Alpha Centauri

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 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 Rijl al-Qinṭūrus, meaning "the Foot of the Centaur". Qinṭūrus is the Arabic transliteration of the Greek (Kentaurus). The name is frequently abbreviated to Rigil Kent () or even Rigil, though the latter name is better known for Rigel ( 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 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 (or stars), 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 "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 (IAU) 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 (). Although its origin is not known, it may have been coined from the Greek letter beta () and Latin 'hoof', originally for Beta Centauri (the other hoof). To the Indigenous Boorong clan of the Wergaia people of northwestern Victoria in Australia, Alpha Centauri and Beta Centauri are , == Observation ==

{{Location map |100x100 open to the night sky, with the Milky Way running diagonally across the sky above it and many southern stars and constellations labelled and connected by lines, including Alpha Centauri and the not visible Proxima Centauri.|alt=Image of a very large telescope dome open to the night sky, with the Milky Way running diagonally across the sky above it and many southern stars and constellations labelled and connected by lines 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 (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. some 4.5° west, 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 (close to Hermosillo and Chihuahua City in Mexico; Galveston, Texas; Ocala, Florida; and Lanzarote, the Canary Islands of Spain), but only for a short time around its culmination. 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 (blue light region). 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 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. 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.{{citation-attribution|1={{cite web|url=http://ad.usno.navy.mil/wds/orb6/orb6ephem.html|title=Sixth Catalogue of Orbits of Visual Binary Stars: Ephemeris (2008)|publisher=U.S. Naval Observatory|access-date=13 August 2008|archive-url=https://web.archive.org/web/20090113210000/http://ad.usno.navy.mil/wds/orb6/orb6ephem.html 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 : objects within 9 light years (ly), arranged clockwise in hours of right ascension, and marked by distance (▬) and position (◆). Distances are marked outward from the Sun (Sol), 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 (doted when positive), 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. 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 (milliarcseconds per year) 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. though later calculations suggest that this will occur in 27,000 AD. At its nearest approach, α Centauri will attain a maximum apparent magnitude of −0.86, comparable to present-day magnitude of Canopus, but it will still not surpass that of Sirius, which will brighten incrementally over the next 60,000 years, and will continue to be the brightest star as seen from Earth (other than the Sun) for the next 210,000 years. == Stellar system ==

Alpha Centauri is a triple star system, with its two main stars, A and B, together comprising 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. the radial separation of A and B along the line of sight had reached a maximum in 2007, with B being further from Earth than A. The orbit is divided here into 80 points: each step refers to a timestep of approx. 0.99888 years or 364.84 days.|alt=Graphic image of a near-circle and a narrow ellipse labelled respectively as "B's real trajectory" and "B's apparent trajectory", with years marked along portions of the ellipses. The A and B components of Alpha Centauri have an orbital period of 79.762 years. Their orbit is moderately eccentric, as it has an eccentricity of almost 0.52; The observed maximum separation of these stars is about 22 arcsec, while the minimum distance is 1.7 arcsec. The widest separation occurred during February 1976, and the next will be in January 2056. Asteroseismic analyses that incorporate tight observational constraints on the stellar parameters for the Alpha Centauri stars have yielded age estimates of Gyr, Gyr, 6.4 Gyr,|alt=Two white disks side by side, each with coloured fringes and prominent diffraction spikes Alpha Centauri A Alpha Centauri A, also known as Rigil Kentaurus, is the principal member, or primary, of the binary system. It is a solar-like main-sequence star with a similar yellowish colour, whose stellar classification is spectral type G2-V; Alpha Centauri B has an apparent magnitude of +1.35, slightly dimmer than Mimosa. It is the closest star to the Sun but is too faint to be visible to the naked eye. == Planetary system ==

