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Alpha Centauri is in the constellation of Centaurus.

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Alpha Centauri is one of the brightest stars in the heavens. Although Alpha Centauri 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 Alpha Centauri is too far south to be visible in most of the northern hemisphere. The two brightest components of the Alpha Centauri 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.

The position of Alpha Centauri.
Observation data
Epoch J2000
constellation Centaurus
Right ascension 14h 39m 36.5/35.1s
Declination -60 50' 02.3/13.8?
apparent magnitude (V) -0.01/+1.34/+11.05
Spectral type G2 V/K1 V/M5.5 Ve
B-V color index 0.65/0.85/1.97
U-B color index 0.24/0.64/1.54
Variable type None
radial velocity (Rv) -21.6 km/s
proper motion () RA: -3678.19 mas/yr
Dec.: 481.84 mas/yr
parallax (p) 747.23  1.17 mas
distance 4.365 0.007 ly
(1.338 0.002 pc)
Absolute magnitude (MV) 4.38/5.71/12.9
mass 1.100/0.907/0.1 M?
radius 1.227/0.865/0.2 R?
luminosity 1.519/0.500/0.00006 L?
temperature 5,800/5,300/2700 K
metallicity 130-230% Sun
rotation  ?
Age 5-6 109 years
Visual binary orbit
Companion Alpha Centauri B
Period (P) 79.24 years
Semimajor axis (a) 17.59"
eccentricity (e) 0.516
inclination (i) 79.24
Node (O) 204.87
Periastron Epoch (T) 1955.56
Other designations
Rigil Kentaurus, Rigil Kent, Toliman, Bungula, FK5 538, CP(D)-605483, GC 19728, CCDM J14396-6050

a Cen A

Gl 559 A, HR 5459, HD 128620, GCTP 3309.00, LHS 50, SAO 252838, HIP 71683

a Cen B

Gl 559 B, HR 5460, HD 128621, LHS 51, HIP 71681

Proxima Cen

LHS 49, HIP 70890
Database references

Alpha Centauri is the closest star system outside our own Solar System, being 4.39 light-years distant (about 25.8 trillion miles or 277,600 AU). Proxima Centauri, often regarded as part of the system, is 4.26 light-years distant. Alpha and Beta Centauri are near to each other in angular distance as seen from the Earth, but Beta Centauri is almost 120 times more distant. Alpha and Beta Centauri are the second closest pair of first magnitude stars as seen from the Earth, and due to the effects of proper motion, they will become the closest pair around 2166, overtaking Acrux and Becrux.

Alpha Centauri's relative proximity makes it a logical choice as "first port of call" in speculative fiction about interstellar travel, which assumes eventual human exploration, and even the discovery and colonization of imagined planetary systems. These themes are common to countless video games and works of science fiction.

Names of Alpha Centauri.

The system bears the proper name Rigil Kentaurus (often shortened to Rigil Kent), derived from the Arabic phrase Al 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, and also the Japanese version of this article.) Finally, it is sometimes called Bungula, possibly from the Latin ungula meaning "hoof".

In Chinese, Alpha Centauri is called Nnmn'r "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-605483, GCTP 3309.00A, and LHS 50.

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

System Components

Alpha Centauri system.
Size and color of the Sun compared to the stars in the Alpha Centauri system.

Alpha Centauri is a triple star system. It consists of two main stars, Alpha Centauri A and Alpha Centauri B (which form a binary system together) at a distance of 4.36 LY, and a dimmer Red Dwarf named Proxima Centauri at a distance of 4.22 ly (Hipparcos distances). Both of the two main stars are rather similar to the Sun. The larger member of the binary star, Alpha Centauri A, is the most similar to the Sun, but a little larger and brighter. Like the Sun, its Spectral type is G2 V, and, like the sun, shines in a yellowish-white light. The smaller of the two, Alpha Centauri B, is dimmer, with a spectral type of K1 V, somewhat smaller and dimmer than the sun, but astronomically similar enough, shining with more of an orangish-yellow-white light. The two orbit one another elliptically (e=0.52), approaching as close as 11.2 astronomical units (210-4 ly) and receding to 35.6 AU (610-4 ly), with a period of just under 80 years. Hence the sum of the two masses is23.43 / 802 = 2.0 times that of the Sun (see formula).

These two stars are about 5 to 6 billion years old. The red dwarf Proxima Centauri is about 13,000 astronomical units away from Alpha Centauri (1 ly = 63,241 AU, hence this is 0.21 ly, about 1/20 of the distance between Alpha Centauri and the Sun) and may be in orbit about it, with a period on the order of 500,000 years or more. For this reason, Proxima is sometimes referred to as Alpha Centauri C. However, it is not clear if it really is in orbit, although the association is unlikely to be entirely accidental as it shares approximately the same motion through space as the larger star system.

Seen from Earth, Proxima 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.

Alpha Centauri trajectory.
Alpha Centauri apparent and real trajectory of B component relative to A component.

The closest neighbours 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 around Alpha Centauri.

Computer models of planetary formation suggest that terrestrial planets would be able to form close to both Alpha Centauri A and B, but 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 solar 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.

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 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 leftmost end, closest to e Cassiopeiae. The position can easily be plotted as RA 02h39m35s, dec +6050', or antipodal to Alpha Centauri's position as seen from Earth.

Alpha Centauri.
Looking back towards Sol from Alpha Centauri.

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 dim Proxima really is.

A hypothetical planet around either a 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 a Cen A (with an Orbital period of 1.34 a) would get Sun-like illumination from its primary, and a Cen B would appear 5.7 to 8.6 magnitudes dimmer (-21.0 to -18.2), 190 to 2700 times dimmer than a Cen A but still 170 to 2300 times brighter than the full Moon. Conversely, an Earth-like planet at 0.71 AUs from a Cen B (with a revolution period of 0.63 a) would get Sun-like illumination from its primary, and a Cen A would appear 4.6 to 7.3 magnitudes dimmer (-22.1 to -19.4), 70 to 840 times dimmer than a 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. 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 surface of the planet would be illuminated as well as a city street by street lamps.

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, if 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 of Alpha Centauri.

Alpha Centauri.
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 an optical double star. Beta Centauri is in reality far more distant than Alpha Centauri.

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.

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