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Alpha Centauri

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Alpha Centauri

Alpha Centauri

α Centauri and β Centauri, with Proxima circled
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Centaurus
Alpha Centauri A
Right ascension 14h 39m 36.49400s[1]
Declination –60° 50′ 02.3737″[1]
Apparent magnitude (V) +0.01[2]
Alpha Centauri B
Right ascension 14h 39m 35.06311s[1]
Declination –60° 50′ 15.0992″[1]
Apparent magnitude (V) +1.33[2]
Spectral type G2V[3]
U−B color index +0.71[2]
B−V color index +0.24[2]
Spectral type K1V[3]
U−B color index +0.68[2]
B−V color index +0.88[2]
Radial velocity (Rv) −21.4 ± 0.76[4] km/s
Proper motion (μ) RA: −3679.25[1] mas/yr
Dec.: 473.67[1] mas/yr
Parallax (π) 754.81 ± 4.11[1] mas
Distance 4.32 ± 0.02 ly
(1.325 ± 0.007 pc)
Absolute magnitude (MV) 4.38[5]
Radial velocity (Rv) −18.6 ± 1.64[4] km/s
Proper motion (μ) RA: −3614.39[1] mas/yr
Dec.: 802.98[1] mas/yr
Parallax (π) 796.92 ± 25.90[1] mas
Distance 4.1 ± 0.1 ly
(1.25 ± 0.04 pc)
Absolute magnitude (MV) 5.71[5]
Alpha Centauri A
Mass 1.100[6] M
Radius 1.227[7] R
Luminosity 1.519[6] L
Surface gravity (log g) 4.30[8] cgs
Temperature 5,790[6] K
Metallicity 0.20[6]
Rotation 22[7] days
Age 4.5–7[9] Gyr
Alpha Centauri B
Mass 0.907[6] M
Radius 0.865[7] R
Luminosity 0.5002[6] L
Surface gravity (log g) 4.37[8] cgs
Temperature 5,260[6] K
Metallicity 0.23[6]
Rotation 41[7] days
Primary A
Companion B
Period (P) 79.91 ± 0.011 yr
Semi-major axis (a) 17.57 ± 0.022"
Eccentricity (e) 0.5179 ± 0.00076
Inclination (i) 79.205 ± 0.041°
Longitude of the node (Ω) 204.85 ± 0.084°
Periastron epoch (T) 1875.66 ± 0.012
Argument of periastron (ω)
231.65 ± 0.076°
Other designations
Rigil Kentaurus, Rigil Kent, Toliman, Bungula, Gliese 559, FK5 538, CD−60°5483, CCDM J14396-6050, GC 19728
α Cen A: α1 Centauri, HR 5459, HD 128620, GCTP 3309.00, LHS 50, SAO 252838, HIP 71683
α Cen B: α2 Centauri, HR 5460, HD 128621, LHS 51, HIP 71681
Database references
Exoplanet Archive data
Extrasolar Planets

Alpha Centauri (α Cen), also known as Rigil Kent () or Toliman, is the closest star system to the Solar System at 4.37 ly (1.34 pc). It consists of three stars, the pair Alpha Centauri A and Alpha Centauri B and a small and faint red dwarf, Alpha Centauri C—better known as Proxima Centauri—that is probably (but not certainly) gravitationally bound to the other two. To the unaided eye, the two main components appear as a single object of an apparent visual magnitude of −0.27, forming the brightest star in the southern constellation Centaurus and the third-brightest star in the night sky, only outshone by Sirius and Canopus.

Alpha Centauri A (α Cen A) has 110% of the mass and 151.9% the luminosity of the Sun, and Alpha Centauri B (α Cen B) is smaller and cooler, at 90.7% of the Sun's mass and 44.5% of its luminosity. During the pair's 79.91-year orbit about a common center, the distance between them varies from about that between Pluto and the Sun to that between Saturn and the Sun. Proxima is at the slightly smaller distance of 1.29 parsecs or 4.24 light years from the Sun, making it the closest star to the Sun, even though it is not visible to the naked eye. The separation of Proxima from Alpha Centauri AB is about 0.06 parsecs, 0.2 light years or 15,000 astronomical units (AU), equivalent to 500 times the size of Neptune's orbit.

Nature and components

The relative sizes and colors of stars in the Alpha Centauri system, compared to the Sun

Alpha Centauri is the name given to what appears as a single star to the naked eye and the brightest star in the southern constellation of Centaurus. At −0.27 apparent visual magnitude (calculated from A and B magnitudes), it is fainter only than Sirius and Canopus. The next-brightest star in the night sky is Arcturus. Alpha Centauri is a multiple-star system, with its two main stars being Alpha Centauri A (α Cen A) and Alpha Centauri B (α Cen B), usually defined to identify them as the different components of the binary α Cen AB. A third companion—Proxima Centauri (or Proxima or α Cen C)—has a distance much greater than the observed separation between stars A and B and is probably gravitationally associated with the AB system. As viewed from Earth, it is located at an angular separation of 2.2° from the two main stars. If it were bright enough to be seen without a telescope, Proxima Centauri would appear to the naked eye as a star separate from α Cen AB. Alpha Centauri AB and Proxima Centauri form a visual double star. Direct evidence that Proxima Centauri has an elliptical orbit typical of binary stars has yet to be found.[11] Together all three components make a triple star system, referred to by double-star observers as the triple star (or multiple star), α Cen AB-C.

