Alpha Centauri
Alpha Centauri is a star system in the southern constellation of Centaurus. It consists of three stars: Rigil Kentaurus, Toliman, and Proxima Centauri. Proxima Centauri is the closest star to the Sun at 4.2465 light-years, which is 1.3020 parsecs, while Alpha Centauri A and B are the nearest stars visible to the naked eye.
Rigil Kentaurus and Toliman are Sun-like stars that together form the binary star system. 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. are the nearest binary stars to the Sun at a distance of.
Rigil Kentaurus has 1.1 times the mass and 1.5 times the luminosity of the Sun, while Toliman is smaller and cooler, at and less than. 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, or about the distance between Pluto and the Sun, to, or about the distance between Saturn and the Sun.
Proxima Centauri is a small faint red dwarf. Though not visible to the naked eye, Proxima Centauri is the closest star to the Sun at a distance of, slightly closer than. The distance between Proxima Centauri and is about, equivalent to about 430 times the radius of Neptune's orbit.
Proxima Centauri has two confirmed planets — Proxima b and Proxima d. The former is an Earth-sized planet in the habitable zone while the latter is a sub-Earth which orbits very closely to the star. A possible but disputed third planet, Proxima c, is a mini-Neptune astronomical units away. Rigil Kentaurus may have a Saturn-mass planet in the habitable zone, though it is not yet known with certainty to be planetary in nature. Toliman has no known planets.
Etymology and nomenclature
α Centauri is the system's designation given by J. Bayer in 1603. It belongs to the constellation Centaurus, named after the part human, part horse creature in Greek mythology; Heracles accidentally wounded the centaur and placed him in the sky after his death. Alpha Centauri marks the right front hoof of the Centaur. The common name Rigil Kentaurus is a Latinisation of the Arabic translation رجل القنطورس Rijl al-Qinṭūrus, meaning "the Foot of the Centaur". Qinṭūrus is the Arabic transliteration of the Greek Κένταυρος. The name is frequently abbreviated to Rigil Kent or even Rigil, though the latter name is better known for Rigel.An alternative name found in European sources, Toliman, is an approximation of the Arabic الظليمان aẓ-Ẓalīmān, meaning 'the Ostriches', an appellation Zakariya al-Qazwini had applied to the pair of stars Lambda and Mu Sagittarii; it was often unclear on old star maps which name was intended to go with which star, and the referents changed over time. The name Toliman originates with Jacob 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.
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 as the approved proper name; it is frequently abbreviated to Proxima.
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 and the name Proxima Centauri for On 10 August 2018, the IAU approved the name Toliman for
Other names
During the 19th century, the northern amateur popularist E.H. Burritt used the now-obscure name Bungula. Its origin is not known, but it may have been coined from the Greek letter beta and Latin ungula 'hoof', originally for Beta Centauri.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.
To the Indigenous Boorong people of northwestern Victoria in Australia, Alpha Centauri and Beta Centauri are Bermbermgle, two brothers noted for their courage and destructiveness, who speared and killed Tchingal "The Emu". The form in Wotjobaluk is.
The Mursi people of Ethiopia call this star Sholbi; it forms an asterism with δ Crucis, β Crucis, and β Centauri.
Observation
To the naked eye, appear 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, but through much of the orbit, both are easily resolved in binoculars or small telescopes. At −0.27 apparent magnitude, 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, 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° South latitude, is circumpolar and never sets below the horizon. North of about 29° N latitude, Alpha Centauri never rises. Alpha Centauri lies close to the southern horizon when viewed from latitude 29° N to the equator, but only for a short time around its culmination. The star culminates each year at local midnight on 24 April and at local 9 p.m. on 8 June.
As seen from Earth, Proxima Centauri is 2.2° southwest from this distance is about four times the angular diameter of the Moon. Proxima Centauri appears as a deep-red star of a typical apparent magnitude of 11.1 in a sparsely populated star field, requiring moderately sized telescopes to be seen. Listed as V645 Cen in the General Catalogue of Variable Stars, version 4.2, this UV Ceti star or "flare star" can unexpectedly brighten rapidly by as much as 0.6 magnitude at visual wavelengths, then fade after only a few minutes. Some amateur and professional astronomers regularly monitor for outbursts using either optical or radio telescopes. In August 2015, the largest recorded flares of the star occurred, with the star becoming 8.3 times brighter than normal on 13 August, in the B band.
Observational history
Alpha Centauri is listed in the 2nd century star catalog appended to Ptolemy's Almagest. Ptolemy gave its ecliptic coordinates, but texts differ as to whether the ecliptic latitude reads or . In Ptolemy's time, Alpha Centauri was visible from Alexandria, Egypt, at but, due to precession, its declination is now, 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:The binary nature of Alpha Centauri AB was recognized in December 1689 by Jean Richaud, while observing a passing comet from his station in Puducherry. Alpha Centauri was only the third binary star to be discovered, preceded by Mizar AB and Acrux.
The large proper motion of Alpha Centauri AB was discovered by Manuel John Johnson, observing from Saint Helena, who informed Thomas Henderson at the Royal Observatory, Cape of Good Hope of it. The parallax of Alpha Centauri was subsequently determined by Henderson from many exacting positional observations of the AB system between April 1832 and May 1833. He withheld his results, however, because he suspected they were too large to be true, but eventually published them in 1839 after Bessel released his own accurately determined parallax for in 1838. For this reason, Alpha Centauri is sometimes considered as the second star to have its distance measured because Henderson's work was not fully acknowledged at first.
John Herschel made the first micrometrical observations in 1834. Since the early 20th century, measures have been made with photographic plates.
By 1926, William Stephen Finsen calculated the approximate orbit elements close to those now accepted for this system. All future positions are now sufficiently accurate for visual observers to determine the relative places of the stars from a binary star ephemeris. Others, like D. Pourbaix, have regularly refined the precision of new published orbital elements.
Robert T. A. Innes discovered Proxima Centauri in 1915 by blinking photographic plates taken at different times during a proper motion survey. These showed large proper motion and parallax similar in both size and direction to those of which suggested that Proxima Centauri is part of the system and slightly closer to Earth than. As a result, Innes concluded that Proxima Centauri was the closest star to Earth yet discovered.
Location and motion
Alpha Centauri may be inside the G-cloud of the Local Bubble, and its nearest known system is the binary brown dwarf system Luhman 16, at distance.Historical distance estimates
File:Angular map of fusors around Sol within 9ly.png|thumb|upright=1.2|Alpha Centauri on a radar map of all known stellar and substellar objects within 9 light years, arranged clockwise in hours of right ascension, and marked by distance and position. Distances are marked outward from the Sun, with concentric circles indicating the distance in one ly steps. Positions are marked inward from their distance markings, connected by lines according to their declinations, representing the arcs of the declinations viewed edge-on.Kinematics
All components of 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 were 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 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 is about 3620 mas/y toward the west and 694 mas/y toward the north, giving an overall motion of 3686 mas/y in a direction 11° north of west. The motion of the centre of mass is about 6.1 arcmin each century, or 1.02° each millennium. The speed in the western direction is and in the northerly direction. Using spectroscopy the mean radial velocity has been determined to be around towards the Solar System. This gives a speed with respect to the Sun of, very close to the peak in the distribution of speeds of nearby stars.
Since is almost exactly in the plane of the Milky Way as viewed from Earth, many stars appear behind it. In early May 2028, will pass between the Earth and the distant red star 2MASS 14392160-6049528, when there is a 45% probability that an Einstein ring will be observed. Other conjunctions will also occur in the coming decades, allowing accurate measurement of proper motions and possibly giving information on planets.