Alpha Centauri
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).[14] Proxima Centauri is the closest star to the Sun at 4.2465 light-years (1.3020 pc).
For other uses, see Alpha Centauri (disambiguation). "α Centauri" redirects here. Not to be confused with a Centauri, A Centauri or Centaurus A. "Toliman" redirects here. For other uses, see Toliman (disambiguation).
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 has 1.1 times the mass and 1.5 times the luminosity of the Sun, while α Centauri B is smaller and cooler, at 0.9 solar mass and less than 0.5 solar luminosity.[15] The pair orbit around a common centre with an orbital period of 79 years.[16] 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.
α 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 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),[17] 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.[18] The existence of Proxima c, a mini-Neptune 1.5 AU away discovered in 2019, is controversial.[19] 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.[20] α Centauri B has no known planets: Planet α Cen Bb, purportedly discovered in 2012, was later disproven,[21] and no other planet has yet been confirmed.
Etymology and nomenclature[edit]
α Centauri (Latinised to Alpha Centauri) is the system's designation given by J. Bayer in 1603. It belongs to the constellation Centaurus, named after the half human, half horse creature in Greek mythology. Hercules accidentally wounded the centaur and placed him in the sky after his death. Alpha Centauri marks the right front hoof of the Centaur.[22] The common name Rigil Kentaurus is a Latinisation of the Arabic translation رجل القنطورس Rijl al-Qinṭūrus, meaning "the Foot of the Centaur".[23][24] Qinṭūrus is the Arabic transliteration of the Greek Κένταυρος (Kentaurus).[25] The name is frequently abbreviated to Rigil Kent or even Rigil, though the latter name is better known for Rigel (β Orionis).[26]
An alternative name found in European sources, Toliman, is an approximation of the Arabic الظليمان aẓ-Ẓalīmān (in older transcription, aṭ-Ṭhalīmān), meaning 'the (two male) Ostriches', an appellation Zakariya al-Qazwini had applied to the pair of stars Lambda and Mu Sagittarii; it was often not clear on old star maps which name was intended to go with which star (or stars), and the referents changed over time.[27]
A third name that has been used is Bungula (/ˈbʌŋɡjuːlə/). Its origin is not known, but it may have been coined from the Greek letter beta (β) and Latin ungula 'hoof', originally for Beta Centauri (the other hoof).[26]
α Centauri C was discovered in 1915 by Robert T. A. Innes,[28] who suggested that it be named Proxima Centaurus,[29] from Latin 'the nearest [star] of Centaurus'.[30] The name Proxima Centauri later became more widely used and is now listed by the International Astronomical Union (IAU) as the approved proper name;[31][32] commonly, it is frequently abbreviated to Proxima.
In 2016, the Working Group on Star Names of the IAU,[14] having decided to attribute proper names to individual component stars rather than to multiple systems,[33] approved the name Rigil Kentaurus (/ˈraɪdʒəl kɛnˈtɔːrəs/) as being restricted to α Centauri A and the name Proxima Centauri (/ˈprɒksɪmə sɛnˈtɔːraɪ/) for α Centauri C .[34] On 10 August 2018, the IAU approved the name Toliman (/ˈtɒlɪmæn/) for α Centauri B .[35]
Other names[edit]
In modern literature, colloquial alternative names of Alpha Centauri include Rigil Kent[125] (also Rigel Kent and variants;[f]
/ˈraɪdʒəl ˈkɛnt/)[23][129] and Toliman[130] (the latter of which became the proper name of Alpha Centauri B on 10 August 2018 by approval of the International Astronomical Union).
Rigil Kent is short for Rigil Kentaurus,[131] which is sometimes further abbreviated to Rigil or Rigel, though that is ambiguous with β Orionis, which is also called Rigel.
The name Toliman originates with Jacobus Golius' 1669 edition of Al-Farghani's Compendium. Tolimân is Golius' latinisation of the Arabic name الظلمان al-Ẓulmān "the ostriches", the name of an asterism of which Alpha Centauri formed the main star.[132][133][134]
During the 19th century, the northern amateur popularist E.H. Burritt used the now-obscure name Bungula,[135] possibly coined from "β" and the Latin ungula ("hoof").[23]
Together, Alpha and Beta Centauri form the "Southern Pointers" or "The Pointers", as they point towards the Southern Cross, the asterism of the constellation of Crux.[67]
In Chinese astronomy, 南門 Nán Mén, meaning Southern Gate, refers to an asterism consisting of Alpha Centauri and Epsilon Centauri. Consequently, the Chinese name for Alpha Centauri itself is 南門二 Nán Mén Èr, the Second Star of the Southern Gate.[136]
To the Indigenous Boorong people of northwestern Victoria in Australia, Alpha Centauri and Beta Centauri are Bermbermgle,[137] two brothers noted for their courage and destructiveness, who speared and killed Tchingal "The Emu" (the Coalsack Nebula).[138] The form in Wotjobaluk is Bram-bram-bult.[137]
In culture[edit]
Alpha Centauri has been recognized and associated throughout history, particularly on the southern hemisphere. Polynesians have been using Alpha Centauri for their star navigation and have called it Kamailehope. In Aboriginal culture Alpha Centauri represents with Beta Centauri a shark chasing a sting ray, the Southern Cross, and in Incan culture it with Beta Centauri the eyes of a Llama shaped dark nebula constellation embedded in the band of stars that the visible Milky Way forms in the sky. More northern ancient cultures, like in ancient Egypt it was also revered and in China it is known as part of the South Gate constellation.[160]