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Alpha Centauri (Latinized from α Centauri , abbreviated Alpha Cen or α Cen ) is the closest star system and closest planetary system to Earth's Solar System at 4.37 light-years (1.34 parsecs) from the Sun. It is a triple star system, consisting of three stars: α Centauri A (officially Rigil Kentaurus ), α Centauri B (officially Toliman ), and α Centauri C (officially Proxima Centauri). Alpha Centauri A and B are Sun-like stars (Class G and K), and together they form the binary star Alpha Centauri AB. To the naked eye, the two main components appear to be a single star with an apparent magnitude of −0.27, the brightest star in the southern constellation of Centaurus and the third-brightest in the night sky, outshone only by Sirius and Canopus. Alpha Centauri A has 1.1 times the mass and 1.519 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 around a common centre with an

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 رِجْل القِنْطورُس ‎ 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 Beta Orionis (Rigel). An alternative name found in European sources, Toliman , is an approximation of the Arabic الظَّلِيمَان ‎ 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 applied is Bungula ( / ˈ b ʌ ŋ ɡ juː l ə / ), of obscure origin. Allen can only surmise it may have been coined from the Greek letter beta (β) and Latin ungula 'hoof'. Alpha Centauri C was discovered in 1915 by Robert T. A. Innes, who suggest

Alpha Centauri is a triple star system, with its two main stars, Alpha Centauri A and Alpha Centauri B, being a binary component. The AB designation, or older A×B , denotes the mass centre of a main binary system relative to companion star(s) in a multiple star system. AB-C refers to the component of Proxima Centauri in relation to the central binary, being the distance between the centre of mass and the outlying companion. Because the distance between Proxima (C) and either of Alpha Centauri A or B is similar, the AB binary system is sometimes treated as a single gravitational object. Orbital properties edit The A and B components of Alpha Centauri have an orbital period of 79.91 years. Their orbit is moderately eccentric, e  = 0.5179; their closest approach or periastron is 11.2 AU (1.68 billion km), or about the distance between the Sun and Saturn; and their furthest separation or apastron is 35.6 AU (5.33 billion km), about the distance between the Sun and Pluto. The most recent

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 arcsec (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 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. South of about 29° S latitude, Alpha Centauri is circumpolar and never sets below the horizon.note North of about 29° N latitude, Alpha Centauri never rises. Alpha Centauri lies close to the southern horizon when viewed from the

Alpha Centauri is listed in the 2nd-century star catalog of Ptolemy. He gave its ecliptic coordinates, but texts differ as to whether the ecliptic latitude reads 44° 10′ South or 41° 10′ South . (Presently the ecliptic latitude is 43.5° South , but it has decreased by a fraction of a degree since Ptolemy's time due to proper motion.) In Ptolemy's time, Alpha Centauri was visible from Alexandria, Egypt, at 31° N, but, due to precession, its declination is now –60° 51′ South , and it can no longer be seen at that latitude. English explorer Robert Hues brought Alpha Centauri to the attention of European observers in his 1592 work Tractatus de Globis , along with Canopus and Achernar, noting: Now, therefore, there are but three Stars of the first magnitude that I could perceive in all those parts which are never seene here in England. The first of these is that bright Star in the sterne of Argo which they call Canobus. The second Achernar is in the end of Eridanus. The third Alph

All components of Alpha Centauri display significant proper motion against the background sky. Over centuries, this causes their apparent positions to slowly change. Proper motion was unknown to ancient astronomers. Most assumed that the stars are permanently fixed on the celestial sphere, as stated in the works of the philosopher Aristotle. In 1718, Edmond Halley found that some stars had significantly moved from their ancient astrometric positions. In the 1830s, Thomas Henderson discovered the true distance to Alpha Centauri by analysing his many astrometric mural circle observations. He then realised this system also likely had a high proper motion. In this case, the apparent stellar motion was found using Nicolas Louis de Lacaille's astrometric observations of 1751–1752, by the observed differences between the two measured positions in different epochs. Calculated proper motion of the centre of mass for Alpha Centauri AB is about 3620 mas (milli-arcseconds) per year toward the

