Alpha Centauri
1979
226012543
2008-07-16T13:03:19Z
Ashmoo
194089
/* Alpha Centauri From a Hypothetical Planet */
{{about}}
{{Starbox begin
| name =Alpha Centauri A<ref name=simbad_a>{{cite web
| url=http://simbad.u-strasbg.fr/simbad/sim-id?Ident=*%20alf%20Cen%20A
| title=LHS 50 -- High proper-motion Star
| publisher=Centre de Données astronomiques de Strasbourg
| accessdate=2008-06-06 }}</ref>/B<ref name=simbad_b>{{cite web
| url=http://simbad.u-strasbg.fr/simbad/sim-id?Ident=*%20alf%20Cen%20B
| title=LHS 51 -- High proper-motion Star
| publisher=Centre de Données astronomiques de Strasbourg
| accessdate=2008-06-06 }}</ref>
}}
{{Starbox image
| image = [[Image:Position Alpha Cen.png|275px]]
| caption = The position of Alpha Centauri.
}}
{{Starbox observe
| epoch = J2000
| ra = {{RA|14|39|36.4951/35.0803}}
| ra_info = [http://nstars.nau.edu/index.cfm?fuseaction=singleSearch.astrometry&SystemID=1439m6050]
| dec = {{DEC|-60|50|02.308/13.761}}
| appmag_v = -0.01/+1.33
| constell = [[Centaurus]]
}}
{{Starbox character
| class =G2V/K2IV
| b-v = +0.69/+0.90
| u-b = +0.23/+0.63
| 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
}}
{{Starbox detail
| age = 4.85{{e|9}}<ref name=eso>{{cite web
| last=Kervella | first=Pierre | coauthors=Thevenin, Frederic
| date=[[March 15]], [[2003]]
| url=http://www.eso.org/public/outreach/press-rel/pr-2003/pr-05-03.html
| title=A Family Portrait of the Alpha Centauri System
| publisher=ESO | accessdate=2008-06-06 }}</ref>
| metal=151/160%<ref name=eso/> Sun
| mass=1.100/0.907<ref name=eso/>
| radius=1.227/0.865<ref name=eso/>
| 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
| doi = 10.1051/0004-6361:20053850 <!--Retrieved from url by DOI bot-->
| accessdate=2007-06-01 }}</ref>
| rotation =
| luminosity = 1.519/0.500<ref name=eso/>
| temperature = 5,790/5,260<ref name=eso/>
}}
{{Starbox visbin
| name=Alpha Centauri AB
| period=79.91
| axis=17.57
| eccentricity=0.5179
| inclination=79.205
| node=204.85
| periastron=1875.66
| periarg=231.65
| reference=<ref name=Pourbaix>{{cite journal
| last=Pourbaix | first=D.
| coauthors=Nidever, D.; McCarthy, C.; Butler, R. P.; Tinney, C. G.; Marcy, G. W.; Jones, H. R. A.; Penny, A. J.; Carter, B. D.; Bouchy, F.;+ 6 more
| title=Constraining the difference in convective blueshift between the components of alpha Centauri with precise radial velocities
| journal=Astronomy and Astrophysics
| year=2002 | volume=386 | issue=1 | pages=208–285
| url=http://adsabs.harvard.edu/abs/2002A%26A...386..280P
| accessdate=2008-06-15
| doi=10.1051/0004-6361:20020287 }}</ref>
}}
{{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
'''α Cen C''' (= '''Proxima Cen''')
[[Luyten Half-Second catalogue|LHS]] 49, [[Hipparcos catalogue|HIP]] 70890
}}
{{Starbox end}}
'''Alpha Centauri''' / α Centauri / α Cen, also known as '''Rigil Kentaurus''', '''Rigel Kent''', or '''Toliman''', is the brightest [[star]] in the southern [[constellation]] of [[Centaurus]] and an established [[binary star]] system, Alpha Centauri AB (α Cen AB). It appears to the naked-eye as the third brightest star in the entire night sky, being only outshone by [[Sirius]] and [[Canopus]]. By total [[Apparent magnitude|visual magnitude]] Alpha Centauri AB is −0.27, which is just fractionally brighter than the fourth brightest individual star in the night sky, [[Arcturus]].
In the southern hemisphere Alpha Centauri is known as one of ''The Pointers'', along with [[Beta Centauri]] or Agena / Hadar, as both stars directly point towards the [[constellation]] [[Crux]] - the famous [[Southern Cross]]. The Pointers were so named because they easily distinguish the true Southern Cross from the less bright [[Asterism (astronomy)|asterism]] known as the [[Vela (constellation)|False Cross]]. The star [[Beta Centauri]] lies some 4.4 [[degree (angle)|degrees]] further west from Alpha Centauri, mid-way between the Southern Cross and α Centauri.
