Alpha Centauri

Alpha Centauri
Alpha Centauri A[1]/B[2]
Alpha centauri.png
The position of Alpha Centauri A and Alpha Centauri B
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Centaurus
Alpha Centauri A
Right ascension 14h 39m 36.4951s
Declination -60° 50′ 02.308″
Apparent magnitude (V) −0.01
Alpha Centauri B
Right ascension 14h 39m 35.0803s
Declination -60° 50′ 13.761″
Apparent magnitude (V) +1.33
Spectral type G2V / K1V[3][4]
U−B color index +0.23 / +0.63
B−V color index +0.69 / +0.90
Radial velocity (Rv) −21.6 km/s
Proper motion (μ) RA: −3678.19 mas/yr
Dec.: 481.84 mas/yr
Parallax (π) 747.1 ± 1.2[5] mas
Distance 4.366 ± 0.007 ly
(1.339 ± 0.002 pc)
Absolute magnitude (MV) 4.38 / 5.71
Alpha Centauri A
Mass 1.100[6] M
Radius 1.227[6] R
Surface gravity (log g) 4.30[7]
Luminosity 1.519[6] L
Temperature 5790[6] K
Metallicity 151%[6] Sun
Rotation 22 days[8]
Age 4.85×109[6] years
Alpha Centauri B
Mass 0.907[6] M
Radius 0.865[6] R
Surface gravity (log g) 4.37[7]
Luminosity 0.500[6] L
Temperature 5260[6] K
Metallicity 160%[6] Sun
Rotation 41 days[8]
Age 4.85×109[6] years
Companion Alpha Centauri AB
Period (P) 79.91 ± 0.011 yr
Semimajor 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, FK5 538, CP(D)−60°5483, GC 19728, CCDM J14396-6050

α Cen A

α1 Centauri, GJ 559 A, HR 5459, HD 128620, GCTP 3309.00, LHS 50, SAO 252838, HIP 71683

α Cen B

α2 Centauri, GJ 559 B, HR 5460, HD 128621, LHS 51, HIP 71681

α Cen C (= Proxima Cen)

LHS 49, HIP 70890
Database references

Alpha Centauri (α Centauri, α Cen; also known as Rigil Kentaurus, Rigil Kent play /ˈrəl/, or Toliman) is the brightest star in the southern constellation of Centaurus. Although it appears to the unaided eye as a single object, Alpha Centauri is actually a binary star system (designated Alpha Centauri AB or α Cen AB) whose combined visual magnitude of -0.27 would qualify it as the third single brightest star in the night sky after -0.72 magnitude Canopus.

Its individual component stars are named Alpha Centauri A (α Cen A), with 110% of the mass and 151.9% the luminosity of our Sun, and Alpha Centauri B (α Cen B), at 90.7% of the Sun's mass and 50.0% of its luminosity. During the stars' 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. They average 1.34 parsecs or 4.37 light years away from the Sun.[10]

A third star, known as Proxima Centauri, Proxima or Alpha Centauri C (α Cen C), is probably gravitationally associated with Alpha Centauri AB. Proxima is now placed 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 true separation of Proxima from Alpha Centauri AB is about 0.06 parsecs, 0.2 light years or 13,000 astronomical units (AU), equivalent to 400 times the size of Neptune's orbit.


Component designations

"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. With the aid of a telescope, Alpha Centauri can be resolved into a binary star system in close orbit. This is known as the Alpha Centauri AB system, often abbreviated as α Centauri AB or α Cen AB.

Alpha Centauri A (α Cen A) and Alpha Centauri B (α Cen B) are the individual stars of the binary system, usually defined to identify them as the different component of the binary α Cen AB. As viewed from Earth, there is probably an additional companion located 2.2° away from the AB star system, whose distance is much greater than the observed separation between stars A and B. This companion is Proxima Centauri, Proxima, or α Cen C. 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, and they are assumed to be gravitationally associated with each other. Direct evidence that Proxima Centauri has an elliptical orbit typical of binary stars has yet to be determined.[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.

Nature of the system

At −0.27v visual magnitude,[12] Alpha Centauri appears to the naked eye as a single star and is fainter than Sirius and Canopus. The next brightest star in the night sky is Arcturus. When considered among the individual brightest stars in the sky (excluding the Sun), Alpha Centauri A is the fourth brightest at −0.01 magnitude,[13] being only fractionally fainter than Arcturus at −0.04v magnitude. Alpha Centauri B at 1.33v magnitude is twenty-first in brightness.

Component Sizes and Colours. Shows the relative sizes and colours of stars in the Alpha Centauri system and compares them to the Sun.

