Alpha Centauri (α Centauri,α Cen, orAlpha Cen) is astar system in the southernconstellation ofCentaurus. It consists of threestars: Rigil Kentaurus (α Centauri A), Toliman (α Centauri B), andProxima Centauri (α Centauri C).[14] Proxima Centauri is theclosest star to theSun at 4.2465 light-years (ly), which is 1.3020parsecs (pc), while Alpha Centauri A and B are the nearest stars visible to the naked eye.
Rigil Kentaurus and Toliman areSun-like stars (class G andK, respectively) that together form thebinary star systemα Centauri AB. To thenaked eye, these two main components appear to be a single star with anapparent magnitude of −0.27. It is the brightest star in the constellation and thethird-brightest in thenight sky, outshone by onlySirius andCanopus.α Centauri AB are the nearest binary stars to the Sun at a distance of 4.344 ly (1.33 pc).
Rigil Kentaurus has 1.1 times themass (M☉) and 1.5 times theluminosity of the Sun (L☉), while Toliman is smaller and cooler, at 0.9 M☉ and less than 0.5 L☉.[15] The pair orbit around acommon centre with an orbital period of 79 years.[16] Their elliptical orbit iseccentric, so that the distance between A and B varies from 35.6 astronomical units (AU), or about the distance betweenPluto and the Sun, to11.2 AU, or about the distance betweenSaturn and the Sun.
Proxima Centauri is a small faintred 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 thanα Centauri AB. The distance between Proxima Centauri andα Centauri AB is about 13,000 AU (0.21 ly),[17] equivalent to about 430 times the radius ofNeptune's orbit.
α Centauri (Latinised to Alpha Centauri) is the system'sdesignation given byJ. Bayer in 1603. It belongs to the constellationCentaurus, named after thepart 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.[24] The common name Rigil Kentaurus is a Latinisation of the Arabic translationرجل القنطورسRijl al-Qinṭūrus, meaning "the Foot of the Centaur".[25][26]Qinṭūrus is the Arabic transliteration of the GreekΚένταυρος (Kentaurus).[27] The name is frequently abbreviated to Rigil Kent (/ˈraɪdʒəlˈkɛnt/) or even Rigil, though the latter name is better known forRigel (β Orionis).[28][29][30][25][31][b]
An alternative name found in European sources, Toliman, is an approximation of the Arabicالظليمانaẓ-Ẓalīmān (in older transcription,aṭ-Ṭhalīmān), meaning 'the (two male) Ostriches', an appellationZakariya al-Qazwini had applied to the pair of starsLambda andMu 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.[35] The name Toliman originates withJacob Golius' 1669 edition ofAl-Farghani'sCompendium. Tolimân is Golius' Latinisation of the Arabic nameالظلمانal-Ẓulmān "the ostriches", the name of an asterism of which Alpha Centauri formed the main star.[36][37][38][39]
α Centauri C was discovered in 1915 byRobert T. A. Innes,[40] who suggested that it be named Proxima Centaurus,[41] from Latin'the nearest [star] ofCentaurus'.[42] The name Proxima Centauri later became more widely used and is now listed by theInternational Astronomical Union (IAU) as the approved proper name;[43][44] it is frequently abbreviated to Proxima.
During the 19th century, the northern amateur popularist E.H. Burritt used the now-obscure name Bungula (/ˈbʌŋɡjuːlə/).[48] Its origin is not known, but it may have been coined from the Greek letterbeta (β) and Latinungula 'hoof', originally forBeta Centauri (the other hoof).[28][25]
To the IndigenousBoorong people of northwesternVictoria in Australia, Alpha Centauri andBeta Centauri are Bermbermgle,[50] two brothers noted for their courage and destructiveness, who speared and killed Tchingal "The Emu" (theCoalsack Nebula).[51] The form inWotjobaluk is Bram-bram-bult.[50]
TheVery Large Telescope 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.
Alpha Centauri AB (left) forms a triple star system withProxima Centauri (below, south of,α Centauri AB), circled in red. The bright star to the right isBeta Centauri.