Both stars are presently too far south to be visible for most northern hemisphere observers. Those below the northern latitude of +33 degrees who can see α Centauri, will find it lying close to their southern horizon during the northern summer. South from about -33 degrees latitude, α Centauri is [[Circumpolar star|circumpolar]] and never sets below the horizon.
Alpha Centauri has the primary distinction of being the closest of all the stars visible to the naked-eye in the night sky. Its true distance is about 1.3 [[parsec]]s, or 4.37 [[light years]].
==Nature of Alpha Centauri ==
Alpha Centauri is a [[star system|triple star system]]. The brightest two stars make a close orbiting [[binary star|binary]]; an additional much more distant and fainter companion is called [[Proxima Centauri]], '''Proxima''' or α Cen C. Individually, the two bright stars rate, respectively, as the ''fourth'' and ''twenty-first'' [[list of brightest stars|brightest stars]] in the sky (excluding the Sun).
Observationally, α Centauri AB is too close to be resolved by the naked eye. However, for more than two centuries it has been very well observed and measured by telescopes as small as 5 cm in aperture.
==Observational History==
Its duplicity, according to the renowned double star observer [[Robert Grant Aitken| Robert Aitken]] (1961), and as now stated in the 6th Catalog of Binary Stars (2008), was originally discovered in December 1689 by Father Richaud from the city of [[Pondicherry]] in India while he was observing a comet. [[Sir John Herschel]] in 1834 was then the first to make micrometrical observations, though [[astrometric]] positions were made as early as 1752 by [[Nicolas Louis de Lacaille| Abbé Nicolas Louis de Lacaillé]] using a [[meridian circle]]. Since the early 20th Century, measures have been made with photographic plates.
By 1926, [[William Stephen Finsen]] produced the general [[Orbital elements|orbit elements]] now accepted for this system. Future positions could then be calculated from a binary star [[Ephemeris|ephemeris]] with sufficiently accuracy for visual observers. Others (e.g. the French astronomer D. Pourbaix in 2002) have slightly refined the orbital elements. (See the second ''Visual binary orbit'' in the ''Observational data'' side-box.) The established eighty-odd year orbital period for α Centauri AB is therefore reasonably accurate.
[[Image:Alpha Centauri AB over limb of Saturn PIA10406.jpg|thumb|left|Alpha Centauri A and B resolved over the limb of Saturn, as seen by [[Cassini–Huygens]].]]
Alpha Centauri is popularly known as the ''closest star system'' to our [[Solar System]] at about 4.37 [[light-year]]s distant, or about 41.5 trillion kilometres, 25.8 trillion miles or 277,600 AU. This was discovered by [[Thomas James Henderson| Thomas Henderson]], who made many exacting observations of both stars in the AB system. He obtained their trigonometric [[parallax]]es (their tiny annual circular movements against the background stars) between April 1832 and May 1833, but did not release them because he seriously doubted his own results, feeling they were too large to be true. Henderson eventually published his results in 1839, after [[Friedrich Wilhelm Bessel]] had released his own accurately determined parallax for the star [[61 Cygni]] in 1838. For this reason, Alpha Centauri is considered to be the ''second'' star to have its distance measured.
[[Proxima Centauri]] was discovered by R.T.A. [[Innes]] in 1915 from South Africa, being detected by blinking two separate photographic plates taken at different times during one of his dedicated [[proper motion]] surveys. It showed very large proper motions similar in both size and direction to those of α Centauri AB, which suggested they were associated. It is usually regarded as part of the system, and presently in its orbit is slightly closer to us than α Centauri AB. Lying 4.22 light-years away, Proxima Centauri is the ''closest star'' to the Sun. Most of the modern calculated distances for all three stars are derived from the parallaxes in the [[Hipparcos]] star catalog (HIP).
==Alpha Centauri : The Gravitational System==
[[Image:Alpha Centauri relative sizes.svg|thumb|left|300px|<b>Component Sizes and Colours</b>. Shows the relative sizes and colours stars in the Alpha Centauri system and compares them to the Sun. ]]
Alpha Centauri is the collective name of this triple star system. It consists of two main stars, α Cen A and α Cen B, together often labelled as α Cen AB, which is the established [[binary star]] system. A third component, α Cen C is the much smaller and dimmer [[red dwarf]] star named [[Proxima Centauri]].
The designation as "AB" has important significance for [[binary stars]]. In double star astronomy, and in most astronomical references, this suggests the central gravitational point of the binary relative to companion star(s) in multiple systems.<ref name=DoubleStarsHeintz>{{cite book | last=Heintz | first=W.D. | year=1978 | page=19 | title=Double Stars | publisher=D. Reidel Publishing Company, Dordrecht}}</ref> "AB-C" usage refers to the orbit of Proxima around the central binary, being the distance between the centre of gravity and the outlying companion. Sometimes older references use A×B, but this usage has been discontinued for decades. Since the distance from the Sun to α Cen AB does not differ significantly from that to either star in the binary system, it makes sense to refer to the binary as a solitary object.