Alpha Centauri A is the principal member, or primary, of the binary system, being slightly larger and more luminous than the Sun. It is a solar-like main-sequence star with a similar yellowish color,[12] whose stellar classification is spectral type G2 V. From the determined mutual orbital parameters, Alpha Centauri A is about 10% more massive than the Sun, with a radius about 23% larger. The projected rotational velocity ( v·sin i ) of this star is 2.7 ± 0.7 km·s−1, resulting in an estimated rotational period of 22 days,[13] which gives it a slightly faster rotational period than the Sun's 25 days. When considered among the individual brightest stars in the sky (excluding the Sun), Alpha Centauri A is the fourth brightest at an apparent visual magnitude of +0.01, being fractionally fainter than Arcturus at an apparent visual magnitude of −0.04.

Alpha Centauri B is the companion star, or secondary, of the binary system, and is slightly smaller and less luminous than the Sun. It is a main-sequence star of spectral type K1 V, making it more an orange color than the primary star.[12] Alpha Centauri B is about 90% the mass of the Sun and 14% smaller in radius. The projected rotational velocity ( v·sin i ) is 1.1 ± 0.8 km·s−1, resulting in an estimated rotational period of 41 days. (An earlier, 1995 estimate gave a similar rotation period of 36.8 days.)[14] Although it has a lower luminosity than component A, star B emits more energy in the X-ray band. The light curve of B varies on a short time scale and there has been at least one observed flare.[15] Alpha Centauri B at an apparent visual magnitude of 1.33 would be twenty-first in brightness if it could be seen independently of Alpha Centauri A.

Alpha Centauri C, also known as Proxima Centauri, is of spectral class M6 Ve, a small main-sequence star (Type V) with emission lines. Its B−V color index is +1.82 and its mass is about 0.123 solar masses (M), or 129 Jupiter masses.

Together, the bright visible components of the binary star system are called Alpha Centauri AB (α Cen AB). This "AB" designation denotes the apparent gravitational centre of the main binary system relative to other companion star(s) in any multiple star system.[16] "AB-C" refers to the orbit of Proxima around the central binary, being the distance between the centre of gravity and the outlying companion. Some older references use the confusing and now discontinued designation of A×B. Because the distance between the Sun and Alpha Centauri AB does not differ significantly from either star, gravitationally this binary system is considered as if it were one object.[17]

Asteroseismic studies, chromospheric activity, and stellar rotation (gyrochronology), are all consistent with the α Cen system being similar in age to, or slightly older than, the Sun, with typical ages quoted between 4.5 and 7 billion years (Gyr).[9] Asteroseismic analyses that incorporate the tight observational constraints on the stellar parameters for α Cen A and/or B have yielded age estimates of 4.85 ± 0.5 Gyr,[6] 5.0 ± 0.5 Gyr,[18] 5.2–7.1 Gyr,[19] 6.4 Gyr,[20] and 6.52 ± 0.3 Gyr.[21] Age estimates for stars A and B based on chromospheric activity (Calcium H & K emission) yield 4.4–6.5 Gyr, whereas gyrochronology yields 5.0 ± 0.3 Gyr.[9]


Alpha Centauri is located in 100x100
Alpha Centauri
Location of Alpha Centauri in Centaurus

The two Alpha Centauri AB binary stars are too close together to be resolved by the naked eye, because the angular separation varies between 2 and 22 arcsec,[22] but through much of the orbit, both are easily resolved in binoculars or small 5 cm (2 in) telescopes.[23]

In the southern hemisphere, Alpha Centauri forms the outer star of The Pointers or The Southern Pointers,[23] so called because the line through Beta Centauri (Hadar/Agena),[24] some 4.5° west,[23] points directly to the constellation Crux — the Southern Cross.[23] The Pointers easily distinguish the true Southern Cross from the fainter asterism known as the False Cross.[25]

South of about 29° S latitude, Alpha Centauri is circumpolar and never sets below the horizon.[26] Both stars, including Crux, are too far south to be visible for mid-latitude northern observers. Below about 29° N latitude to the equator (roughly Hermosillo, Chihuahua City in Mexico, Galveston, Texas, Ocala, Florida and Lanzarote, the Canary Islands of Spain) during the northern summer, Alpha Centauri lies close to the southern horizon.[24] The star culminates each year at midnight on 24 April or 9 p.m. on 8 June.[24][27]