As seen from Earth, Proxima Centauri is 2.2° southwest fromα Centauri AB; this distance is about four times theangular diameter of theMoon.[59] 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 theGeneral Catalogue of Variable Stars, version 4.2, thisUV Ceti star or "flare star" can unexpectedly brighten rapidly by as much as 0.6magnitude at visual wavelengths, then fade after only a few minutes.[60] Some amateur and professional astronomers regularly monitor for outbursts using either optical or radio telescopes.[61] 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 theB band (blue light region).[62]
Alpha Centauri is listed in the 2nd century star catalog appended toPtolemy'sAlmagest. Ptolemy gave itsecliptic coordinates, but texts differ as to whether the ecliptic latitude reads44° 10′ south or41° 10′ south[63] (presently the ecliptic latitude is43.5° south, but it has decreased by a fraction of a degree sincePtolemy's time due toproper motion). In Ptolemy's time, Alpha Centauri was visible fromAlexandria, Egypt, at31° N, but, due toprecession, its declination is now–60° 51′ South, and it can no longer be seen at that latitude. English explorerRobert Hues brought Alpha Centauri to the attention of European observers in his 1592 workTractatus de Globis, along with Canopus andAchernar, noting:
Now, therefore, there are but threeStars of the first magnitude that I could perceive in all those parts which are never seene here inEngland. The first of these is that bright Star in the sterne ofArgo which they call Canobus [Canopus]. The second [Achernar] is in the end ofEridanus. The third [Alpha Centauri] is in the right foote of theCentaure.[64]
Thebinary nature of Alpha Centauri AB was recognized in December 1689 by Jean Richaud, while observing a passingcomet from his station inPuducherry. Alpha Centauri was only the third binary star to be discovered, preceded byMizar AB andAcrux.[65]
The large proper motion of Alpha Centauri AB was discovered byManuel John Johnson, observing fromSaint Helena, who informedThomas Henderson at theRoyal Observatory, Cape of Good Hope of it. Theparallax 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 afterBessel released his own accurately determined parallax for61 Cygni in 1838.[66] 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.[66] (The distance of Alpha Centauri from the Earth is now reckoned at 4.396 light-years or 4.159×1013 km.)
By 1926,William Stephen Finsen calculated the approximateorbit elements close to those now accepted for this system.[69] All future positions are now sufficiently accurate for visual observers to determine the relative places of the stars from a binary starephemeris.[70] Others, like D. Pourbaix (2002), have regularly refined the precision of new published orbital elements.[16]
Robert T. A. Innes discoveredProxima Centauri in 1915 by blinking photographic plates taken at different times during aproper motion survey. These showed large proper motion and parallax similar in both size and direction to those ofα Centauri AB, which suggested that Proxima Centauri is part of theα Centauri system and slightly closer to Earth thanα Centauri AB. As a result, Innes concluded that Proxima Centauri was the closest star to Earth yet discovered.
Relative positions of Sun, Alpha Centauri AB and Proxima Centauri. Grey dot is projection of Proxima Centauri, located at the same distance as Alpha Centauri AB.
Alpha Centauri (with unlabeled Proxima Centauri) on a radar map of all known stellar andsubstellar objects within 9 light years (ly), arranged clockwise inhours ofright ascension, and marked by distance (▬) and position (◆). Distances are marked outward from theSun (Sol), with concentric circles indicating the distance in one ly steps. Positions are marked inward from their distance markings, connected by lines according to theirdeclinations (doted when positive), representing thearcs of the declinations viewed edge-on.
Animation (in Italian) showing motion ofα Centauri through the sky. (The other stars are held fixed for didactic reasons.) "Oggi" means today; "anni" means years.
All components ofα Centauri display significantproper motion against the background sky. Over centuries, this causes their apparent positions to slowly change.[88] Proper motion was unknown to ancient astronomers. Most assumed that the stars were permanently fixed on thecelestial sphere, as stated in the works of the philosopher Aristotle.[89] In 1718,Edmond Halley found that some stars had significantly moved from their ancientastrometric positions.[90]
In the 1830s,Thomas Henderson discovered the true distance toα Centauri by analysing his many astrometric mural circle observations.[73][91] He then realised this system also likely had a high proper motion.[92][93][69] In this case, the apparent stellar motion was found usingNicolas Louis de Lacaille's astrometric observations of 1751–1752,[94] by the observed differences between the two measured positions in different epochs.