These binary components and Proxima Centauri are described individually as follows;
<br clear="left">
===Alpha Centauri AB===
[[Image:AlphaCentauri AB Trajectory.gif|thumb|right|300px|'''Apparent and True Orbits of Alpha Centauri'''. Motion is shown from the A component against the relative orbital motion of B component. The ''Apparent Orbit'' (thin ellipse) is the shape of the orbit as seen by the observer on Earth. The ''True Orbit'' is the shape of the orbit viewed perpendicular to the plane of the orbital motion.]]
Alpha Centauri A is the principal member or ''primary'' of the binary system, being both slightly larger and more luminous than our [[Sun]]. Like the Sun, it is a [[main sequence]] star with a similar yellowish-white colour, whose [[stellar classification]] is [[spectral type]] G2 V. From the determined mutual orbital parameters, Alpha Centauri A is about 10% more massive than our Sun, with a radius about 23% larger.<ref name=eso/>
Alpha Centauri B is the companion star or ''secondary'' to the primary star, appearing slightly smaller and less luminous than the Sun. This [[main sequence]] star displays the spectral type of K1 V, being an observed deeper orangish-yellow colour than the primary star. By mass, α Cen B is about 90% of the Sun, and is 14% smaller in radius.<ref name=eso/> A likely solar-like rotation period of some 36.8 days has been determined.<ref name=iau6259>{{cite news
| last=Guinan | first=E. | coauthors=Messina, S.
| year=1995 | title=IAU Circular 6259, Alpha Centauri B
| publisher=Central Bureau for Astronomical Telegrams }}</ref> Although it has a lower luminosity than component A, star B emits a higher level of energy in the [[X-ray]] part of the spectrum. The light curve of B varies on a short time scale and at least one flare has been observed.<ref>{{cite journal
| last=Robrade | first=J.
| coauthors=Schmitt, J.H.M.M., Favata, F.
| title=X-rays from α Centauri - The darkening of the solar twin
| journal=Astronomy and Astrophysics
| year=2005 | volume=442 | issue=1 | pages=315–321
| url=http://adsabs.harvard.edu/abs/2005A&A...442..315R
| accessdate=2008-06-27 }}</ref>
The stars revolve about each other in a moderately elliptical ([[Eccentricity (orbit)|e]] = 0.5179) 79.91-year-long orbit.<ref name=SixthCatOrbVisBin>{{cite news | last=Hartkopf | first=W. | coauthors=Mason, D. M. | year=2008 | title=Sixth Catalog of Orbits of Visual Binaries | publisher=U.S.Naval Observatory, Washingto D.C.}}</ref> Unlike most of the planetary orbits in the Solar System, these stars can approach to 11.2 [[astronomical unit]]s (1.67 billion kilometres - roughly the distance between the Sun and [[Saturn]]) of each other, or can recede to a separation of 35.6 AU (5.9 billion kilometres, or approximately the distance from the Sun to [[Pluto (Planet)|Pluto]]). [http://ad.usno.navy.mil/wds/orb6.html] From this one can calculate the total mass (ΣM<sub>ʘ</sub>) of both stars to be about twice that of the Sun (<math>[(11.2+35.6)/2]^3/79.91^2=2.0</math>, see [[Standard gravitational parameter#Two bodies orbiting each other|formula]]). Both these stars, according to [[stellar evolution]] theory, are slightly older than the Sun<ref name=eso/>; some sources quote 5 to 6 billion years (as implied by their derived mass and spectral characteristics).
In their ''true orbit'', both stars last made their closest approach ([[apsis|periastron]]) of 11.5 A.U. in August [[1955]]; this will not occur again until May [[2035]]. [[apsis|Apastron]], when the stars are furthest apart in the orbit at 35.6 A.U., last occurred in May [[1995]]. The two stars are presently approaching each other. The next apastron will be in A.D. [[2075]]. <ref name=SixthCatOrbVisBin>{{cite news | last=Hartkopf | first=W. | coauthors=Mason, D. M. | year=2008 | title=Sixth Catalog of Orbits of Visual Binaries | publisher=U.S.Naval Observatory, Washingto D.C.}}</ref>
As seen from the Earth, the ''apparent orbit'' of this binary star means that the [[separation]] and [[position angle]] are in continuous change throughout the projected eighty-odd year period. According to the current version of the [[United States Naval Observatory| U.S.N.O.]]'s ''6th Binary Star Catalogue : Ephemeris'', the observed distance between the stars is now 8.29 [[arcsec]] through [[P.A.]] 237 degrees (2008) reducing in the next year to 7.53 [[arcsec]] through [[P.A.]] 241 degrees (2009). The next apparent closest approach of both these stars will be seen during February [[2016]], when the distance reduces to 4.0 [[arcsec]] through P.A. 300 degrees. (See [http://homepage.mac.com/andjames/PageAlphaCen006.htm External Reference.] ) The maximum separation of these stars is about 22 [[arcsec]], which last happened in February [[1976]]. This will not occur again until January [[2056]].