Alpha Centauri A is the principal member or primary of the binary system, being slightly larger and more luminous than our Sun. It is a solar-like main sequence star with a similar yellowish-white color, whose stellar classification is spectral type G2 V.[13] From the determined mutual orbital parameters, Alpha Centauri A is about 10% more massive than our Sun, with a radius about 23% larger.[6] The projected rotational velocityv·sin i ) of this star is 2.7±0.7 km·s−1, resulting in an estimated rotational period of 22 days,[8] which gives it a slightly faster rotational period than our Sun's 25 days.

Alpha Centauri B is the companion star or secondary, slightly smaller and less luminous than our Sun. This main sequence star is of spectral type K1 V,[4][13] making it more an orange-yellow color than the whiter primary star. Alpha Centauri B is about 90% the mass of the Sun and 14% smaller in radius.[6] The projected rotational velocity ( v·sin i ) is 1.1±0.8 km·s−1, resulting in an estimated rotational period of 41 days.[8] (An earlier estimate gave a similar rotation period of 36.8 days.)[14] Although it has a lower luminosity than component A, star B's spectrum emits higher energies in X-rays. The light curve of B varies on a short time scale and there has been at least one observed flare.[15]

Alpha Centauri C, also known as Proxima Centauri, is of spectral class M5Ve[13] or M5VIe, suggesting this is either a small main sequence star (Type V) or sub-dwarf (VI) with emission lines, whose B-V color index is +1.90. Its mass is about 0.12 M,[16] only 12.3% of a solar mass, or 129 Jupiter masses.[17]

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.[18] "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. Since 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.[19]

Alpha Centauri A and B are believed to have formed around the same timeframe, and are estimated to be approximately 4.85 billion years old, around 250 million years older than the Sun.[6]


The Alpha Centauri AB binary is too close to be resolved by the naked eye, as the angular separation varies between 2 and 22 arcsec,[20] but through much of the orbit, both are easily resolved in binoculars or small 5 cm (2 in) telescopes.[21]

In the southern hemisphere, Alpha Centauri forms the outer star of The Pointers or The Southern Pointers[21], so called because the line through Beta Centauri (Hadar/Agena),[22] some 4.5° west,[21] points directly to the constellation Crux—the Southern Cross.[21] The Pointers easily distinguish the true Southern Cross from the fainter asterism known as the False Cross.[23]

South of about −29° S latitude, Alpha Centauri is circumpolar and never sets below the horizon.[24] 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 and Chihuahua in Mexico, Galveston, Texas, and Ocala, Florida) during the northern summer, Alpha Centauri lies close to the southern horizon.[22] The star culminates each year at midnight on 24 April or 9 p.m. on 8 June.[22][25]

As seen from Earth, Proxima Centauri lies 2.2° southwest from Alpha Centauri AB.[26] 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 13.1v visual magnitude in a poorly 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 to about 11.0v or 11.09V magnitude.[13] Some amateur and professional astronomers regularly monitor for outbursts using either optical or radio telescopes.[27]

Observational history

According to the renowned double star observer Robert Aitken (1961), Father Richaud discovered Alpha Centauri AB's duplicity from the Indian city of Pondicherry in December 1689 while observing a comet.[28][29] By 1752, French astronomer Abbé Nicolas Louis de Lacaille made astrometric positional measurements using a meridian circle while John Herschel, in 1834, made the first micrometrical observations.[30] Since the early 20th Century, measures have been made with photographic plates.[31]

By 1926, South African astronomer William Stephen Finsen calculated the approximate orbit elements close to those now accepted for this system.[32] All future positions are now sufficiently accurate for visual observers to determine the relative places of the stars from a binary star ephemeris.[33] Others, like the Belgian astronomer D. Pourbaix (2002), have regularly refined the precision of any new published orbital elements.[29]

Alpha Centauri A and B resolved over the limb of Saturn, as seen by Cassini–Huygens

Alpha Centauri is the closest star system to our Solar System. It lies about 4.37 light-years in distance, or about 41.5 trillion kilometres, 25.8 trillion miles or 277,600 AU. Astronomer Thomas James Henderson made the original discovery from many exacting observations of the trigonometric parallaxes of the AB system between April 1832 and May 1833. He withheld the results because he suspected they were too large to be true, but eventually published in 1839 after Friedrich Wilhelm Bessel released his own accurately determined parallax for 61 Cygni in 1838.[34] For this reason, Alpha Centauri is considered as the second star to have its distance measured because it was not formally recognized first.[34]

