Alpha Centauri (α Centauri, abbreviated Alf Cen or α Cen) is the star system closest to the Solar System, being 4.37 light-years (1.34 pc) from the Sun. It consists of three stars: Alpha Centauri A (also named Rigil Kentaurus) and Alpha Centauri B (also named Toliman) are Sun-like stars (Class G and K), which form the binary star Alpha Centauri AB, and a small and faint red dwarf (Class M), Alpha Centauri C (also named Proxima Centauri). To the unaided eye, the two main components appear as a single point of light with an apparent magnitude of −0.27, forming the brightest star in the southern constellation of Centaurus and is the third-brightest star in the night sky, outshone only by Sirius and Canopus. Alpha Centauri A has 1.1 times the mass and 1.519 times the luminosity of the Sun, while Alpha Centauri B is smaller and cooler, at 0.907 times the Sun's mass and 0.445 times its luminosity. The pair orbit about a common centre with an orbital period of 79.91 years. The elliptical orbits have a marked eccentricity, so that the distance between them varies from nearly that between Pluto and the Sun (35.6 astronomical units), to that between Saturn and the Sun (11.2 AU). Though not visible to the naked eye, Proxima Centauri is the closest star to the Sun, at a distance of 4.24 light-years (1.30 pc). The distance between Proxima Centauri and Alpha Centauri AB is about 13,000 astronomical units (0.21 ly), equivalent to about 430 times the radius of Neptune's orbit. Proxima Centauri b, an Earth-sized exoplanet in the habitable zone of Proxima Centauri, was discovered in 2016.
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