===Alpha Centauri C / Proxima Centauri / V645 Centauri===
{{main|Proxima Centauri}}
The much fainter [[red dwarf]] star named '''Alpha Centauri C''', "α Cen C", '''Proxima Centauri''', or simply "Proxima", is about 13,000 [[astronomical units|A.U.]] away from Alpha Centauri AB (1.94 trillion kilometres or 0.21 [[Light year|ly]] – about ''one-twentieth'' the distance between Alpha Centauri AB and the Sun). It may be in orbit around it, though the period must be in the order of 100,000 to 500,000 years or more. It is possible that the orbit might be [[Hyperbolic trajectory|hyperbolic]], similar to the planetary sling-shot effect adopted by interplanetary spacecraft to change direction and velocity to a second planetary body, and so Proxima may leave the system after a few million years. Association with Alpha Centauri AB is unlikely to be entirely accidental, as it shares approximately the same motion through space as the inner binary star system. However, true gravitational binding is yet to be proven.
Seen from Earth, Proxima Centauri is 2.2 degrees south-west from Alpha Centauri AB. This is about ''four'' times the [[angular diameter]] of the Full Moon, and almost exactly half the distance between Alpha Centauri and Beta Centauri. A moderate-sized telescope is required to see Proxima.
Proxima usually appears as a 13.1 visual [[magnitude]] deep-red star in a poor star field of only several stars. The star is listed in the ''[[General Catalogue of Variable Stars]]'' (G.C.V.S. Version 4.2) as '''V645 Cen''', being a known [[UV Ceti]]-type [[flare star]], which may suddenly and unexpectedly brighten by about two magnitudes or so. (A visual magnitude of 11.0 is often quoted.) Both amateur and professional astronomers monitor this star from time to time - with both optical and radio telescopes. Proxima is of [[spectral class]] M5Ve or M5VIe, whose B-V [[colour index]] is +1.81. The spectral class suggest this is either a small [[main sequence]] star (Type V) or sub-dwarf (VI) with [[Emission lines|emission lines]]. Its mass is about 0.4 [[solar masses]].
The Sun is the closest star to the Alpha Centauri system.
==Alpha Centauri: A High Proper Motion System==
[[Image:Motion-of-Alpha-Cen.jpg|thumb|170px|Apparent motion of Alpha Centauri relative to [[Beta Centauri]].]]
Alpha Centauri, like the ''first magnitude'' stars of [[Sirius]] and [[Arcturus]], shows high proper motions against the background sky, which causes its sky position to gradually change over the centuries. These slow motions were unknown to ancient astronomers, like [[Aristotle]], who stated that all stars were permanently fixed to their places on the [[Celestial Sphere|celestial sphere]]. [[Edmond Halley]] in [[1718]] first found that contemporary [[astrometric]] sky positions, especially of the bright star Arcturus, differed significantly from those given by [[Ptolemy]] (probably measured earlier by [[Hipparchos]]) during the 1st Century B.C. (In Arcturus' case, the star moved almost ½ degree in 1800 years.) These motions were found mainly for northern stars, and thus the motion of the southern star Alpha Centauri was not found until the early 17th Century.
As stated above, [[Thomas Henderson]] in the 1830s was the first to discover Alpha Centauri's true distance, but also soon realised that this system was likely to have a high proper motion. Because of the proximity of these stars, their true velocity through space would appear to be much larger. In Alpha Centauri's case, the apparent motion of these stars were found from the astrometric observations made by [[Abbé Nicolas Louis de Lacaille]] during 1751-52.
Using the [[Hipparcos Catalogue|Hipparcos Star Catalogue]] (HIP) data, the mean individual proper motions (in milli [[arcsec]]) are -3678 mas.yr<sup>-1</sup> (mas/yr) or 3.678 arcsec per year in [[right ascension]] (the negative value indicating the sky motion is east to west) and +481.84 mas.yr<sup>-1</sup> (mas/yr) or 0.48184 arcsec per year in [[declination]]. As proper motions are cumulative, the motion of Alpha Centauri is about 6.1 [[arcmin]]/[[century]] (367.8 arcsec/ century) equivalent to 1.02 [[degree (angle)|degree]]s/[[millennia]] or 61.3 [[arcmin]]/[[millennia]]. These motions are about one-fifth and twice, respectively, the diameter of the [[Moon|full moon]].
A rough calculation, for example, shows that the current distance of 4.4 degrees between Alpha and Beta Centauri will take about 4200 years by proper motion to cross, therefore these two naked-eye stars will be closest in about A.D. 6200. Calculating backwards in time, say to the time of 2000 years ago, the pointers of Alpha and Beta Centauri were about 6½ degrees apart, and Alpha Centauri was instead lying in the present day southern constellation of [[Circinus]].