As seen from Earth, Proxima Centauri lies 2.2° southwest from Alpha Centauri AB.[28] This is about four times the angular diameter of the Full Moon, and almost exactly half the distance between Alpha Centauri AB and Beta Centauri. Proxima usually appears as a deep-red star of an apparent visual magnitude of 13.1 in a sparsely populated star field, requiring moderately sized telescopes to see. Listed as V645 Cen in the General Catalogue of Variable Stars (G.C.V.S.) Version 4.2, this UV Ceti-type flare star can unexpectedly brighten rapidly by as much as 0.6 magnitudes at visual wavelengths, then fade after only a few minutes.[29] Some amateur and professional astronomers regularly monitor for outbursts using either optical or radio telescopes.[30]

Observational history

View of Alpha Centauri from the Digitized Sky Survey 2

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 is in the end of Eridanus. The third [Alpha Centauri] is in the right foote of the Centaure."[31]

The binary nature of Alpha Centauri AB was first recognized in December 1689 by astronomer and Jesuit priest Jean Richaud. The finding was made incidentally while observing a passing comet from his station in Puducherry. Alpha Centauri was only the second binary star system to be discovered, preceded only by Alpha Crucis.[32] By 1752, French astronomer Abbé Nicolas Louis de Lacaille made astrometric positional measurements using state-of-the-art instruments of that time.[33] Its large proper motion 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 of the AB system between April 1832 and May 1833. However, he withheld his results because he suspected they were too large to be true, but eventually published them in 1839 after Friedrich Wilhelm Bessel released his own accurately determined parallax for 61 Cygni in 1838.[34] For this reason, Alpha Centauri is sometimes considered as the second star to have its distance measured because Henderson's work was not fully recognized at first.[34] (The distance of Alpha is now reckoned at 4.396 light-years in distance, or about 41.6 trillion kilometres).

Later, John Herschel made the first micrometrical observations in 1834.[35] Since the early 20th century, measures have been made with photographic plates.[36]

By 1926, South African astronomer William Stephen Finsen calculated the approximate orbit elements close to those now accepted for this system.[37] All future positions are now sufficiently accurate for visual observers to determine the relative places of the stars from a binary star ephemeris.[38] Others, like the Belgian astronomer D. Pourbaix (2002), have regularly refined the precision of any new published orbital elements.[39] Alpha Centauri is inside the G-cloud, and the nearest known system to it is Luhman 16 at 3.6 light years.[40]

Scottish astronomer Robert Innes discovered Proxima Centauri in 1915 by blinking photographic plates taken at different times during a dedicated proper motion survey. This showed the large proper motion and parallax of the star was similar in both size and direction to those of Alpha Centauri AB, suggesting immediately it was part of the system and slightly closer to us than Alpha Centauri AB. Lying 4.24 light-years away, Proxima Centauri is the nearest star to the Sun. All current derived distances for the three stars are from the parallaxes obtained from the Hipparcos star catalog (HIP)[41][42][43][44] and the Hubble Space Telescope[45]

Binary system

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 vs. time [10] the radial separation of A and B along the line of sight had reached a maximum in 2007 with B being behind A. The orbit is divided here into 80 points, each step refers to a timestep of approx. 0.99888 years or 364.84 days.

With the orbital period of 79.91 years,[39] the A and B components of this binary star can approach each other to 11.2 astronomical units, equivalent to 1.67 billion km or about the mean distance between the Sun and Saturn, or may recede as far as 35.6 AU (5.3 billion km—approximately the distance from the Sun to Pluto).[39][46] This is a consequence of the binary's moderate orbital eccentricity e = 0.5179.[39] From the orbital elements, the total mass of both stars is about 2.0 M[47]—or twice that of the Sun.[46] The average individual stellar masses are 1.09 M and 0.90 M, respectively,[48] though slightly higher masses have been quoted in recent years, such as 1.14 M and 0.92 M,[49] or totalling 2.06 M. Alpha Centauri A and B have absolute magnitudes of +4.38 and +5.71, respectively. Stellar evolution theory implies both stars are slightly older than the Sun at 5 to 6 billion years, as derived by both mass and their spectral characteristics.[28][48]

Viewed from Earth, the apparent orbit of this binary star means that its separation and position angle (P.A.) are in continuous change throughout its projected orbit. Observed stellar positions in 2010 are separated by 6.74 arcsec through the P.A. of 245.7°, reducing to 6.04 arcsec through 251.8° in 2011.[39] The closest approach in the future will be in February 2016, at 4.0 arcsec through 300°.[39][50] The observed maximum separation of these stars is about 22 arcsec, while the minimum distance is 1.7 arcsec.[51] The widest separation occurred during February 1976 and the next will be in January 2056.[39]

In the true orbit, closest approach or periastron was in August 1955, and next in May 2035. Furthest orbital separation at apastron last occurred in May 1995 and the next will be in 2075. The apparent distance between the two stars is rapidly decreasing, at least until 2019.[39]

