Alpha Centauri (α Centauri, Alpha Cen, or α Cen) is a triple star system in the southern constellation of Centaurus. It consists of three stars: Rigil Kentaurus (Alpha Centauri A), Toliman (B) and Proxima Centauri (C). Proxima Centauri is the closest star to the Sun at 4.2465 light-years (1.3020 pc).
Alpha Centauri A and B are Sun-like stars (Class G and K, respectively), and together form the binary star system Alpha Centauri AB. To the naked eye, the 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 only by Sirius and Canopus.
Alpha Centauri A has 1.1 times the mass and 1.5 times the luminosity of the Sun, while Alpha Centauri B is smaller and cooler, at 0.9 solar mass and less than 0.5 solar luminosity. 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.
Alpha Centauri C, or 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 , slightly closer than Alpha Centauri AB. Currently, the distance between Proxima Centauri and Alpha Centauri AB is about , equivalent to about 430 times the radius of Neptune's orbit.
Proxima Centauri has two confirmed planets: Proxima b, an Earth-sized planet in the habitable zone discovered in 2016, and Proxima d, a candidate sub-Earth which orbits very closely to the star, announced in 2022. The existence of Proxima c, a mini-Neptune 1.5 AU away discovered in 2019, is controversial. Alpha Centauri A may have a candidate Neptune-sized planet in the habitable zone, though it is not yet known to be planetary in nature and could be an artifact of the discovery mechanism. Alpha Centauri B has no known planets: planet Bb, purportedly discovered in 2012, was later disproven, and no other planet has yet been confirmed.
Etymology and nomenclature
α Centauri (Latinised to Alpha Centauri) is the system's designation given by Johann Bayer in 1603. It bears the traditional name Rigil Kentaurus, which is a Latinisation of the Arabic name Rijl al-Qinṭūrus, meaning 'the Foot of the Centaur'. The name is frequently abbreviated to Rigil Kent or even Rigil, though the latter name is better known for Rigel (Beta Orionis).
An alternative name found in European sources, Toliman, is an approximation of the Arabic aẓ-Ẓalīmān (in older transcription, aṭ-Ṭhalīmān), meaning 'the (two male) Ostriches', an appellation Zakariya al-Qazwini had applied to Lambda and Mu Sagittarii, also in the southern hemisphere.
A third name that has been used is Bungula (). Its origin is not known, but it may have been coined from the Greek letter beta (β) and Latin 'hoof'.
Alpha Centauri C was discovered in 1915 by Robert T. A. Innes, who suggested that it be named Proxima Centaurus, . The name Proxima Centauri later became more widely used and is now listed by the International Astronomical Union (IAU) as the approved proper name.
In 2016, the Working Group on Star Names of the IAU, having decided to attribute proper names to individual component stars rather than to multiple systems, approved the name Rigil Kentaurus () as being restricted to Alpha Centauri A and the name Proxima Centauri () for Alpha Centauri C. On 10 August 2018, the IAU approved the name Toliman () for Alpha Centauri B.
Observation
To the naked eye, Alpha Centauri AB appears to be a single star, the brightest in the southern constellation of Centaurus. Their apparent angular separation varies over about 80 years between 2 and 22 arcseconds (the naked eye has a resolution of 60 arcsec), but through much of the orbit, both are easily resolved in binoculars or small telescopes. At −0.27 apparent magnitude (combined for A and B magnitudes), Alpha Centauri is a first-magnitude star and is fainter only than Sirius and Canopus. It is the outer star of The Pointers or The Southern Pointers, so called because the line through Beta Centauri (Hadar/Agena), some 4.5° west, points to the constellation Crux—the Southern Cross. The Pointers easily distinguish the true Southern Cross from the fainter asterism known as the False Cross.