A more precise calculation can be made that involves taking into account the slight changes in the distance of the star by its own motion, and in Alpha Centauri's case, this means a slow increase in the values of these proper motions. Slight variances are also due to the small difference in measured values of the proper motions of α Cen A and α Cen B, which is roughly about 0.5% in accuracy.
===Future Sky Positions of Alpha Centauri===
Around [[Anno Domini|A.D.]] 5973, the significantly very high [[proper motion]]s observed for the stars of Alpha Centauri will eventually lead to some future observer seeing a brilliant naked-eye visual [[double star]] adjoining the slightly fainter 1st magnitude star [[Beta Centauri]] / β Centauri. At closest approach, the apparent separation of these stars will be only 23 [[arcmin]] or two-thirds the Moon's apparent diameter. {{Fact|date=May 2008}} <ref name=AOST1>{{cite news | last=Hartung | first=E.J. | coauthors=Frew, David Malin, David | year=1994 | title=''Astronomical Objects for Southern Telescopes'' | publisher=Cambridge University Press }}</ref> This spectacular duo will form a very rare optical ''stellar conjunction''. β Centauri has only about one one-hundredth of α Centauri's overall proper motion (-33.96 mas.yr<sup>-1</sup> in [[Right Ascension|R.A.]] and -2.506 mas.yr<sup>-1</sup> in [[Declination|Dec.]]), and as such, β Centauri appears almost stationary over the millennia in its general position against the background stars. This difference in proper motion is primarily caused by β Centauri lying just over 120 times farther away from us than α Centauri itself (530 light-years compared to 4.3 light-years). It is really the overall large apparent motion of α Centauri that will cause this future optical alignment, making the stars more akin to two ships slowly passing one another in the night.
After this, Alpha Centauri will continue to slowly brighten, passing just north of the [[Southern Cross]] or [[Crux]], before moving northwest and up towards the [[celestial equator]] and away from the [[galactic plane]]. By about 29,700 A.D., α Centauri will lie exactly 1.00 [[parsec]]s or 3.26 light-years away, reaching the maximum brightness of -0.86 visual magnitude - similar in brightness to present day [[Canopus]]. At this time it will be placed near the present-day constellation of [[Hydra]]. Soon after this relatively close solar approach, the system will then begin to move away from the Sun. In 43,300 A.D., α Centauri will pass near 2nd magnitude [[Alpha Hydrae]] / [[Alphard]]. Then the apparent visual magnitude will be +1.03 and lie 5.36 ly. away. <ref>{{cite journal | author= Matthews, R.A.J.
| title=[http://adsabs.harvard.edu/abs/1994QJRAS..35....1M''The Close Approach of Stars in the Solar Neighbourhood''] | journal= Quarterly Journal of the Royal Astronomical Society | year=1994 | volume=35|pages=1 – 8}}</ref>
As the star slowly disappears among the stars of the Milky Way, it will reach a final [[vanishing point]] location, due to visual [[Perspective|perspective]], more than 100,000 years from now. Here this once bright star will finally drop below naked-eye visibility somewhere in the faint southern constellation of [[Telescopium]]. This unusual location results from α Centauri's orbit around the galactic center being highly tilted with respect to the plane of our [[Milky Way]] galaxy.
==The Sky from Alpha Centauri==
[[Image:Sol View from AlpCenA.png|thumb|300px|Looking toward [[Sun|Sol]] from Alpha Centauri in [[Celestia]]]]
Viewed from near the Alpha Centauri system, the sky would appear very much as it does to observers on Earth, except being without the three stars of α Centauri. Most of the familiar constellations, such as [[Ursa Major]] and [[Orion (constellation)|Orion]], would appear almost unchanged. However, Centaurus would be missing its brightest star, and our [[Sun]] would appear as star of +0.5 magnitude in the northern Milky Way constellation of [[Cassiopeia (constellation)|Cassiopeia]]. An interstellar observer would find the familiar \/\/ shape of Cassiopeia becoming a /\/\/, with the Sun being at the end closest to the star [[Epsilon Cassiopeiae|ε Cassiopeiae]]. The Sun's position is easily plotted, as it lies at the point [[antipodal point|antipodal]] to Alpha Centauri's current position as seen from Earth. This is at [[right ascension|RA]] 02h 39m 35s, [[declination|Dec.]] +60° 50' (2000)
From Alpha Centauri, the bright stars which are relatively close to us, such as [[Sirius]], [[Procyon]] and [[Altair]], would appear to have quite different sky positions. Sirius, for example, would been seen to become part of the constellation of Orion, appearing some 2 degrees west of [[Betelgeuse]]. It would also be -1.2 magnitude star - some 0.2 magnitudes dimmer than seen from Earth. Other bright stars like [[Fomalhaut]] and [[Vega]], although slightly further away, would appear only slightly displaced from their familiar positions in the sky.
Due to the low luminosity of Proxima Centauri, to the naked-eye it would remain an inconspicuous 4.5 magnitude star even though being merely 0.25 [[light-year]] away. Its slow and gradual movement against the stars of the background sky would probably be detectable to the average person over one or two decades.