Calculated proper motion of the centre of mass forα Centauri AB 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.[95][e] 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 23.0 km/s (14.3 mi/s) and in the northerly direction 4.4 km/s (2.7 mi/s). Usingspectroscopy the mean radial velocity has been determined to be around 22.4 km/s (13.9 mi/s) towards the Solar System.[95] This gives a speed with respect to the Sun of 32.4 km/s (20.1 mi/s), very close to the peak in the distribution of speeds of nearby stars.[96]
Sinceα Centauri AB is almost exactly in the plane of theMilky Way as viewed from Earth, many stars appear behind it. In early May 2028,α Centauri A will pass between the Earth and the distant red star 2MASS 14392160-6049528, when there is a 45% probability that anEinstein ring will be observed. Otherconjunctions will also occur in the coming decades, allowing accurate measurement of proper motions and possibly giving information on planets.[95]
Based on the system's common proper motion and radial velocities,α Centauri will continue to change its position in the sky significantly and will gradually brighten. For example, in about 6,200 CE, α Centauri's true motion will cause an extremely rarefirst-magnitude stellar conjunction withBeta Centauri, forming a brilliant opticaldouble star in the southern sky.[56] It will then pass just north of the Southern Cross orCrux, before moving northwest and up towards the presentcelestial equator and away from thegalactic plane. By about 26,700 CE, in the present-day constellation ofHydra,α Centauri will reach perihelion at 0.90 pc or 2.9 ly away,[97] though later calculations suggest that this will occur in 27,000 AD.[98] At its nearest approach, α Centauri will attain a maximumapparent magnitude of −0.86, comparable to present-day magnitude ofCanopus, but it will still not surpass that ofSirius, 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.[99]
Orbital plot of Proxima Centauri around the bright apparent star Alpha Centauri AB, with position change marked (in thousands of years).
Alpha Centauri is a triple star system, with its two main stars, A and B, together comprising abinary component. TheAB designation, or olderA×B, denotes the mass centre of a main binary system relative to companion star(s) in a multiple star system.[100]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.[101]
Apparent and true orbits of Alpha Centauri. The A component is held stationary, and the relative orbital motion of the B component is shown. The apparent orbit (thin ellipse) is the shape of the orbit as seen by an observer on Earth. The true orbit is the shape of the orbit viewed perpendicular to the plane of the orbital motion. According to the radial velocity versus time,[102] 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.
The A and B components of Alpha Centauri have an orbital period of 79.762 years. Their orbit is moderatelyeccentric, as it has an eccentricity of almost 0.52;[5] their closest approach orperiastron is 11.2 AU (1.68×10^9 km), or about the distance between the Sun and Saturn; and their furthest separation orapastron is 35.6 AU (5.33×10^9 km), about the distance between the Sun and Pluto.[16] The most recentperiastron was in August 1955 and the next will occur in May 2035; the most recentapastron was in May 1995 and will next occur in 2075.
Viewed from Earth, the apparent orbit of A and B means that their separation andposition angle (PA) are in continuous change throughout their projected orbit. Observed stellar positions in 2019 are separated by 4.92arcsec through the PA of 337.1°, increasing to 5.49 arcsec through 345.3° in 2020.[16] The closest recent approach was in February 2016, at 4.0 arcsec through the PA of 300°.[16][103] The observed maximum separation of these stars is about 22 arcsec, while the minimum distance is 1.7 arcsec.[69] The widest separation occurred during February 1976, and the next will be in January 2056.[16]
Alpha Centauri C is about 13,000 AU (0.21 ly; 1.9×10^12 km) from Alpha Centauri AB, equivalent to about 5% of the distance between Alpha Centauri AB and the Sun.[17][59][68] Until 2017, measurements of its small speed and its trajectory were of too little accuracy and duration in years to determine whether it is bound to Alpha Centauri AB or unrelated.
