[Smithsonian Astrophysical Observatory Star Catalog|SAO]] 252838, [[Hipparcos catalogue|HIP]] 71683

'''α Cen B'''

[[Gliese Catalogue of Nearby Stars|Gl]] 559 B, [[Harvard Revised catalogue|HR]] 5460, [[Henry Draper catalogue|HD]] 128621, [[Luyten Half-Second catalogue|LHS]] 51, [[Hipparcos catalogue|HIP]] 71681

'''Proxima Cen'''

[[Luyten Half-Second catalogue|LHS]] 49, [[Hipparcos catalogue|HIP]] 70890
}}
{{Starbox end}}
'''Alpha Centauri''' (α Cen / α Centauri, also known as '''Rigil Kentaurus'''), is the brightest [[star system]] in the southern [[constellation]] of [[Centaurus]]. Although it appears as a single point to the naked eye, Alpha Centauri is actually a system of three [[star|stars]], one of which is the [[list of brightest stars|fourth brightest star]] in the night sky. Alpha Centauri is famous in the Southern Hemisphere as the outermost "pointer" to the [[Crux|Southern Cross]], but it is too far south to be visible in most of the northern hemisphere. The two brightest components of the system are too close to be resolved as separate stars by the naked eye and so are perceived as a single source of light with a total visual magnitude of about −0.27 (brighter than the third brightest star in the night sky, [[Arcturus]]).

Alpha Centauri is the closest star system to our [[Solar System]] at 4.37 [[light-year]]s distant (about 41.5 trillion km, 25.8 trillion miles or 277,600 AU). [[Proxima Centauri]], usually regarded as part of the system, is the closest star at 4.22 light-years distant.<ref name="SolStation">{{cite web
| title = Alpha Centauri 3
| work = SolStation
| url = http://www.solstation.com/stars/alp-cent3.htm
| accessmonthday = 30 November
| accessyear = 2005}}</ref> Alpha Centauri's relative proximity makes it a logical choice as "first port of call" in [[speculative fiction]] about [[interstellar travel]], which predicts eventual human [[space exploration|exploration]], and even the [[Discovery (observation)|discovery]] and [[space colonization|colonization]] of [[planetary system]]s. These themes are common to several works of [[science fiction]] and [[video game]]s.

==System==
Alpha Centauri is a triple star system consisting of two main stars, ''Alpha Centauri A'' and ''Alpha Centauri B'' (which form a [[binary star|binary system]] together), at a distance of 4.36 [[light-year|ly]] from [[Earth]]'s solar system. The third member of the system is a much smaller and dimmer [[red dwarf]] named [[Proxima Centauri]] which is 4.22 ly from Earth. (Distances are from the [[Hipparcos]] catalog.)
[[Image:Alpha Centauri relative sizes.png|thumb|left|300px|Size and color of the Sun compared to the stars in the Alpha Centauri system]]
<br clear="left">
===Alpha Centauri A and B===
Alpha Centauri A is the largest member of the system and is slightly larger and brighter than the [[Sun]]. Like the Sun, its [[Stellar classification|spectral type]] is G2 V, and it shines with a yellowish-white light. Alpha Centauri B is the second-largest member and is slightly smaller and dimmer than the Sun. Its spectral type is K1 V and it shines with an orangish-yellow-white light. The two orbit one another elliptically ([[Eccentricity (orbit)|e]]=0.52), approaching as close as 11.2 [[astronomical unit]]s (1.669 billion kilometers or 1.04 billion miles: roughly the distance from the Sun to [[Saturn]]) and receding to 35.6 AU (5.9 billion km: approximately the distance from the Sun to [[Pluto (Planet)|Pluto]]), with a period of just under 80 years. [http://www.solstation.com/orbits/ac-absys.htm] Hence the sum of the two masses is just over double that of the Sun (<math>[(11.2+35.6)/2]^3/80^2=2.0</math>, see [[Standard gravitational parameter#Two bodies orbiting each other|formula]]). These two stars are about 5 to 6 billion years old.
[[Image:AlphaCentauri_AB_Trajectory.gif|thumb|left|300px|Apparent and real trajectory of B component relative to A component]]
<br clear="left">

===Proxima Centauri===
{{main|Proxima Centauri}}
The red dwarf Proxima Centauri is about 13,000 astronomical units away from Alpha Centauri A and B (1.94 trillon kilometers or 1.2 trillon miles or 0.21 [[Light year|ly]] – this is about 1/20 of the distance between the Alpha Centauri System and the Sun). It may be in orbit around it, with a period on the order of half a million years or more, or it may be in a [[Hyperbolic trajectory|hyperbolic]] orbit and may leave the system after a few million years. For this reason, Proxima is sometimes referred to as ''Alpha Centauri C''.<ref name="SolStation" /> However the association with Alpha Centauri A and B is unlikely to be entirely accidental as it shares approximately the same motion through space as the larger star system.

