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Alpha Centauri

Star system in the southern constellation of Centaurus

Alpha Centauri (α Centauri, α Cen, or Alpha Cen) is a 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 (ly), which is 1.3020 parsecs (pc), while Alpha Centauri A and B are the nearest stars visible to the naked eye. Rigil Kentaurus and Toliman 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 AB are the nearest binary stars to the Sun at a distance of 4.344 ly (1.33 pc). Rigil Kentaurus has 1.1 times the mass (M☉) and 1.5 times the luminosity of the Sun (L☉), while Toliman is smaller and cooler, at 0.9 M☉ and less than 0.5 L☉. 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. Proxima Centauri 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. 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 two confirmed planets—Proxima b and Proxima d. The former is an Earth-sized planet in the habitable zone (though it is unlikely to be habitable) while the latter is a sub-Earth which orbits very closely to the star. A possible but disputed third planet, Proxima c, is a mini-Neptune 1.5 astronomical units away. Rigil Kentaurus may have a Saturn-mass planet (Alpha Centauri Ab) in the habitable zone, though it is not yet known with certainty to be planetary in nature. Toliman has no known planets; the only candidate planet, Alpha Centauri Bb, was disproven in 2015.