The Alpha Centauri system as a whole has two confirmed planets, both of them around Proxima Centauri. While other planets have been claimed to exist around all of the stars, none of the discoveries have been confirmed. Planets of Alpha Centauri A In 2021, a candidate planet named Candidate 1 (or C1) was detected around Alpha Centauri A, thought to orbit at approximately with a period of about one year, and to have a mass between that of Neptune and one-half that of Saturn, though it may be a dust disk or an artefact. The possibility of C1 being a background star has been ruled out. If this candidate is confirmed, the temporary name C1 will most likely be replaced with the scientific designation Alpha Centauri Ab in accordance with current naming conventions. GO Cycle 1 observations are planned for the James Webb Space Telescope (JWST) to search for planets around Alpha Centauri A, as well as observations of Epsilon Muscae. The coronographic observations, which occurred on July 26 and 27, 2023, were failures, though there are follow-up observations in March 2024. Pre-launch estimates predicted that JWST will be able to find planets with a radius of 5 at . Multiple observations every 3–6 months could push the limit down to 3 . Post-launch estimates based on observations of HIP 65426 b find that JWST will be able to find planets even closer to Alpha Centauri A and could find a 5 planet at . Candidate 1 has an estimated radius between If it is an exoplanet, it should have a mass between 90 and 150 Earth masses, a radius between 1.0 and 1.1 and a temperature of . A search for transits of planet Bb was conducted with the Hubble Space Telescope from 2013 to 2014. This search detected one potential transit-like event, which could be associated with a different planet with a radius around . This planet would most likely orbit Alpha Centauri B with an orbital period of 20.4 days or less, with only a 5% chance of it having a longer orbit. The median of the likely orbits is 12.4 days. Its orbit would likely have an eccentricity of 0.24 or less. It could have lakes of molten lava and would be far too close to Alpha Centauri B to harbour life. If confirmed, this planet might be called . However, the name has not been used in the literature, as it is not a claimed discovery. Planets of Proxima Centauri Proxima Centauri b or Alpha Centauri Cb is a terrestrial planet discovered in 2016 by astronomers at the European Southern Observatory (ESO). It has an estimated minimum mass of 1.17 (Earth masses) and orbits approximately 0.049 AU from Proxima Centauri, placing it in the star's habitable zone. It has a mass of roughly 7 and orbits about from Proxima Centauri with a period of . In June 2020, a possible direct imaging detection of the planet hinted at the presence of a large ring system. However, a 2022 study disputed the existence of this planet. , evidence for Proxima c remains inconclusive; observations with the NIRPS spectrograph were unable to confirm it, but found hints of a lower-amplitude signal with a similar period. Hypothetical planets Additional planets may exist in the Alpha Centauri system, either orbiting Alpha Centauri A or Alpha Centauri B individually, or in large orbits around Alpha Centauri AB. Because both stars are fairly similar to the Sun (in age and metallicity, for example), astronomers have been especially interested in making detailed searches for planets in the Alpha Centauri system. Several established planet-hunting teams have used various radial velocity or star transit methods in their searches around these two bright stars. Bodies around Alpha Centauri A would be able to orbit at slightly farther distances due to its stronger gravity. In addition, the lack of any brown dwarfs or gas giants in close orbits around Alpha Centauri make the likelihood of terrestrial planets greater than otherwise. A 2016 estimate placed the probability of finding an Earth-like planet around Alpha Centauri at roughly 75%. The observational thresholds for planet detection in the habitable zones by the radial velocity method are currently (2017) estimated to be about for Alpha Centauri A, for Alpha Centauri B, and for Proxima Centauri. Early computer-generated models of planetary formation predicted the existence of terrestrial planets around both Alpha Centauri A and B, Despite these difficulties, given the similarities to the Sun in spectral types, star type, age and probable stability of the orbits, it has been suggested that this stellar system could hold one of the best possibilities for harbouring extraterrestrial life on a potential planet. In the Solar System, it was once thought that Jupiter and Saturn were probably crucial in perturbing comets into the inner Solar System, providing the inner planets with a source of water and various other ices. However, since isotope measurements of the deuterium to hydrogen (D/H) ratio in comets Halley, Hyakutake, Hale–Bopp, 2002T7, and Tuttle yield values approximately twice that of Earth's oceanic water, more recent models and research predict that less than 10% of Earth's water was supplied from comets. In the system, Proxima Centauri may have influenced the planetary disk as the system was forming, enriching the area around Alpha Centauri with volatile materials. This would be discounted if, for example, happened to have gas giants orbiting (or vice versa), or if and B themselves were able to perturb comets into each other's inner systems, as Jupiter and Saturn presumably have done in the Solar System. For the slightly less luminous and cooler , the habitable zone is between about 0.7 and . The S.I.M. mission, however, was cancelled due to financial issues in 2010. Circumstellar discs Based on observations between 2007 and 2012, a study found a slight excess of emissions in the 24 μm (mid/far-infrared) band surrounding , which may be interpreted as evidence for a sparse circumstellar disc or dense interplanetary dust. == View from this system ==