Seen from Earth, Proxima Centauri is separated by 2 degrees from Alpha Centauri A and B (about 4 times the [[angular diameter]] of the full Moon), and the latter are at an angular distance of up to 40" from each other.
The closest stars to the Alpha Centauri system are the Sun and [[Barnard's star]] (1.98 [[parsec|pc]] or 6.47 ly), which is also the next nearest star from Earth, at a distance of 5.96 ly.

===Possibility of planet formation===
Computer models of planetary formation suggest that [[terrestrial planet]]s would be able to form close to both Alpha Centauri A and B (see Lissauer and Quintana in references below), but that [[gas giant]] planets similar to our [[Jupiter]] and [[Saturn (planet)|Saturn]] would not be able to form because of the binary stars' gravitational effects.<ref>{{cite journal | author= M. Barbier, F. Marzari, H. Scholl | title=Formation of terrestrial planets in close binary systems: The case of α Centauri A | journal= Astronomy & Astrophysics | year=2002 | volume=396|pages=219 – 224 |doi=10.1051/0004-6361:20021357 }}</ref> Given the similarities in star type, age and stability of the orbits it has been suggested that this stellar system may hold one of the best possibilities for [[extraterrestrial life]].<ref>{{cite journal | author= P.A. Wiegert and M.J. Holman | title=[http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1997AJ....113.1445W The stability of planets in the Alpha Centauri system] | journal= The Astronomical Journal | year=1997 | volume=113 | pages=1445 – 1450 |}}</ref>
However, some astronomers have speculated that any terrestrial planets in the Alpha Centauri system may be dry because it is believed that Jupiter and Saturn were crucial at directing [[comet]]s into the inner solar system and providing the inner planets with a source of [[water]]. This would not be a problem, however, if Alpha Centauri B happened to play a similar role for Alpha Centauri A that the gas giants do for the [[Sun]], and vice versa. Both stars are of the right [[spectral type]] to harbor life on a potential planet.

A planet around Alpha Centauri A would be about 1.25 [[astronomical unit|AU]] away from the star if it were to have Earthlike temperatures, or about halfway between the distances of [[Earth]]'s orbit and [[Mars]]' orbit in our own [[solar system]]. For dimmer, cooler Alpha Centauri B, the distance would be about 0.7 [[AU]], or about the distance of [[Venus]] from the [[Sun]].

Proxima Centauri, along with Alpha Centauri A and B, are among the "Tier 1" target stars for [[NASA]]'s [[Space Interferometry Mission]] (SIM). SIM is designed to be able to detect planets as small as three Earth-masses or smaller within two [[Astronomical Units]] of a "Tier 1" target.<ref name=numbers>"[http://www.jpl.nasa.gov/news/features.cfm?feature=1209 Planet Hunting by Numbers]," ([[Press Release]]), NASA, Stars and Galaxies, ''Jet Propulsion Laboratory'', [[18 October]] [[2006]]. Retrieved [[24 April]] [[2007]].</ref>

==Sky appearance from the Alpha Centauri system==
Viewed from near the Alpha Centauri system, the sky (other than the Alpha Centauri stars) would appear very much as it does to observers on Earth, with most of the constellations such as [[Ursa Major]] and [[Orion (constellation)|Orion]] being almost unchanged. However, Centaurus would be missing its brightest star and our [[Sun]] would appear as a 0.5-magnitude star in [[Cassiopeia (constellation)|Cassiopeia]]. Roughly speaking, the \/\/ of Cassiopeia would become a /\/\/, with the Sun at the end closest to [[Epsilon Cassiopeiae|ε Cassiopeiae]]. The position can easily be plotted as [[right ascension|RA]] 02h39m35s, [[declination|dec]] +60°50', or [[antipodal point|antipodal]] to Alpha Centauri's position as seen from Earth.
[[Image:Sol_View_from_AlpCenA.png|thumb|300px|Looking toward [[Sol]] from Alpha Centauri in [[Celestia]]]]
Nearby very bright stars such as [[Sirius]] and [[Procyon]] would appear to be in very different positions, as would [[Altair]] to a lesser extent. Sirius would become part of the constellation of Orion, appearing 2 degrees to the west of [[Betelgeuse]], slightly dimmer than from here (-1.2). The stars [[Fomalhaut]] and [[Vega]], although further away, would appear somewhat displaced as well. Proxima Centauri would be an inconspicuous 4.5 magnitude star, which considering it would only be a quarter of a [[light-year]] away shows just how faint Proxima really is.