Confirmed planets edit Proxima Centauri b edit Proxima Centauri b is a terrestrial planet discovered in 2016 by astronomers at the European Southern Observatory. It has a minimum mass of 1.17 M ⊕ (Earth masses) and orbits approximately 0.049 AU from Proxima Centauri, placing it in the star's habitable zone. Proxima Centauri c edit Proxima Centauri c was formally discovered and confirmed in 2020 and is a likely super-Earth or mini-Neptune. It has a mass of roughly 7 M ⊕ and orbits about 1.49 AU from Proxima Centauri with a period of 1,928 days (5.28 yr). In June 2020, a large ring system encircling the planet was possibly detected. Controversial and hypothetical planets edit Alpha Centauri Bb edit In 2012, a planet around Alpha Centauri B was announced, Alpha Centauri Bb, but in 2015 a new analysis concluded that it almost certainly does not exist and was just a spurious artefact of the data analysis. Unconfirmed discovery edit Whilst ruling out the existence of Alpha Centauri Bb,

The sky from Alpha Centauri AB would appear much as it does from the Earth, except that Centaurus would be missing its brightest star. The Sun would appear as a yellow star of apparent magnitude +0.47, roughly the same as the average brightness of Betelgeuse from Earth. It would be at the antipodal point of Alpha Centauri AB's current right ascension and declination, at 02h 39m 35s +60° 50′ (2000), in eastern Cassiopeia, easily outshining all the rest of the stars in the constellation. With the placement of the Sun east of the magnitude 3.4 star Epsilon Cassiopeiae, nearly in front of the Heart Nebula, the "W" line of stars of Cassiopeia would have a "/W" shape.note

In modern literature, Rigil Kent (also Rigel Kent and variants;note / ˈ r aɪ dʒ əl ˈ k ɛ n t / ) and Toliman , are used as colloquial alternative names of Alpha Centauri (then became the proper name of Alpha Centauri B on 10 August 2018 by approval of IAU). Rigil Kent is short for Rigil Kentaurus , which is sometimes further abbreviated to Rigil or Rigel , though that is ambiguous with Beta 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. During the 19th century, the northern amateur popularist Elijah H. Burritt used the now-obscure name Bungula , possibly coined from "β" and the Latin ungula ("hoof"). Together, Alpha and Beta Centauri form the "Southern Pointers" or "The Pointers"

Alpha Centauri is a likely first target for crewed or robotic interstellar exploration. Using current spacecraft technologies, crossing the distance between the Sun and Alpha Centauri would take several millennia, though the possibility of nuclear pulse propulsion or laser light sail technology, as considered in the Breakthrough Starshot program, could reduce the journey time to decades. An objective of such a mission would be to make a fly-by of, and possibly photograph, planets that might exist in the system. The existence of Proxima Centauri b, announced by the European Southern Observatory (ESO) in August 2016, would be a target for the Starshot program. In January 2017, Breakthrough Initiatives and the ESO entered a collaboration to search for habitable planets in the Alpha Centauri system. The agreement involves Breakthrough Initiatives providing funding for an upgrade to the VISIR (VLT Imager and Spectrometer for mid-Infrared) instrument on ESO's Very Large Telescope (VLT) i

Alpha Centauri AB distance estimates Source Parallax (mas) Distance (pc) Distance (ly) Distance (Pm) References Henderson (1839) 1160 ± 110 0.86 +0.09 −0.07 2.57 ± 0.53 26.6 +2.8 −2.3 Henderson (1842) 912.8 ± 64 1.03 ± 0.15 3.34 ± 0.5 33.8 +2.5 −2.2 Maclear (1851) 918.7 ± 34 1.09 ± 0.04 3.55 +0.14 −0.13 32.4 ± 2.5 Moesta (1868) 880 ± 68 1.14 +0.10 −0.08 3.71 +0.31 −0.27 35.1 +2.9 −2.5 Gill & Elkin (1885) 750 ± 10 1.333 ± 0.018 4.35 ± 0.06 41.1 +0.6 −0.5 Roberts (1895) 710 ± 50 1.32 ± 0.2 4.29 ± 0.65 43.5 +3.3 −2.9 Woolley et al. (1970) 743 ± 7 1.346 ± 0.013 4.39 ± 0.04 41.5 ± 0.4 Gliese & Jahreiß (1991) 749.0 ± 4.7 1.335 ± 0.008 4.355 ± 0.027 41.20 ± 0.26 van Altena et al. (1995) 749.9 ± 5.4 1.334 ± 0.010 4.349 +0.032 −0.031 41.15 +0.30 −0.29 Perryman et al. (1997) (A and B) 742.12 ± 1.40 1.3475 ± 0.0025 4.395 ± 0.008 41.58 ± 0.08 Söderhjelm (1999)