from Alpha Centauri with Sirius near Betelgeuse, Procyon in Gemini, and the Sun in Cassiopeia generated by Celestia |alt=Simulated night-sky image centred on Orion labelled with constellation names in red and star names in yellow, including Sirius very close to Betelgeuse and the Sun near Cassiopeia. connected by lines, and the Sun, labeled "Sol", as it would appear to the left of the "W" The sky from would appear much as it does from the Earth, except that Centaurus's brightest star, being itself, would be absent from the constellation. The Sun would appear as a white star of apparent magnitude +0.5, roughly the same as the average brightness of Betelgeuse from Earth. It would be at the antipodal point of current right ascension and declination, at (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. Other nearby stars' placements may be affected somewhat drastically. Sirius, at 9.2 light years away from the system, would still be the brightest star in the night sky, with a magnitude of -1.2, but would be located in Orion less than a degree away from Betelgeuse. Procyon, which would also be at a slightly further distance than from the Sun, would move to outshine Pollux in the middle of Gemini. A planet around either or B would see the other star as a very bright secondary. For example, an Earth-like planet at from (with a revolution period of 1.34 years) would get Sun-like illumination from its primary, and would appear 5.7–8.6 magnitudes dimmer (−21.0 to −18.2), 190–2,700 times dimmer than but still 150–2,100 times brighter than the full Moon. Conversely, an Earth-like planet at from (with a revolution period of 0.63 years) would get nearly Sun-like illumination from its primary, and would appear 4.6–7.3 magnitudes dimmer (−22.1 to −19.4), 70 to 840 times dimmer than but still 470–5,700 times brighter than the full Moon. Proxima Centauri would appear dim as one of many stars, being magnitude 4.5 at its current distance, and magnitude 2.6 at periastron. == Future exploration ==

to the Sun, within 7.5 light years|alt=Series of partial circles centred on a small yellow disk labelled "Sun", each circle labelled with a distance, and several other small disks labelled with the names of stars Alpha Centauri is a 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 make the journey to Alpha Centauri in 20 years. 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. NASA released a mission concept in 2017 that would send a spacecraft to Alpha Centauri in 2069, scheduled to coincide with the 100th anniversary of the first crewed lunar landing in 1969, Even at 10% of the speed of light (about 108 million km/h), which NASA experts say may be possible, it would take a spacecraft 44 years to reach the system, by the year 2113, and would take another 4 years for a signal, by the year 2117 to reach Earth. The concept received no further funding or development. ==In culture==

Alpha Centauri has been recognized and associated throughout history, particularly in the Southern Hemisphere. Polynesians have been using Alpha Centauri for their star navigation and have called it Kamailehope. In the Ngarrindjeri culture of Australia, Alpha Centauri represents with Beta Centauri two sharks chasing a stingray, the Southern Cross, and in Incan culture it with Beta Centauri form the eyes of a llama-shaped dark constellation embedded in the band of stars that the visible Milky Way forms in the sky. In ancient Egypt it was also revered and in China it is known as part of the South Gate asterism. The Sagan Planet Walk in Ithaca, New York, is a walkable scale model of the solar system. An obelisk representing the scaled position of Alpha Centauri has been added at ʻImiloa Astronomy Center in Hawaii. == See also ==