The position of Alpha Centauri.
Alpha Centauri A/B/C
||14h 39m 36.5/35.1s
||-60° 50′ 02.3/13.8″
|Apparent magnitude (V)
||G2 V/K1 V/M5.5 Ve
|U-B color index
|B-V color index
|Radial velocity (Rv)
|Proper motion (μ)
||RA: -3678.19 mas/yr
Dec.: 481.84 mas/yr
||747.23 ± 1.17 mas
||4.365 ± 0.007 ly
(1.338 ± 0.002 pc)
|Absolute magnitude (MV)
|Surface gravity (log g)
|Visual binary orbit
||Alpha Centauri B
|Semimajor axis (a)
|Longitude of the node (Ω)
|Periastron epoch (T)
Rigil Kentaurus, Rigil Kent, Toliman, Bungula,
FK5 538, CP(D)−60°5483, GC 19728,
α Cen A
Gl 559 A, HR 5459, HD 128620, GCTP 3309.00,
LHS 50, SAO 252838, HIP 71683
α Cen B
Gl 559 B, HR 5460, HD 128621, LHS 51, HIP 71681
LHS 49, HIP 70890
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 stars, one of which is the fourth brightest star in the night sky. Alpha Centauri is famous in the Southern Hemisphere as the outermost "pointer" to the 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-years 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. 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 exploration, and even the discovery and colonization of planetary systems.
Alpha Centauri is a triple star system consisting of two main stars, Alpha Centauri A and Alpha Centauri B (which form a multiple starsystem together), at a distance of 4.36 ly from Earth's solar system. These stars have an estimated age of 6 billion years, or 30% older than the Sun. 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.)
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 spectral type is G2 V, and it shines with a yellowish-white light. However, it has about 10% more mass than the Sun.
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. It only has 90% of the Sun's mass, and a rotation period of 36.8 days.
The two orbit one another elliptically (e=0.52), approaching as close as 11.2 astronomical units (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), with a period of just under 80 years.  Hence the sum of the two masses is just over double that of the Sun ([(11.2 + 35.6) / 2]3 / 802 = 2.0, see formula). These two stars are about 5 to 6 billion years old.
Main article: Proxima Centauri
The red dwarf Proxima Centauri is about 13,000 astronomical units away from Alpha Centauri A and B (1.94 trillion kilometers or 1.2 trillion miles or 0.21 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 orbit and may leave the system after a few million years. For this reason, Proxima is sometimes referred to as Alpha Centauri C. 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 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 predict terrestrial planets around both Alpha Centauri A and B, but suggest that gas giant planets similar to our Jupiter and Saturn would not be able to form because of the binary stars' gravitational effects. 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. 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 comets 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 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.
Sky appearance from the Alpha Centauri system
Size and color of the Sun compared to the stars in 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 being almost unchanged. However, Centaurus would be missing its brightest star and our Sun would appear as a 0.5-magnitude star in Cassiopeia. Roughly speaking, the \/\/ of Cassiopeia would become a /\/\/, with the Sun at the end closest to ε Cassiopeiae. The position can easily be plotted as RA 02h39m35s, dec +60°50', or antipodal to Alpha Centauri's position as seen from Earth.
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 Units from α Cen A (with an orbital period of 1.34 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 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 would appear a dark blue, instead of being pitch black, and a person could walk around and even read a book 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. If technology advances enough for humans to start sending interstellar robotic probes, Alpha Centauri may very likely be at the top of the list for exploration.
Around 5973 C.E, the proper motion of Alpha Centauri will lead to a situation where from an observer on Earth see the stars appear close enough to Beta Centauri to form a naked-eye visual double star 23 arcmin apart. This will be an example of a rare stellar conjunction, but in reality, Beta Centauri is just over 120 times more distant than Alpha Centauri. (530 to 4.3 light-years)
Etymology and cultural significance
The system bears the proper name Rigil Kentaurus (often shortened to Rigil Kent), derived from the 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 (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". This latter name in modern times is, however, rarely used.
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 HD 128620, HR 5459, CP-60°5483, GCTP 3309.00A, and LHS 50.
Alpha Centauri B is HD 128621, HR 5460, GCTP 3309.00B, and LHS 51.
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.
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* 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
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