A hypothetical planet around either α Centauri A or B would see the other star as a very bright secondary. For example, an Earth-like planet at 1.25 [[Astronomical Unit]]s from α Cen A (with an [[orbital period]] of 1.34 [[year|a]]) would get Sun-like illumination from its primary, and α Cen B would appear 5.7 to 8.6 magnitudes dimmer (−21.0 to −18.2), 190 to 2700 times dimmer than α Cen A but still 170 to 2300 times brighter than the full Moon. Conversely, an Earth-like planet at 0.71 AUs from α Cen B (with a revolution period of 0.63 [[year|a]]) would get Sun-like illumination from its primary, and α Cen A would appear 4.6 to 7.3 magnitudes dimmer (−22.1 to −19.4), 70 to 840 times dimmer than α Cen B but still 520 to 6300 times brighter than the full Moon. In both cases the secondary sun would, in the course of the planet's year, appear to circle the sky. If a low orbital inclination against the orbit of α Cen A and B around each other is assumed it would start off right beside the primary and end up, half a period later, opposite it in the sky (a "midnight sun"). After another half period, it would complete the cycle. For a hypothetical Earthlike planet around either star, the secondary sun would not be bright enough to adversely affect climate or plant [[photosynthesis]] (being as far away as [[Saturn]] is from our [[Sun]]), but would mean that for about half the year, the night sky, instead of a pitch black, would appear a dark blue, and one could walk around and even read rather easily without artificial light.



The discovery of planets in binary star systems such as [[Gamma Cephei]], the high metallicity of the Alpha Centauri system, and the mere existence of the extensive satellite systems around all the giant planets in our own Solar System suggest that the existence of rocky Earthlike planets around the two stars in the system is not unlikely. [[Radial velocity]] methods by various planet-hunting teams have failed to find any giant planets or brown dwarfs in the system, which (if they existed) could disrupt the orbits of any potential terrestrial planets orbiting in or near the stars' habitable zones. Certainly, when technology advances enough for humans to start sending interstellar robotic probes, Alpha Centauri will be near the top of the list for exploration.

==Apparent movement==
[[Image:Motion-of-Alpha-Cen.jpg|thumb|170px|Apparent motion of Alpha Centauri relative to [[Beta Centauri]].]]

In about 4000 years, the [[proper motion]] of Alpha Centauri will mean that from the point of view of Earth it will appear close enough to [[Beta Centauri]] to form a visual [[double star]]. Beta Centauri is in reality far more distant than Alpha Centauri.

==Etymology and cultural significance==
The system bears the proper name '''Rigil Kentaurus''' (often shortened to '''Rigil Kent'''), derived from the [[Arabic language|Arabic]] phrase ''Rijl al Kentaurus'', meaning "foot of the centaur," but is most often referred to by its [[Bayer designation]] ''Alpha Centauri''. An alternative name is '''Toliman''', whose etymology may be Arabic (''Al-Thalimain'' meaning "Ostriches"), or [[Hebrew language|Hebrew]] (meaning "The Heretofore and the Hereafter" and/or "Shoot of the Vine"). (See [[Centaurus]]) Finally, it is sometimes called '''Bungula''', possibly from the [[Latin]] ''ungula'' meaning "hoof".

In Chinese, Alpha Centauri is called Nánmén'èr (南門二) "Second Star of the Southern Gate". As mentioned, Alpha and Beta Centauri together form the "Pointers" to ''[[Crux]]'', the Southern Cross.

''Alpha Centauri A'' is [[Henry Draper Catalogue|HD]] 128620, [[Harvard Revised catalogue|HR]] 5459, [[Cape Photographic Durchmusterung|CP]]-60°5483, [[General Catalogue of Trigonometric Parallaxes|GCTP]] 3309.00A, and [[Luyten Half-Second Catalogue|LHS]] 50.

