The nearest star is invisible to the naked eye, but its neighbours shine brightly
Alpha Centauri looks like one brilliant star, the third brightest in the night sky. It is really three. Two Sun-like stars circle each other every 79 years, and a faint red dwarf called Proxima, too dim to see without a telescope, is the closest star to the Sun, about 4.25 light-years away.
The bright pair, Rigil Kentaurus and Toliman, sit about 4.34 light-years off and together shine at magnitude −0.27, outdone only by Sirius and Canopus. Rigil Kentaurus has 1.1 times the Sun's mass and half again its brightness; Toliman is smaller and cooler, below half the Sun's output. Their stretched orbit brings them as near as 11.2 astronomical units, roughly Saturn's distance from the Sun, and as far as 35.6, about Pluto's. Binoculars or a small telescope usually split them.
Proxima lies about 13,000 astronomical units from the pair, some 430 times the radius of Neptune's orbit. It has two confirmed planets: Proxima b, Earth-sized and in the zone where liquid water could exist though probably not habitable, and Proxima d, a small world hugging the star. A third, Proxima c, is disputed. Rigil Kentaurus may host a Saturn-mass planet, while Toliman's only candidate was ruled out in 2015. Robert Innes found Proxima in 1915 and suggested its Latin name, meaning nearest.
Johann Bayer labelled the system Alpha Centauri in 1603. It marks the right front hoof of the centaur, and Rigil Kentaurus comes from Arabic for the centaur's foot. Toliman traces to an Arabic name for ostriches, popularised in Jacob Golius's 1669 edition of Al-Farghani. The IAU fixed names on the individual stars in 2016 and approved Toliman on 10 August 2018.
Other cultures saw different pictures. Chinese astronomers called it the Second Star of the Southern Gate. For the Boorong people of northwestern Victoria in Australia, it and Beta Centauri were Bermbermgle, two bold brothers who speared the Emu, represented by the dark Coalsack Nebula. The Mursi of Ethiopia know it as Sholbi. Southern stargazers use it and Beta Centauri as the Pointers, whose line leads to the true Southern Cross rather than the dimmer False Cross.
Source: Alpha Centauri