Radial velocity measurements made in 2017 were precise enough to show that Proxima Centauri and Alpha Centauri AB are gravitationally bound.[17] The orbital period of Proxima Centauri is approximately511000+41000 −30000 years, with an eccentricity of 0.5, much more eccentric thanMercury's. Proxima Centauri comes within4100+700 −600AU of AB at periastron, and its apastron occurs at12300+200 −100AU.[5]
ESO - Alpha Centauri in the HR-System (by)Relative sizes and colour of the Alpha Centauri A, B and C (Proxima) and otherlocal stars, incl. the Sun and Jupiter (artist's impression)
Asteroseismic studies,chromospheric activity, and stellar rotation (gyrochronology) are all consistent with the Alpha Centauri system being similar in age to, or slightly older than, the Sun.[104] Asteroseismic analyses that incorporate tight observational constraints on the stellar parameters for the Alpha Centauri stars have yielded age estimates of4.85±0.5 Gyr,[105]5.0±0.5 Gyr,[106]5.2 ± 1.9 Gyr,[107] 6.4 Gyr,[108] and6.52±0.3 Gyr.[109] Age estimates for the stars based on chromospheric activity (Calcium H & K emission) yield4.4 ± 2.1 Gyr, whereas gyrochronology yields5.0±0.3 Gyr.[104]Stellar evolution theory implies both stars are slightly older than the Sun at 5 to 6 billion years, as derived by their mass and spectral characteristics.[59][110]
From theorbital elements, the total mass of Alpha Centauri AB is about 2.0 M☉[f]– or twice that of the Sun.[69] The average individual stellar masses are about 1.08 M☉ and 0.91 M☉, respectively,[5] though slightly different masses have also been quoted in recent years, such as 1.14 M☉ and 0.92 M☉,[87] totaling 2.06 M☉. Alpha Centauri A and B haveabsolute magnitudes of +4.38 and +5.71, respectively.
Alpha Centauri A, also known asRigil Kentaurus, is the principal member, or primary, of the binary system. It is a solar-likemain-sequence star with a similar yellowish colour,[112] whosestellar classification isspectral type G2-V;[3] it is about 10% more massive than the Sun,[105] with a radius about 22% larger.[113] When considered among the individualbrightest stars in the night sky, it is the fourth-brightest at an apparent magnitude of +0.01,[2] being slightly fainter thanArcturus at anapparent magnitude of −0.05.
The type ofmagnetic activity on Alpha Centauri A is comparable to that of the Sun, showingcoronal variability due tostar spots, as modulated by the rotation of the star. However, since 2005 the activity level has fallen into a deep minimum that might be similar to the Sun's historicalMaunder Minimum. Alternatively, it may have a very long stellar activity cycle and is slowly recovering from a minimum phase.[114]
Alpha Centauri B, also known asToliman, is the secondary star of the binary system. It is a main-sequence star of spectral type K1-V, making it more an orange colour than Alpha Centauri A;[112] it has around 90% of the mass of the Sun and a 14% smaller diameter. Although it has a lower luminosity than A, Alpha Centauri B emits more energy in theX-ray band.[115] Itslight curve varies on a short time scale, and there has been at least one observedflare.[115] It is more magnetically active than Alpha Centauri A, showing a cycle of8.2±0.2 yr compared to 11 years for the Sun, and has about half the minimum-to-peak variation in coronal luminosity of the Sun.[114] This cycle was recently re-estimated based on more than 20 years of high-resolution spectroscopic observations of the CaIIH&K lines showing a cycle of7.8±0.2 yr.[116] Alpha Centauri B has an apparent magnitude of +1.35, slightly dimmer thanMimosa.[46]
Alpha Centauri C, better known asProxima Centauri, is a small main-sequencered dwarf of spectral class M6-Ve. It has anabsolute magnitude of +15.60, over 20,000 times fainter than the Sun. Its mass is calculated to be0.1221M☉.[117] It is the closest star to the Sun but is too faint to be visible to the naked eye.[118]
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.
