Alpha Centauri

Alternate meaning: Alpha Centauri computer game

Alpha Centauri is the brightest star system in the southern constellation of Centaurus, and is the third brightest in the entire night sky (although too far south to be visible in most of the northern hemisphere). It is also the closest star system to Earth, at 4.4 light-years. It bears the proper name Rigil Kentaurus (often shortened to Rigil Kent), derived from the Arabic phrase for "foot of the centaur," but is nonetheless usually referred to by its Bayer designation Alpha Centauri. Another alternative name is Toliman.

Alpha Centauri is a triple star system. It consists of two main stars, Alpha Centauri A and Alpha Centauri B (which form a binary star together), and a dimmer red dwarf named Proxima Centauri. The larger member of the binary star, Alpha Centauri A, is similar to the Sun, but a little larger and brighter. Like the sun, its spectral type is G2 V. The smaller of the two, Alpha Centauri B, is dimmer, with a spectral type of K1 V. The two orbit one another elliptically (e=0.52), approaching as close as 11.2 astronomical units and receding to 35.6 AU with a period of just under 80 years. These two stars are about 5 to 6 billion years old.

The red dwarf Proxima Centauri is only about 13,000 astronomical units away from Alpha Centauri (this is 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.

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 giants 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.

Viewed from Alpha Centauri, the sky would appear very much as it does to us with most of the constellations such as Ursa Major and Orion being unchanged. However, Centaurus would be missing its brightest star and Cassiopeia would be home to a bright star in the form of Sol, our Sun. Nearby very bright stars such as Sirius and Procyon would be in very different positions, as would Altair to a lesser extent. Fomalhaut and Vega would appear somewhat displaced as well.

In about 4000 years, the proper motion of Alpha Centauri will take it close enough to (the much more distant) Beta Centauri to form an optical double star.

See also: List of nearest stars

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