Proxima Centauri

The much fainter red dwarf Proxima Centauri, or simply Proxima, is about 15,000 AU away from Alpha Centauri AB.[16][28][36] This is equivalent to 0.24 light years or 2.2 trillion kilometres—about 5% the distance between Alpha Centauri AB and the Sun. Proxima is likely gravitationally bound to Alpha Centauri AB, orbiting it with a period between 100,000 and 500,000 years.[28] However, it is also possible that Proxima is not gravitationally bound and thus moving along a hyperbolic trajectory[52] with respect to Alpha Centauri AB.[16] The main evidence for a bound orbit is that Proxima's association with Alpha Centauri AB is unlikely to be coincidental, because they share approximately the same motion through space.[28] Theoretically, Proxima could leave the system after several million years.[53] It is not yet certain whether Proxima and Alpha Centauri are truly gravitationally bound.[54]

Relative positions of Sun, Alpha Centauri AB and Proxima Centauri. Gray dot is projection of Proxima Centauri, located at the same distance as Alpha Centauri AB.

Proxima is a red dwarf of spectral class M6 Ve with an absolute magnitude of +15.60, which is only a small fraction of the Sun's luminosity. By mass, Proxima is calculated as 0.123 ± 0.06 M (rounded to 0.12 M) or about one-eighth that of the Sun.[55]


Stars closest to the Sun, including Alpha Centauri (25 April 2014).[56]

All components of Alpha Centauri display significant proper motions against the background sky, similar to the first-magnitude stars Sirius and Arcturus. Over the centuries, this causes the apparent stellar positions to slowly change. Such motions define the high-proper-motion stars.[57] These stellar motions were unknown to ancient astronomers. Most assumed that all stars were immortal and permanently fixed on the celestial sphere, as stated in the works of the philosopher Aristotle.[58]

Edmond Halley in 1718 found that some stars had significantly moved from their ancient astrometric positions.[59] For example, the bright star Arcturus (α Boo) in the constellation of Boötes showed an almost 0.5° difference in 1800 years,[60] as did the brightest star, Sirius, in Canis Major (α CMa).[61] Halley's positional comparison was Ptolemy's catalogue of stars contained in the Almagest[62] whose original data included portions from an earlier catalog by Hipparchos during the 1st century BCE.[63][64][65] Halley's proper motions were mostly for northern stars, so the southern star Alpha Centauri was not determined until the early 19th century.[51]

Scottish-born observer Thomas James Henderson in the 1830s at the Royal Observatory at the Cape of Good Hope discovered the true distance to Alpha Centauri.[66][67] He soon realised this system displayed an unusually high proper motion,[68] and therefore its observed true velocity through space should be much larger.[69][51] In this case, the apparent stellar motion was found using Abbé Nicolas Louis de Lacaille's astrometric observations of 1751–1752,[70] by the observed differences between the two measured positions in different epochs. Using the Hipparcos Star Catalogue (HIP) data, the mean individual proper motions are −3678 mas/yr or −3.678 arcsec per year in right ascension and +481.84 mas/yr or 0.48184 arcsec per year in declination.[71][72] As proper motions are cumulative, the motion of Alpha Centauri is about 6.1 arcmin each century, and 61.3 arcmin or 1.02° each millennium. These motions are about one-fifth and twice, respectively, the diameter of the full moon.[53] Using spectroscopy the mean radial velocity has been determined to be around 20 km/s towards the Solar System.

As the stars of Alpha Centauri approach us, the measured proper motion and trigonometric parallax slowly increase.[28][53][53][71] Changes are also observed in the size of the semi-major axis of the orbital ellipse, increasing by 0.03 arcsec per century.[16] This change slightly shortens the observed orbital period of Alpha Centauri AB by some 0.006 years per century. This small effect is gradually decreasing until the star system is at its closest to us, and is then reversed as the distance increases again.[16] Consequently, the observed position angles of the stars are subject to changes in the orbital elements over time, as first determined by W. H. van den Bos in 1926.[73][74][75] Some slight differences of about 0.5% in the measured proper motions are caused by Alpha Centauri AB's orbital motion.[71]

Apparent motion of Alpha Centauri relative to Beta Centauri

Based on these observed proper motions and radial velocities, Alpha Centauri will continue to gradually brighten, passing just north of the Southern Cross or Crux, before moving northwest and up towards the celestial equator and away from the galactic plane. By about 29,700 AD, in the present-day constellation of Hydra, Alpha Centauri will be 1.00 pc or 3.26 ly away.[53] Then it will reach the stationary radial velocity (RVel) of 0.0 km/s and the maximum apparent magnitude of −0.86v (which is comparable to present-day magnitude of Canopus). However, even during the time of this nearest approach, the apparent magnitude of Alpha Centauri 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 for the next 210,000 years).[76]

The Alpha Centauri system will then begin to move away from the Solar System, showing a positive radial velocity.[53] Due to visual perspective, about 100,000 years from now, these stars will reach a final vanishing point and slowly disappear among the countless stars of the Milky Way. Here this once bright yellow star will fall below naked-eye visibility somewhere in the faint present day southern constellation of Telescopium (this unusual location results from the fact that Alpha Centauri's orbit around the galactic centre is highly tilted with respect to the plane of the Milky Way).[53]

In about 4000 years, the proper motion of Alpha Centauri will mean that from the point of view of Earth it will appear close enough to Beta Centauri to form an optical double star. Beta Centauri is in reality far more distant than Alpha Centauri.