The discovery image of Alpha Centauri's candidate Neptunian planet, marked here as "C1"
In 2021, a candidate planet named Candidate 1 (or C1) was detected around Alpha Centauri A, thought to orbit at approximately1.1 AU 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.[119][20] 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.[120]
GO Cycle 1 observations are planned for theJames Webb Space Telescope (JWST) to search for planets around Alpha Centauri A, as well as observations ofEpsilon Muscae.[121] The coronographic observations, which occurred on July 26 and 27, 2023, were failures, though there are follow-up observations in March 2024.[122] Pre-launch estimates predicted that JWST will be able to find planets with a radius of 5R🜨 at1–3 AU. Multiple observations every 3–6 months could push the limit down to 3R🜨.[123] Post-launch estimates based on observations ofHIP 65426 b find that JWST will be able to find planets even closer to Alpha Centauri A and could find a 5R🜨 planet at0.5–2.5 AU.[124] Candidate 1 has an estimated radius between3.3–11R🜨[20] and orbits at1.1 AU.
Observations with the James Webb Space Telescope in August 2024 uncovered a point source which might be an exoplanet at a separation of 2 astronomical units, believed to be the same object detected in 2021. This object is confirmed to be not a background object, and is unlikely to be an instrumental artifact, and therefore might be an exoplanet. It was not recovered and needs additional observations to be confirmed as a planet; there is a 52% chance it was not re-observed due to the orbital motion.[22][21] If it is an exoplanet, it should have a mass between 90 and 150Earth masses, a radius between 1.0 and 1.1RJ and a temperature of 225 K (−48 °C; −55 °F).[22]
The first claim of a planet around Alpha Centauri B was that ofAlpha Centauri Bb in 2012, which was proposed to be an Earth-mass planet in a 3.2-day orbit.[125] This was refuted in 2015 when the apparent planet was shown to be an artefact of the way theradial velocity data was processed.[126][127][23]
A search fortransits of planet Bb was conducted with theHubble 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 0.92 R🜨. 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.[128] It could have lakes of molten lava and would be far too close to Alpha Centauri B to harbourlife.[129] If confirmed, this planet might be calledAlpha Centauri Bc. However, the name has not been used in the literature, as it is not a claimed discovery.
The discovery ofProxima Centauri c was formally published in 2020 and could be asuper-Earth ormini-Neptune.[132][133] It has a mass of roughly 7M🜨 and orbits about1.49 AU from Proxima Centauri with a period of 1,928 days (5.28 yr).[134] In June 2020, a possible direct imaging detection of the planet hinted at the presence of a large ring system.[135] However, a 2022 study disputed the existence of this planet.[19] As of 2025[update], evidence for Proxima c remains inconclusive; observations with theNIRPS spectrograph were unable to confirm it, but found hints of a lower-amplitude signal with a similar period.[136]
A 2020 paper refining Proxima b's mass excludes the presence of extra companions with masses above0.6M🜨 at periods shorter than 50 days, but the authors detected a radial-velocity curve with a periodicity of 5.15 days, suggesting the presence of a planet with a mass of about0.29M🜨.[131] This planet,Proxima Centauri d, was detected in 2022[18][19] and later confirmed in 2025.[136]
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 (for example, in age andmetallicity), astronomers have been especially interested in making detailed searches for planets in the Alpha Centauri system. Several established planet-hunting teams have used variousradial velocity or startransit methods in their searches around these two bright stars.[137] All the observational studies have so far failed to find evidence forbrown dwarfs orgas giants.[137][138]
In 2009, computer simulations showed that a planet might have been able to form near the inner edge of Alpha Centauri B's habitable zone, which extends from0.5–0.9 AU from the star. Certain special assumptions, such as considering that the Alpha Centauri pair may have initially formed with a wider separation and later moved closer to each other (as might be possible if they formed in a densestar cluster), would permit an accretion-friendly environment farther from the star.[139] 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.[140] A theoretical study indicates that a radial velocity analysis might detect a hypothetical planet of1.8M🜨 in Alpha Centauri B'shabitable zone.[141]
Radial velocity measurements of Alpha Centauri B made with theHigh Accuracy Radial Velocity Planet Searcherspectrograph were sufficiently sensitive to detect a4M🜨 planet within the habitable zone of the star (i.e. with an orbital period P = 200 days), but no planets were detected.[125]
