Alpha Centauri
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Alpha Centauri (α Centauri, α Cen, or Alpha Cen) is a triple star system in the southern constellation of Centaurus. It consists of three stars: Rigil Kentaurus (α Centauri A), Toliman (α Centauri B), and Proxima Centauri (α Centauri C). Proxima Centauri is the closest star to the Sun at 4.2465 light-years (1.3020 pc). α Centauri A and B are Sun-like stars (class G and K, respectively) that together form the binary star system α Centauri AB. To the naked eye, these two main components appear to be a single star with an apparent magnitude of −0.27. It is the brightest star in the constellation and the third-brightest in the night sky, outshone by only Sirius and Canopus. α Centauri A (Rigil Kentaurus) has 1.1 times the mass and 1.5 times the luminosity of the Sun, while α Centauri B (Toliman) is smaller and cooler, at 0.9 solar masses and less than 0.5 solar luminosities. The pair orbit around a common centre with an orbital period of 79 years. Their elliptical orbit is eccentric, so that the distance between A and B varies from 35.6 astronomical units (AU), or about the distance between Pluto and the Sun, to 11.2 AU, or about the distance between Saturn and the Sun. One astronomical unit is the distance from Earth to the Sun, 150 million kilometers. Proxima Centauri, or α Centauri C, is a small faint red dwarf (class M). Though not visible to the naked eye, Proxima Centauri is the closest star to the Sun at a distance of 4.24 ly (1.30 pc), slightly closer than α Centauri AB. Currently, the distance between Proxima Centauri and α Centauri AB is about 13,000 AU (0.21 ly), equivalent to about 430 times the radius of Neptune's orbit. Proxima Centauri has one confirmed planet: Proxima b, an Earth-sized planet in the habitable zone (though it is unlikely to be habitable), one candidate planet, Proxima d, sub-Earth which orbits very closely to the star, and the controversial Proxima c, a mini-Neptune 1.5 astronomical units away. α Centauri A may have a Neptune-sized planet in the habitable zone, though it is not yet known with certainty to be planetary in nature and could be an artifact of the discovery mechanism. α Centauri B has no known planets: Planet α Cen Bb, purportedly discovered in 2012, was later disproven, and no other planet has yet been confirmed.