Until the 1990s, technologies did not exist that could detect planets outside the Solar System.[77] Since then, exoplanet-detection capabilities have steadily improved to the point where Earth-mass planets can be detected.

Other possibly detected planets

On 25 March 2015, a scientific paper by Demory and colleagues published transit results for Alpha Centauri B using the Hubble Space Telescope for a total of 40 hours.[78] They evidenced a transit event possibly corresponding to a planetary body. 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 average of the likely orbits being 12.4 days with an impact parameter of around 0–0.3. Its orbit would likely have an eccentricity of 0.24 or less. If confirmed, this planet would be called Alpha Centauri Bc. This planet would also still be far too close to its parent star to harbour life.[79]

Possibility of additional planets

The discovery of planets orbiting other star systems, including similar binary systems (Gamma Cephei), raises the possibility that additional planets may exist in the Alpha Centauri system. Such planets could orbit Alpha Centauri A or Alpha Centauri B individually, or be on large orbits around the binary Alpha Centauri AB. Because both the principal stars are fairly similar to the Sun (for example, in age and metallicity), 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.[80] All the observational studies have so far failed to find any evidence for brown dwarfs or gas giants.[80][81]

In 2009, computer simulations (then unaware of the close-in planet Bb) showed that a planet might have been able to form near the inner edge of Alpha Centauri B's habitable zone, which extends from 0.5 to 0.9 AU from the star. Certain special assumptions, such as considering that Alpha Centauri A and B may have initially formed with a wider separation and later moved closer to each other (as might be possible if they formed in a dense star cluster) would permit an accretion-friendly environment farther from the star.[82] Bodies around A would be able to orbit at slightly farther distances due to A's stronger gravity. In addition, the lack of any brown dwarfs or gas giants in close orbits around A or B make the likelihood of terrestrial planets greater than otherwise.[77] Theoretical studies on the detectability via radial velocity analysis have shown that a dedicated campaign of high-cadence observations with a 1–m class telescope can reliably detect a hypothetical planet of 1.8 M in the habitable zone of B within three years.[83]

Radial velocity measurements of Alpha Centauri B with High Accuracy Radial Velocity Planet Searcher spectrograph ruled out planets of more than 4 M to the distance of the habitable zone of the star (orbital period P = 200 days).[84]

Alpha Centauri is envisioned as the first target for unmanned interstellar exploration. Crossing the huge distance between the Sun and Alpha Centauri using current spacecraft technologies would take several millennia, though the possibility of solar sail or nuclear pulse propulsion technology could cut this down to a matter of decades.[85]

Theoretical planets

Early computer-generated models of planetary formation predicted the existence of terrestrial planets around both Alpha Centauri A and B,[83][86][87] but most recent numerical investigations have shown that the gravitational pull of the companion star renders the accretion of planets very difficult.[82][88] 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.[5][77][89][90]

In the Solar System both Jupiter and Saturn were probably crucial in perturbing comets into the inner Solar System. Here, the comets provided the inner planets with their own source of water and various other ices.[91] In the Alpha Centauri system Proxima Centauri may have influenced the planetary disk as the Alpha Centauri system was forming, enriching the area around Alpha Centauri A and B with volatile materials.[92] This would be discounted if, for example, Alpha Centauri B happened to have gas giants orbiting Alpha Centauri A (or conversely, Alpha Centauri A for Alpha Centauri B), or if the stars B and A themselves were able to perturb comets into each other's inner system as Jupiter and Saturn presumably have done in the Solar System. Such icy bodies probably also reside in Oort 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 starwards.[53] There is no direct evidence yet of the existence of such an Oort cloud around Alpha Centauri AB, and theoretically this may have been totally destroyed during the system's formation.[53]

To be in the star's habitable zone, any suspected planet around Alpha Centauri A would have to be placed about 1.25 AU away – about halfway between the distances of Earth's orbit and Mars's orbit in the Solar System – so as to have similar planetary temperatures and conditions for liquid water to exist. For the slightly less luminous and cooler Alpha Centauri B, the habitable zone would lie closer at about 0.7 AU (100 million km), approximately the distance that Venus is from the Sun.[91][93]

With the goal of finding evidence of such planets, both Proxima Centauri and Alpha Centauri AB were among the listed "Tier 1" target stars for NASA's Space Interferometry Mission (SIM). Detecting planets as small as three Earth-masses or smaller within two astronomical units of a "Tier 1" target would have been possible with this new instrument.[94] The SIM mission, however, was cancelled due to financial issues in 2010.[95]

View from this system

Looking toward the sky around Orion from Alpha Centauri with Sirius near Betelgeuse, Procyon in Gemini, and the Sun between Perseus and Cassiopeia generated by Celestia