From Proxima itself, α Centauri AB would appear like two close very bright stars with the combined magnitude of −6.80. Depending on the position of the binary in their orbit, the stars would appear noticeably double to the naked eye or occasionally, for a short time, as one unresolved star. In [[visual magnitude]], α Cen A would be −6.52, and α Cen B −5.19.
<!--- To compute the following figures, use masses 1.1 and 0.92 Suns, luminosities 1.57 and 0.51 Suns (Sun visual magnitude -26.73), and an orbit of 11.2 to 35.6 AUs; min luminosity adds the planet's orbital radius to the max A-B distance (conjunction), max luminosity subtracts the planet's orbital radius from the min A-B distance (opposition) --->
==Does Alpha Centauri Have Planets?==
Discovery of additional planets orbiting both single stars ''and'' binary star systems, leaves the real possibility of finding either new planets in the Alpha Centauri AB system or planets revolving close to either α Cen A or α Cen B. With additional evidence, like both the principle stars being similar in nature to the Sun, I.e. high [[metallicity]] and similar ages, simply reinforces the astronomers view that it is very worthwhile to make detailed searches for planetary bodies around Alpha Centauri. Additionally, planets have also been found in other similar binary systems. I.e. [[Gamma Cephei]]. Several established planet-hunting teams have used various [[radial velocity]] or star [[Astronomical transit|transit]] methods in their searches around these two bright stars. All observational studies have so far failed to find any suggestion of either [[brown dwarf]]s, [[Gas giants|gas giants]] (planets) or small extrasolar [[Terrestrial planets| terrestrial planets]].
Based on theoretical computer simulations, other planetary astronomers consider that any potential terrestrial planets that did once orbit near the stars' habitable zones are now likely no longer located there. The loss several billion years ago of these small bodies probably happened during the system's formation. All may have since been ejected by significant disruptions caused by strong gravitational or [[perturbation]] effects generated between the two main stellar components.
In the not too distant future, assuming our human technology advances enough to enable voyages for interstellar robotic probes, Alpha Centauri may be first on the list for exo-planetary exploration. Such lengthy trips to cross the huge empty gulfs between the stars would likely still take several centuries, and this still assumes that some interstellar spacecraft could obtain high enough velocities to get there. If present ground or orbit based observatories are unable to detect planets, future unmanned exploratory journeys will be the only means of obtaining direct evidence that such planets do exist. <ref>{{cite web | url = http://www.universetoday.com/2008/07/08/how-long-would-it-take-to-travel-to-the-nearest-star | title=How Long Would it Take to Travel to the Nearest Star? 08 July 2008| author = Ian O'NeilL }}</ref>
===Alpha Centauri From a Hypothetical Planet===
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Any hypothetical planet orbiting around either α Centauri A or α Centauri B would see the other star as an intensely bright star in the sky with a discernible disk. For example, an Earth-like planet about 1.25 [[Astronomical Unit]] (A.U.) from α Cen A (with an [[orbital period]] of about one year three months or 1.3(4) [[year|a]]) would get Sun-like illumination from its primary, where 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|moon]]. Conversely, a similar Earth-like planet at 0.71 A.U. from α Cen B (with the yearly revolution period of about 0.6(3) [[year|a]]) would get Sun-like illumination from its primary, where 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 these cases, in the course of the planet's year, the secondary sun would appear to circle the whole sky. Either way, any such hypothetical Earth-like planet would find that the secondary sun would not be bright enough to significantly influence climate or plant [[photosynthesis]].
<!--- Sun gives us about 1 kW.m<sup>-2</sup> of energy at ground level, and about 60% of it is in visible light, so we can count 600 W.m<sup>-2</sup>. Street lamps placed on each tenth meter have to illuminate about 100 m<sup>2</sup> each. Power of lamps vary. Let's assume them to emit 50 W each (we count light emission, not energy consumption). Then we have 1 W.m<sup>-2</sup>, which is 1200 times less than the Sun. This is somewhere between 70 and 2700 (ranges given above are 70-840 for one star and 190-2700 for another). There are various cities and various streets with various levels of illumination, but arguably most of them fits in the given ranges (70 to 2700 dimmer than the Sun). One can also argue that illumination from the other companion would depend on local weather and on its elevation above the horizon. It's true, and it expands the latter bound from 2700 to maybe even 27000, but it doesn't change much about the comparison usefulness here - it was meant primarily to give an idea of how much is several hundred times dimmer than Sun, but several hundred times brighter than Moon. --->
If one assumes the planet has a low orbital inclination with respect to the mutual orbit of α Cen A and B, then the secondary star would start beside the primary at ''[[Conjunction (astronomy and astrology)|conjunction]]''. Half the period later (about forty years), at ''[[Opposition (astronomy and astrology)|opposition]]'', both stars would be opposite each other in the sky. At this time, for about half the planetary year the appearance of the night sky would be dark blue - similar to our sky during totality at any total [[solar eclipse]]. People could easily walk around and clearly see the surrounding terrain, and even reading a book would be quite possible without any artificial light. After another half period in the stellar orbit, the stars would complete their cycle and return to the next stellar conjunction. At this time a familiar Earth-like day and night cycle would return.