R.T.A. Innes from South Africa 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 4.22 light-years away, Proxima Centauri is the nearest star to the Sun. All current derived distances for the three stars are presently from the parallaxes obtained from the Hipparcos star catalog (HIP).[35][36][37][38]

Binary system

Apparent and True Orbits of Alpha Centauri. Motion is shown from the A component against the relative orbital motion of B component. The Apparent Orbit (thin ellipse) is the shape of the orbit as seen by the 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 [9] the radial separation of A and B along the line of sight had reached a maximum in 2007 with B being behind A. Since 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,[29] 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).[29][39] This is a consequence of the binary's moderate orbital eccentricity e = 0.5179 [29] From the orbital elements, the total mass of both stars is about 2.0 M[40]—or twice that of the Sun.[39] The average individual stellar masses are 1.09 M and 0.90 M, respectively,[41] though slightly higher masses have been quoted in recent years, such as 1.14 M and 0.92 M,[13] or totalling 2.06 M. Alpha Centauri A and B have absolute magnitudes of +4.38 and +5.71, respectively.[13][31] Stellar evolution theory implies both stars are slightly older than the Sun[6] at 5 to 6 billion years, as derived by both mass and their spectral characteristics.[26][42]

Viewed from Earth, the apparent orbit of this binary star means that the separation and position angle (P.A.) are in continuous change throughout the 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.[29] Next closest approach will be in February 2016, at 4.0 arcsec through 300°.[29][43] Observed maximum separation of these stars is about 22 arcsec, while the minimum distance is 1.7 arcsec.[44] Widest separation occurred during February 1976 and the next will be in January 2056.[29]

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 presently rapidly decreasing, at least until 2019.[29]

Companion: Proxima Centauri

The much fainter red dwarf star named Proxima Centauri, or simply Proxima, is about 12,000 to 13,000 A.U. away from Alpha Centauri AB.[18][26][31] This is equivalent to 0.21 light years or 1.94 trillion kilometres—about 5% the distance between the Sun and Alpha Centauri AB. Proxima may be gravitationally bound to Alpha Centauri AB, orbiting it with a period between 100,000 and 500,000 years.[26] However, it is also possible that Proxima is not gravitationally bound and thus is moving along a hyperbolic trajectory[45] around Alpha Centauri AB.[18] The main evidence for a bound orbit is that Proxima's association with Alpha Centauri AB is unlikely to be accidental, since they share approximately the same motion through space.[26] Theoretically, Proxima could leave the system after several million years.[46] It is not yet certain whether Proxima and Alpha are truly gravitationally bound.[47]

Proxima is an M5.5V spectral class red dwarf with an absolute magnitude of +15.53, which is considerably less than the Sun. By mass, Proxima is presently calculated as 0.123±0.06 M (rounded to 0.12 M) or about one-eighth that of the Sun.[48]

High proper motion star

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.[49] 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.[50]

Edmond Halley in 1718 found that some stars had significantly moved from their ancient astrometric positions.[51] For example, the bright star Arcturus (α Boo) in the constellation of Boŏtes showed an almost ½° difference in 1800 years,[52] as did the brightest star, Sirius, in Canis Major (α CMa).[53] Halley's positional comparison was Ptolemy's catalogue of stars contained in the Almagest[54] whose original data included portions from an earlier catalog by Hipparchos during the 1st century BCE.[55][56][57] Halley's proper motions were mostly for northern stars, so the southern star Alpha Centauri was not determined until the early 19th century.[44]

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.[58][59] He soon realised this system displayed an unusually high proper motion,[60] and therefore its observed true velocity through space should be much larger.[61][44] In this case, the apparent stellar motion was found using Abbé Nicolas Louis de Lacaille's astrometric observations of 1751–1752,[62] 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.[63][64] 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[46]. Using spectroscopy the mean radial velocity has been determined to be 25.1 ± 0.3 km/s towards the solar system.[65][66]

A more precise calculation involves taking into account the slight changes in the stellar distance by the star's own motion.[26][46] Alpha Centauri at present is slowly increasing the measured proper motion and trigonometric parallax as the stars approach us.[46][63] Changes are also observed in the size of the semi-major axis 'a' of the orbital ellipse increase by 0.03 arcsec per century as the stars currently approach us.[18][67] Also the orbital period of Alpha Centauri AB is slightly shorter by some 0.006 years per century, caused by the reduced time required for the light to travel to Earth as the distance reduces.[18] 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.[68][69][70] Some slight differences of about 0.5% in the measured proper motions are caused by Alpha Centauri AB's orbital motion.[63]