Viewed from near the Alpha Centauri system, the sky would appear very much as it does for an observer on Earth, except that Centaurus would be missing its brightest star. The Sun would be a yellow star of an apparent visual magnitude of +0.5 in eastern Cassiopeia, at the antipodal point of Alpha Centauri's current right ascension and declination, at 02h 39m 35s +60° 50′ (2000). This place is close to the 3.4-magnitude star ε Cassiopeiae. An interstellar or alien observer would find the \/\/ of Cassiopeia had become a /\/\/ shape[note 1] nearly in front of the Heart Nebula in Cassiopeia. Sirius lies less than a degree from Betelgeuse in the otherwise unmodified Orion and is with −1.2 a little fainter than from Earth but still the brightest star in the Alpha Centauri sky. Procyon is also displaced into the middle of Gemini, outshining Pollux, whereas both Vega and Altair are shifted northwestward relative to Deneb (which barely moves, due to its great distance)- giving the Summer Triangle a more equilateral appearance.

From Proxima itself, Alpha Centauri AB would appear like two close bright stars with the combined apparent magnitude of −6.8. Depending on the binary's orbital position, the bright stars would appear noticeably divisible to the naked eye, or occasionally, but briefly, as single unresolved star. Based on the calculated absolute magnitudes, the visual apparent magnitudes of Alpha Centauri A and B would be −6.5 and −5.2, respectively.[96]

View from a hypothetical planet

Artist's rendition of the view from a hypothetical airless planet orbiting Alpha Centauri A

An observer on a hypothetical planet orbiting around either Alpha Centauri A or Alpha Centauri B would see the other star of the binary system as an intensely bright object in the night sky, showing a small but discernible disk.

For example, some theoretical planet orbiting about 1.25 AU from Alpha Centauri A (so that the star appears roughly as bright as the Sun viewed from the Earth) would see Alpha Centauri B orbit the entire sky once roughly every one year and three months (or 1.3(4) a), the planet's own orbital period. Added to this would be the changing apparent position of Alpha Centauri B during its long eighty-year elliptical orbit with respect to Alpha Centauri A (comparable in speed to Uranus here). Depending on the position on its orbit, Alpha Centauri B would vary in apparent magnitude between −18.2 (dimmest) and −21.0 (brightest). These visual apparent magnitudes are much dimmer than the apparent magnitude of the Sun as viewed from the Earth (−26.7). The difference of 5.7 to 8.6 magnitudes means Alpha Centauri B would appear, on a linear scale, 2500 to 190 times dimmer than Alpha Centauri A (or the Sun viewed from the Earth), but also 190 to 2500 times brighter than the full Moon as seen from the Earth (−12.5).

Also, if another similar planet orbited at 0.71 AU from Alpha Centauri B (so that in turn Alpha Centauri B appeared as bright as the Sun seen from the Earth), this hypothetical planet would receive slightly more light from the more luminous Alpha Centauri A, which would shine 4.7 to 7.3 magnitudes dimmer than Alpha Centauri B (or the Sun seen from the Earth), ranging in apparent magnitude between −19.4 (dimmest) and −22.1 (brightest). Thus Alpha Centauri A would appear between 830 and 70 times dimmer than the Sun but some 580 to 6900 times brighter than the full Moon. During such planet's orbital period of 0.6(3) a, an observer on the planet would see this intensely bright companion star circle the sky just as we see with the Solar System's planets. Furthermore, Alpha Centauri A's sidereal period of approximately eighty years means that this star would move through the local ecliptic as slowly as Uranus with its eighty-four year period, but as the orbit of Alpha Centauri A is more elliptical, its apparent magnitude will be far more variable. Although intensely bright to the eye, the overall illumination would not significantly affect climate nor influence normal plant photosynthesis.[91]

An observer on the hypothetical planet would notice a change in orientation to VLBI reference points commensurate with the binary orbit periodicity plus or minus any local effects such as precession or nutation.

Assuming this hypothetical planet had a low orbital inclination with respect to the mutual orbit of Alpha Centauri A and B, then the secondary star would start beside the primary at 'stellar' conjunction. Half the period later, at 'stellar' opposition, both stars would be opposite each other in the sky. Then, for about half the planetary year the appearance of the night sky would be a darker blue – similar to the sky during totality at any total solar eclipse. Humans could easily walk around and clearly see the surrounding terrain, and reading a book would be quite possible without any artificial light.[91] After another half period in the stellar orbit, the stars would complete their orbital cycle and return to the next stellar conjunction, and the familiar day and night cycle would return.

Traditional names

The colloquial name of Alpha Centauri is Rigel Kent or Rigil Kent,[97] short for Rigil/Rigel Kentaurus,[98][note 2] the romanization of the Arabic name رجل القنطورس Rijl Qanṭūris,[97] from the phrase Rijl al-Qanṭūris "the foot of the Centaur".[99] This is sometimes further abbreviated to Rigel, though that is ambiguous with Beta Orionis, which is also called Rigel. Although the short form Rigel Kent is common in English, the stars are most often referred to by their Bayer designation Alpha Centauri.