Current estimates place the probability of finding an Earth-like planet around Alpha Centauri at roughly 75%.[142] The observational thresholds for planet detection in the habitable zones by the radial velocity method are currently (2017) estimated to be about53M🜨 for Alpha Centauri A,8.4M🜨 for Alpha Centauri B, and0.47M🜨 forProxima Centauri.[143]
Early computer-generated models of planetary formation predicted the existence ofterrestrial planets aroundboth Alpha Centauri A and B,[141][144] but most recent numerical investigations have shown that the gravitational pull of the companion star renders the accretion of planets difficult.[139][145] Despite these difficulties, given the similarities to the Sun inspectral 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 harbouringextraterrestrial life on a potential planet.[6][140][146][144]
In theSolar System, it was once thought thatJupiter andSaturn were probably crucial in perturbingcomets into the inner Solar System, providing the inner planets with a source of water and various other ices.[147] However, since isotope measurements of thedeuterium tohydrogen (D/H) ratio in cometsHalley,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 theα Centauri system, Proxima Centauri may have influenced the planetary disk as theα Centauri system was forming, enriching the area around Alpha Centauri with volatile materials.[148] This would be discounted if, for example,α Centauri B happened to havegas giants orbitingα Centauri A (or vice versa), or ifα Centauri A 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.[147] Such icy bodies probably also reside inOort clouds of other planetary systems. When they are influenced gravitationally by either the gas giants or disruptions by passing nearby stars, many of these icy bodies then travel star-wards.[147] Such ideas also apply to the close approach of Alpha Centauri or other stars to the Solar system, when, in the distant future, theOort Cloud might be disrupted enough to increase the number of active comets.[97]
To be in thehabitable zone, a planet around Alpha Centauri A would have an orbital radius of between about 1.2 and2.1 AU so as to have similar planetary temperatures and conditions for liquid water to exist.[149] For the slightly less luminous and coolerα Centauri B, the habitable zone is between about 0.7 and1.2 AU.[149]
With the goal of finding evidence of such planets, both Proxima Centauri andα Centauri AB were among the listed "Tier-1" target stars forNASA'sSpace Interferometry Mission (S.I.M.). Detecting planets as small as three Earth-masses or smaller within two AU of a "Tier-1" target would have been possible with this new instrument.[150] The S.I.M. mission, however, was cancelled due to financial issues in 2010.[151]
Based on observations between 2007 and 2012, a study found a slight excess of emissions in the 24 μm (mid/far-infrared) band surroundingα Centauri AB, which may be interpreted as evidence for a sparsecircumstellar disc or denseinterplanetary dust.[152] The total mass was estimated to be between 10−7 to 10−6 the mass of theMoon, or 10–100 times the mass of the Solar System'szodiacal cloud.[152] If such a disc existed around both stars,α Centauri A's disc would likely be stable to2.8 AU, andα Centauri B's would likely be stable to2.5 AU[152] This would put A's disc entirely within thefrost line, and a small part of B's outer disc just outside.[152]
Looking towards the sky aroundOrion from Alpha Centauri withSirius nearBetelgeuse,Procyon inGemini, and the Sun inCassiopeia generated byCelestiaSimulated night-sky image with a "W" of stars fromCassiopeia connected by lines, and the Sun, labeled "Sol", as it would appear to the left of the "W"
The sky fromα Centauri AB would appear much as it does from the Earth, except thatCentaurus's brightest star, beingα Centauri AB itself, would be absent from the constellation. The Sun would appear as a white star ofapparent magnitude +0.5,[153] roughly the same as the average brightness ofBetelgeuse from Earth. It would be at theantipodal point ofα Centauri AB's currentright ascension anddeclination, at02h 39m 36s +60° 50′ 02.308″ (2000), in easternCassiopeia, easily outshining all the rest of the stars in theconstellation. With the placement of the Sun east of the magnitude 3.4 starEpsilon Cassiopeiae, nearly in front of theHeart Nebula, the "W" line of stars of Cassiopeia would have a "/W" shape.[154]
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 inOrion less than a degree away from Betelgeuse.Procyon, which would also be at a slightly further distance than from the Sun, would move to outshinePollux in the middle ofGemini.