===Possibilities In Planet Formation===
Some recent computer models regarding planetary formation, do predict the possibility of [[terrestrial planet]]s existing around both Alpha Centauri A and B. <ref>Javiera Guedes, [http://www.ucolick.org/~javiera/alphacen.shtml Terrestrial Planet Formation Around Alpha Cen B]</ref><ref>see Lissauer and Quintana in references below</ref><ref>{{cite web | url = http://front.math.ucdavis.edu/0802.3482 | title = Formation and Detectability of Terrestrial Planets Around Alpha Centauri B | author = Javiera M. Guedes, Eugenio J. Rivera, Erica Davis, Gregory Laughlin, Elisa V. Quintana, Debra A. Fischer | date = to be published in 2008 | work = Astrophysical Journal }}</ref> Other models also strongly suggested that formation of [[gas giant]] planets similar to our [[Jupiter]] and [[Saturn (planet)|Saturn]] remain unlikely because of the significant gravitational and angular momentum effects of this binary system.<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> Although highly speculative, given the similarities to the Sun in [[spectral type|spectral types]], star type, age and probable stability of the orbits, it has been sometimes suggested that this stellar system could hold one of the best possibilities for harbouring [[extraterrestrial life]] on some potential planet.<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><ref>{{cite journal | author=Lissauer, J. J., E. V. Quintana, J. E. Chambers, M. J. Duncan, and F. C. Adams. | title= ''Terrestrial Planet Formation in Binary Star Systems''. | journal= "''Revista Mexicana de Astronomia y Astrofisica (Serie de Conferencias); First Astrophysics meeting of the Observatorio Astronomico Nacional: Gravitational Collapse: From Massive Stars to Planets''"; | year=2004 | volume=22 | pages=99 – 103 | unused_data= |G. Garca-Segura, G. Tenorio-Tagle, J. Franco, and H. W. Yorke (Ed.)l|}}</ref><ref>{{cite journal | author=Quintana, E. V.; Lissauer, J. J.; Chambers, J. E.; Duncan, M. J.; | title=''Terrestrial Planet Formation in the Alpha Centauri System.'' | journal=Astrophysical Journal | year=2002 | volume=2 | part=1 | pages=982–996 | doi=10.1086/341808 | unused_data=|Bulletin of the American Astronomical Society|}}</ref><ref>{{cite journal | author=Quintana, E. V.; Lissauer, J. J.; | title=''Terrestrial Planet Formation in Binary Star Systems.'' | journal="''Planets in Binary Star Systems.''" | unused_data=|Springer publishing company year=2007|}}</ref>
However, some astronomers have speculated that any possible terrestrial planets in the Alpha Centauri system may be bone dry or lack significant atmospheres. This is because theoretically, both Jupiter and Saturn were very crucial in perturbing [[comet]]s into the inner solar system, thus providing the inner planets with their own source of [[water]] and various other ices. This significant issue might not be as problematic, if for example, α Centauri B happened to have played a similar role as any hypothetical giant gas planets orbiting α Centauri A. (or conversely, α Cen A for α Cen B.) Also against this view is that most comets are considered to reside in the outer regions of the stellar system in some huge [[Oort Cloud]], and these comets can only sent sun-wards by the perturbations of gas planets or disruptions by the passing of nearby stars. As yet there is no direct evidence that such regions do exist around α Centauri AB, and it is indeed possible that this theoretical Oort Cloud region was totally destroyed during the system's own formation.
Any suspected Earth-like planet around Alpha Centauri A would have to be placed about 1.25 [[astronomical unit|AU]] away - about halfway between the distances of [[Earth]]'s orbit and [[Mars]]' orbit in our own [[Solar System]], so as to have similar planetary temperatures and conditions for liquid water to exist. For the slightly less luminous and cooler Alpha Centauri B, this distance would be closer to its star at about 0.7 [[astronomical unit| AU]], being about the distance that [[Venus]] is from the [[Sun]].