Distances of the nearest stars from 20,000 years ago until 80,000 years in the future

A medieval name is Toliman, whose etymology may be Arabic الظلمان al-Ẓulmān "the ostriches".[97] During the 19th century, the northern amateur popularist Elijah H. Burritt used the now-obscure name Bungula,[100] possibly coined from "β" and the Latin ungula ("hoof").[97] 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.[23]

In Mandarin, 南門 Nán Mén, meaning Southern Gate, refers to an asterism consisting of α Centauri and ε Centauri. Consequently, α Centauri itself is known as 南門二 Nán Mén Èr, the Second Star of the Southern Gate.[101]

To the Australian aboriginal Boorong people of northwestern Victoria, Alpha and Beta Centauri are Bermbermgle,[102] two brothers noted for their courage and destructiveness, who speared and killed Tchingal "The Emu" (the Coalsack Nebula).[103] The form in Wotjobaluk is Bram-bram-bult.[102]


Alpha Centauri AB distance estimates
Source Parallax, mas Distance, pc Distance, ly Distance, Pm Ref.
Henderson (1839) 1160 ± 110 0.86 +0.09
2.81 +0.29
26.6 +2.8
Henderson (1842) 912.8 ± 64 1.10 +0.08
3.57 +0.27
33.8 +2.5
Maclear (1851) 918.7 ± 34 1.09 ± 0.04 3.55 +0.14
33.6 +1.3
Moesta (1868) 880 ± 68 1.14 +0.10
3.71 +0.31
35.1 +2.9
Gill & Elkin (1885) 750 ± 10 1.333 ± 0.018 4.35 ± 0.06 41.1 +0.6
Roberts (1895) 710 ± 50 1.41 +0.11
4.59 +0.35
43.5 +3.3
Woolley et al. (1970) 743 ± 7 1.346 ± 0.013 4.39 ± 0.04 41.5 ± 0.4 [109]
Gliese & Jahreiß (1991) 749.0 ± 4.7 1.335 ± 0.008 4.355 ± 0.027 41.20 ± 0.26 [110]
van Altena et al. (1995) 749.9 ± 5.4 1.334 ± 0.010 4.349 +0.032
41.15 +0.30
Perryman et al. (1997) (A and B) 742.12 ± 1.40 1.3475 ± 0.0025 4.395 ± 0.008 41.58 ± 0.08 [112][113][114][115]
Söderhjelm (1999) 747.1 ± 1.2 1.3385 +0.0022
4.366 ± 0.007 41.30 ± 0.07 [116]
van Leeuwen (2007) (A) 754.81 ± 4.11 1.325 ± 0.007 4.321 +0.024
40.88 ± 0.22 [117]
van Leeuwen (2007) (B) 796.92 ± 25.90 1.25 ± 0.04 4.09 +0.14
38.7 +1.3
RECONS TOP100 (2012) 747.23 ± 1.17[note 3] 1.3383 ± 0.0021 4.365 ± 0.007 41.29 ± 0.06 [49]

See also


  1. ^ The coordinates of the Sun would be diametrically opposite Alpha Centauri AB, at α=02h 39m 36.4951s, δ=+60° 50′ 02.308″
  2. ^ Spellings include Rigjl Kentaurus, Hyde T., "Ulugh Beighi Tabulae Stellarum Fixarum", Tabulae Long. ac Lat. Stellarum Fixarum ex Observatione Ulugh Beighi, Oxford, 1665, p. 142., Hyde T., "In Ulugh Beighi Tabulae Stellarum Fixarum Commentarii", op. cit., p. 67., Portuguese Riguel Kentaurus da Silva Oliveira, R., "Crux Australis: o Cruzeiro do Sul", Artigos: Planetario Movel Inflavel AsterDomus.
  3. ^ Weighted parallax based on parallaxes from van Altena et al. (1995) and Söderhjelm (1999).