''Alpha Centauri B'' is HD 128621, HR 5460, GCTP 3309.00B, and LHS 51.

===Modern fiction===
{{main|Alpha Centauri in fiction}}
Because of its status as our star's nearest galactic neighbor, Alpha Centauri has frequently been referred to in [[science fiction]] stories involving [[interstellar travel]].

==See also==
* [[List of nearest stars]]
* [[List of brightest stars]]

==References==

<div class="references-small">
<references />
</div>

* Lissauer, J. J., E. V. Quintana, J. E. Chambers, M. J. Duncan, and F. C. Adams. 2004. Terrestrial Planet Formation in Binary Star Systems. In "Revista Mexicana de Astronomia y Astrofisica (Serie de Conferencias); First Astrophysics meeting of the Observatorio Astronomico Nacional: Gravitational Collapse: From Massive Stars to Planets", G. Garca-Segura, G. Tenorio-Tagle, J. Franco, and H. W. Yorke (Ed.), 22, 99-103.

* Quintana, E. V.; Lissauer, J. J.; Chambers, J. E.; Duncan, M. J. 2002. Terrestrial Planet Formation in the Alpha Centauri System. ASTROPHYSICAL JOURNAL, Bulletin of the American Astronomical Society, VOL 576, NUMB 2, PART 1, pages 982-996. ISSN: 0004-637X

* Quintana, E. V.; Lissauer, J. J. 2007. Terrestrial Planet Formation in Binary Star Systems. In: "Planets in Binary Star Systems". Springer publishing company (to be published in 2007).

==External links==
* [http://www.glyphweb.com/esky/stars/alphacentauri.html Alpha Centauri]
* [http://simbad.u-strasbg.fr/sim-id.pl?protocol=html&Ident=alpha+centauri SIMBAD observational data]
* {{cite web
| title = A Family Portrait of the Alpha Centauri System
| work = SpaceRef.com
| url = http://www.spaceref.com/news/viewpr.html?pid=11016
| accessmonthday = 21 March
| accessyear = 2003}}
{{memoryalpha}}
* [http://jumk.de/astronomie/near-stars/alpha-centauri.shtml Alpha Centauri System]
* [http://www.uranometrianova.pro.br/astronomia/AA002/alphacen.htm O Sistema Alpha Centauri (portuguese)]

[[Category:Triple star systems]]
[[Category:Centaurus constellation]]
[[Category:Yellow dwarfs|Alpha Centauri]]
[[Category:Orange dwarfs|Alpha Centauri]]
[[Category:Red dwarfs|Alpha Centauri]]
[[Category:Bayer objects|Centauri, Alpha]]
[[Category:HD and HDE objects|128620]]
[[Category:HD and HDE objects|128621]]
[[Category:HIP objects|71683]]
[[Category:HIP objects|71681]]
[[Category:Stars with proper names]]

[[ar:القنطوري الألفا]]
[[bn:আলফা সেন্টরাই]]
[[bg:Алфа Центавър]]
[[ca:Alfa Centauri]]
[[cs:Alfa Centauri]]
[[de:Alpha Centauri]]
[[es:Alfa Centauri]]
[[eo:Alfa Centaŭro]]
[[eu:Alpha Centauri]]
[[fa:رجل قنطورس]]
[[fr:Alpha Centauri]]
[[gl:Alfa Centauri]]
[[ko:켄타우루스자리 알파]]
[[hr:Alpha Centauri]]
[[io:Alpha Centauri]]
[[id:Alpha Centauri]]
[[it:Alfa Centauri]]
[[he:אלפא קנטאורי]]
[[sw:Alpha Centauri]]
[[la:Alpha Centauri]]
[[lt:Kentauro alfa]]
[[hu:Alfa Centauri]]
[[nl:Alpha Centauri]]
[[ja:ケンタウルス座アルファ星]]
[[no:Alfa Centauri]]
[[oc:Alfa Centauri]]
[[pl:Alfa Centauri]]
[[pt:Alfa Centauri]]
[[ro:Alfa Centauri]]
[[qu:Llama Ñawi]]
[[ru:Альфа Центавра]]
[[simple:Alpha Centauri]]
[[sk:Alfa Centauri]]
[[sr:Алфа Кентаури]]
[[fi:Alfa Centauri]]
[[sv:Alfa Centauri]]
[[to:Ongo tangata]]
[[tr:Alfa Centauri]]
[[uk:Альфа Центавра]]
[[zh:南門二]]