To find evidence of such planets, currently both Proxima Centauri and α Centauri AB are among the listed "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>
==Origin of Name and Cultural Significance==
The system bears the proper name '''Rigil Kentaurus'''<ref> [[Francis Bailey|Bailey, F.]], "The Catalogues of Ptolemy, Ulugh Beigh, Tycho Brahe, Halley, and Hevelius," ''Memoirs of Royal Astronomical Society'', vol. XIII, London, 1843.</ref> (often shortened to '''Rigil Kent.'''<ref name=K&S>Kunitzsch P., & Smart, T., ''A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations'', Cambride, Sky Pub. Corp., 2006, p. 27</ref>, former '''Rigjl Kentaurus'''<ref>[[Thomas Hyde|Hyde T.]], "Ulugh Beighi Tabulae Stellarum Fixarum", ''Tabulae Long. ac Lat. Stellarum Fixarum ex Observatione Ulugh Beighi'', Oxford, 1665, p. 142.</ref><ref>Hyde T., "In Ulugh Beighi Tabulae Stellarum Fixarum Commentarii", ''op. cit.'', p. 67.</ref>, and '''Riguel Kentaurus'''<ref>da Silva Oliveira, R., [http://www.asterdomus.com.br/principal_1.htm "Crux Australis: o Cruzeiro do Sul",] Artigos: Planetario Movel Inflavel AsterDomus.</ref> in [[Portuguese language|Portuguese]]), derived from the [[Arabic language|Arabic]] phrase ''Rijl Qan<u>t</u>ūris''<ref name=K&S/> (or ''Rijl al-Qan<u>t</u>ūris'',<ref>Davis Jr., G. A., [http://adsabs.harvard.edu/abs/1944PA.....52....8D "The Pronunciations, Derivations, and Meanings of a Selected List of Star Names,"]''Popular Astronomy'', Vol. LII, No. 3, Oct. 1944, p. 16.</ref> meaning "Foot of the [[Centaurus|Centaur]])," but is most often referred to by its [[Bayer designation]] ''Alpha Centauri''. An alternative name is '''Toliman''', whose etymology may be Arabic ''al-<u>Z</u>ulmān'' (meaning "the Ostriches")<ref name=K&S/>, or [[Hebrew language|Hebrew]] (meaning "The Heretofore and the Hereafter" and/or "Shoot of the Vine").{{Fact|date=June 2008}} (See [[Centaurus]]) Finally, it is sometimes called '''Bungula'''<ref>Burritt, E. H., ''[[Star atlas|Atlas]], Designed to Illustrate the Geography of the Heavens'', (New Edition), New York, F. J. Huntington and Co., 1835, pl. VII.</ref>, possibly coined from "β" and the [[Latin]] ''ungula'' (meaning "hoof").<ref name=K&S/> This latter name in modern times is, however, rarely used.
In Chinese, Alpha Centauri was called Nánmén'èr (南門二) "Second Star of the Southern Gate". As mentioned, Alpha and Beta Centauri together form the "Southern Pointers" to ''[[Crux]]'', the Southern Cross.
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''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.
Represented in the table of this page.-->
==Alpha Centauri in Modern Fiction==
{{main|Alpha Centauri in fiction}}
Alpha Centauri's relative proximity makes it in some ways likely the logical choice as "first port of call". [[Speculative fiction]] about [[interstellar travel]] 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 many works of [[science fiction]] and [[video game]]s. Because of its status as our nearest galactic neighbor, [[Alpha Centauri]] has frequently been used in [[science fiction]] stories involving [[interstellar travel]].
==References==
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==External links==
* [http://simbad.u-strasbg.fr/sim-id.pl?protocol=html&Ident=alpha+centauri SIMBAD observational data]
* [http://ad.usno.navy.mil/wds/orb6.html Sixth Catalogue of Orbits of Visual Binary Stars U.S.N.O.]
* [http://ad.usno.navy.mil/wds/orb6/orb6frames.html Sixth Catalogue of Orbits of Visual Binary Stars : Orbital Elements U.S.N.O.]
* [http://ad.usno.navy.mil/wds/orb6/orb6notes.html Sixth Catalogue of Orbits of Visual Binary Stars : Orbit Notes U.S.N.O.]
* [http://ad.usno.navy.mil/wds/orb6/orb6ephem.html Sixth Catalogue of Orbits of Visual Binary Stars : Orbit Notes U.S.N.O.]
* [http://homepage.mac.com/andjames/PageAlphaCen001.htm The Imperial Star - Alpha Centauri]
* [http://homepage.mac.com/andjames/PageAlphaCen006.htm Alpha Centauri - A Voyage to Alpha Centauri]
* [http://homepage.mac.com/andjames/PageAlphaCen006.htm Immediate History of Alpha Centauri]
* [http://www.glyphweb.com/esky/stars/alphacentauri.html eSky : Alpha Centauri]
==External Links : Hypothetical Planets or Exploration==
* {{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)]
* [http://www.alpha-centauri.pt Alpha Centauri - Associação de Astronomia (portuguese)]
* http://www.space.com/scienceastronomy/080307-another-earth.html
==See Also==
* [[List of nearest stars]]
* [[List of brightest stars]]
* [[Project Longshot]]
[[Category:Triple star systems]]
[[Category:Centaurus constellation]]
[[Category:G-type main sequence stars|Alpha Centauri]]
[[Category:K-type main sequence stars|Alpha Centauri]]
[[Category:M-type main sequence stars|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]]
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