  1. ^ a b c d e f g h i j
  2. ^ a b c d e f
  3. ^ a b
  4. ^ a b
  5. ^ a b c
  6. ^ a b c d e f g h i
  7. ^ a b c d
  8. ^ a b
  9. ^ a b c
  10. ^ a b
  11. ^
  12. ^ a b
  13. ^
  14. ^
  15. ^
  16. ^ a b c d e
  17. ^
  18. ^
  19. ^
  20. ^
  21. ^
  22. ^
  23. ^ a b c d e
  24. ^ a b c
  25. ^
  26. ^ This is calculated for a fixed latitude by knowing the star's declination (δ) using the formulae (90°+ δ). Alpha Centauri's declination is −60° 50′, so the latitude where the star is circumpolar will be south of −29° 10′S or 29°. Similarly, the place where Alpha Centauri never rises for northern observers is north of the latitude (90°+ δ) N or +29°N.
  27. ^
  28. ^ a b c d e f
  29. ^
  30. ^
  31. ^ Knobel, p. 416.
  32. ^
  33. ^
  34. ^ a b Pannekoek, A., "A Short History of Astronomy", Dover, 1989, pp. 345–6
  35. ^
  36. ^ a b
  37. ^ Aitken, R.G., "The Binary Stars", Dover, 1961, pp. 236–237.
  38. ^
  39. ^ a b c d e f g h
  40. ^
  41. ^
  42. ^
  43. ^
  44. ^
  45. ^ ,
  46. ^ a b Aitken, R.G., "The Binary Stars", Dover, 1961, p. 236.
  47. ^ \begin{smallmatrix}[(11.2+35.6)/2]^3/79.91^2=2.0\end{smallmatrix}, see formula
  48. ^ a b
  49. ^ a b
  50. ^
  51. ^ a b c Aitken, R.G., "The Binary Stars", Dover, 1961, p. 235.
  52. ^
  53. ^ a b c d e f g h i
  54. ^
  55. ^
  56. ^
  57. ^
  58. ^
  59. ^ Berry, A., "A History of Astronomy", Dover, 1989, pp. 357–358
  60. ^ Pannekoek, A., "A Short History of Astronomy", Dover, 1989
  61. ^
  62. ^
  63. ^ Newton R.R., "The Crime of Claudius Ptolemy", T. Baltimore: Johns Hopkins University Press, (1977)
  64. ^ Pannekoek, A., "A Short History of Astronomy", Dover, 1989, p. 157
  65. ^
  66. ^ a b
  67. ^
  68. ^ Pannekoek, A., "A Short History of Astronomy", Dover, 1989, p. 333
  69. ^
  70. ^
  71. ^ a b c European Space Agency: The Hipparcos and Tycho Catalogues Search facility(2008)
  72. ^ Proper motions are expressed in smaller angular units than arcsec, being measured in milli-arcsec (mas.) or one-thousandth of an arcsec. A negative value for proper motion in RA indicates the sky motion is east to west, in declination north to south.
  73. ^
  74. ^
  75. ^ Calculated as; θ − θo = μα × sin α × (t − to ), where; α = right ascension (in degrees), μα is the common proper motion (cpm.) expressed in degrees, and θ and θo are the current position angle and calculated position angle at the different epochs.
  76. ^ Sky and Telescope, April 1998 (p60), based on computations from HIPPARCOS data.
  77. ^ a b c
  78. ^
  79. ^ Twin Earths may lurk in our nearest star system
  80. ^ a b
  81. ^
  82. ^ a b
  83. ^ a b
  84. ^
  85. ^
  86. ^ Javiera Guedes, Terrestrial Planet Formation Around Alpha Cen B
  87. ^ see Lissauer and Quintana in references below
  88. ^
  89. ^
  90. ^
  91. ^ a b c d
  92. ^
  93. ^
  94. ^ "Planet Hunting by Numbers", (Press Release), NASA, Stars and Galaxies, Jet Propulsion Laboratory, 18 October 2006. Retrieved 24 April 2007.
  95. ^
  96. ^ Computed; using in solar terms: 1.1 M and 0.92 M, luminosities 1.57 and 0.51 L*/L, Sun magnitude −26.73(v), 11.2 to 35.6 AU orbit; The minimum luminosity adds planet's orbital radius to A–B distance (max) (conjunction). Max. luminosity subtracts the planet's orbital radius to A–B distance (min) (opposition).
  97. ^ a b c d Kunitzsch P., & Smart, T., A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations, Cambridge, Sky Pub. Corp., 2006, p. 27
  98. ^ Bailey, F., "The Catalogues of Ptolemy, Ulugh Beigh, Tycho Brahe, Halley, and Hevelius", Memoirs of Royal Astronomical Society, vol. XIII, London, 1843.
  99. ^ Davis Jr., G. A., "The Pronunciations, Derivations, and Meanings of a Selected List of Star Names,"Popular Astronomy, Vol. LII, No. 3, Oct. 1944, p. 16.
  100. ^ Burritt, E. H., Atlas, Designed to Illustrate the Geography of the Heavens, (New Edition), New York, F. J. Huntington and Co., 1835, pl. VII.
  101. ^ (Chinese) [ AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 27 日]
  102. ^ a b
  103. ^
  104. ^ Page 370, Page 371.
  105. ^ Page 98
  106. ^ Page 117/118
  107. ^ Page 188
  108. ^ Page 189/190
  109. ^
  110. ^
  111. ^
  112. ^
  113. ^
  114. ^
  115. ^
  116. ^
  117. ^
  118. ^

External links

  • SIMBAD observational data
  • Sixth Catalogue of Orbits of Visual Binary Stars U.S.N.O.
  • The Imperial Star – Alpha Centauri
  • Alpha Centauri – A Voyage to Alpha Centauri
  • Immediate History of Alpha Centauri
  • eSky : Alpha Centauri
  • Alpha Centauri at Constellation Guide

Hypothetical planets or exploration

  • Alpha Centauri System
  • O Sistema Alpha Centauri (Portuguese)
  • Alpha Centauri – Associação de Astronomia (Portuguese)

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