A planet around eitherα Centauri A or B would see the other star as a very bright secondary. For example, an Earth-like planet at1.25 AU fromα Cen A (with a revolution period of 1.34 years) would get Sun-like illumination from its primary, andα Cen B would appear 5.7–8.6 magnitudes dimmer (−21.0 to −18.2), 190–2,700 times dimmer thanα Cen A but still 150–2,100 times brighter than the full Moon. Conversely, an Earth-like planet at0.71 AU fromα Cen B (with a revolution period of 0.63 years) would get nearly Sun-like illumination from its primary, andα Cen A would appear 4.6–7.3 magnitudes dimmer (−22.1 to −19.4), 70 to 840 times dimmer thanα Cen B 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.[155]
Diagram of theclosest stars to the Sun, within 7.5 light years
Alpha Centauri is a first target for crewed or roboticinterstellar exploration. Using current spacecraft technologies, crossing the distance between the Sun and Alpha Centauri would take several millennia, though the possibility ofnuclear pulse propulsion or laserlight sail technology, as considered in theBreakthrough Starshot program, could make the journey to Alpha Centauri in 20 years.[156][157][158] An objective of such a mission would be to make a fly-by of, and possibly photograph, planets that might exist in the system.[159][160] The existence ofProxima Centauri b, announced by theEuropean Southern Observatory (ESO) in August 2016, would be a target for the Starshot program.[159][161]
NASA released a mission concept in 2017 that wouldsend a spacecraft to Alpha Centauri in 2069, scheduled to coincide with the 100th anniversary of the first crewed lunar landing in 1969,Apollo 11. Even at 10% of thespeed 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.[162][163]
Alpha Centauri has been recognized and associated throughout history, particularly in theSouthern Hemisphere.Polynesians have been using Alpha Centauri for theirstar navigation and have called it Kamailehope. In theNgarrindjeri culture of Australia, Alpha Centauri represents withBeta Centauri two sharks chasing astingray, theSouthern Cross, and inIncan culture it with Beta Centauri form the eyes of allama-shapeddark constellation embedded in the band of stars that the visibleMilky 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.[164]
^Proxima Centauri is gravitationally bound to theα Centauri system, but for practical and historical reasons it is described in detail in its own article.
^Spellings include Rigjl Kentaurus,[32] Portuguese Riguel Kentaurus,[33][34]
^This is calculated for a fixed latitude by knowing the star'sdeclination (δ) using the formulae (90°+δ).α Centauri's declination is −60° 50′, so the observedlatitude where the star is circumpolar will be south of −29° 10′ South or 29°. Similarly, the place where Alpha Centauri never rises for northern observers is north of the latitude (90°+δ) N or +29° North.
^Proper motions are expressed in smaller angular units than arcsec, being measured in milliarcsec (mas.) (thousandths of an arcsec). Negative values for proper motion in RA indicate the sky motion is from east to west, and in declination north to south.
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^Marshall Eubanks, T.; Hein, Andreas M.; Lingam, Manasvi; Hibberd, Adam; Fries, Dan; Perakis, Nikolaos; et al. (2021). "Interstellar objects in the Solar System: 1. Isotropic kinematics from the Gaia early data release 3".arXiv:2103.03289 [astro-ph.EP].
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^Schaaf, Fred (2008)."Alpha Centauri".The Brightest Stars: Discovering the Universe through the Sky's Most Brilliant Stars. Wiley. pp. 122–123.ISBN978-0-471-70410-2.The first great science-fiction story in which Alpha Centauri played a major role may have been a 1944 tale by A. E. van Vogt. I read it in a much later anthology when I was a kid. The title of the tale—including the sound of that title—was what really filled me with admiration and has stuck with me ever since: "Far Centaurus." Although the name Proxima Centauri basically means "near Centaurus," the title of the story is appropriate because the tale tells of a first spaceship journey that would take many generations to complete—"'Tis for far Centaurus we sail!"
^Stableford, Brian (2004)."Barton, William R.".Historical Dictionary of Science Fiction Literature. Scarecrow Press. p. 23.ISBN978-0-8108-4938-9.Alpha Centauri (1997), in which terrorists plague the colony ship which is humankind's last hope
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