Based on these observed proper motions and radial velocities, Alpha Centauri will continue to slowly 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.[46] Then it will reach the stationary radial velocity (RVel) of 0.0 km/s and the maximum apparent magnitude of −0.86V – similar to present day Canopus. Soon after this close approach, the system will begin to move away from us, showing a positive radial velocity.[46]

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 Alpha Centauri's orbit around the galactic centre being highly tilted with respect to the plane of our Milky Way galaxy.[46]

Possibility of planets

The discovery of planets orbiting other star systems, including similar binary systems (Gamma Cephei), raises the possibility that 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. Since 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.[71] All the observational studies have so far failed to find any evidence for brown dwarfs or gas giant planets.[71][72]

However, computer simulations show that a planet might have been able to form within a distance of 1.1 AU (160 million km) of Alpha Centauri B and the orbit of that planet may remain stable for at least 250 million years.[73] 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 around A and B make the likelihood of terrestrial planets greater than otherwise.[74] As of 2002, technologies did not allow for terrestrial planets like Earth to be detected around Alpha Centauri.[74] But 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 Earth masses in the habitable zone of B within three years.[75]

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,[citation needed] though the possibility of space sail, or Nuclear Pulse Fusion technology may cut this down to a matter of decades.[76]

Theoretical planets

Some computer-generated models of planetary formation predict the existence of terrestrial planets around both Alpha Centauri A and B.[75][77][78] Other models also suggested that formation of gas giant planets similar to Jupiter and Saturn is unlikely because of the significant gravitational and angular momentum effects of this binary system.[79] Although highly speculative, 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.[80][81][82][83]

Some astronomers speculated that any possible terrestrial planets in the Alpha Centauri system may be bone dry or lack significant atmospheres. In our 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[84] but Proxima Centauri may have influenced the planetary disk as the Alpha Centauri system was forming enriching the area round Alpha Centauri A and B with volatile materials.[85] This would be discounted, if for example, Alpha Centauri B happened to have giant gas planets orbiting Alpha Centauri A (or conversely, Alpha Centauri A for Alpha Centauri B), or if the stars B and A themselves were able to successfully perturb comets into each other's inner system like Jupiter and Saturn presumably have done here. As comets probably also reside in some huge Oort Cloud located to the outer regions of stellar systems, when they are influenced gravitationally by either the giant gas planets or disruptions by passing nearby stars, many of these comets then travel sun-wards.[46] As yet, there is no direct evidence 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.[46]

To be in the star's habitable zone, any suspected Earth-like 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' orbit in our own 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, this distance would be closer to its star at about 0.7 AU (100 million km), being about the distance that Venus is from the Sun.[84][86]

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.[87] However, the SIM mission was cancelled due to financial issues in 2010.[88]

View from this system

Looking toward the Sun from Alpha Centauri in Celestia

Viewed from near the Alpha Centauri system, the sky would appear very much as it does for earthbound observers, except that Centaurus would be missing its brightest star. Our Sun would be a yellow +0.5 visual magnitude star in eastern Cassiopeia at the antipodal point of Alpha Centauri's current RA and Dec. 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.[89]

From Proxima itself, Alpha Centauri AB would appear like two close bright stars with the combined 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 magnitudes of Alpha Centauri A and B would be −6.5 and −5.2, respectively.[90]

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 an intensely bright star in the night sky showing a small but discernible disk.

For example, some theoretical Earth-like 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 magnitudes are much dimmer than the currently observed −26.7 magnitude for the Sun as viewed from the Earth. 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 −12.5 magnitude full Moon as seen from the Earth.

Also, if another similar Earth-like 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 6900 to 580 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 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.[84]

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. Also reading a book would be quite possible without any artificial light.[84] 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 Earth-like day and night cycle would return.

Origin of name and cultural significance

This prominent southern star commonly bears the proper name Rigil Kentaurus[91] (often shortened to Rigil Kent,[92] former Rigjl Kentaurus;[93][94] Riguel Kentaurus[95] in Portuguese), derived from the Arabic phrase Rijl Qantūris[92] (or Rijl al-Qantūris,[96] (Arabic: رجل أقنطورس) meaning "Foot of the Centaur)", but is most often referred to by its Bayer designation Alpha Centauri. An alternative name is Toliman, whose etymology may be Arabic al-Zulmān (الظلمان, meaning "the ostriches").[92] During the 19th century, the northern amateur popularist Elijah H. Burritt called the star Bungula,[97] possibly coined from "β" and the Latin ungula ("hoof").[92] This latter name is rarely used today.

In Chinese, 南門 (Nán Mén), meaning Southern Gate, refers to an asterism consisting of α Centauri and ε Centauri.[98] Consequently, α Centauri itself is known as 南門二 (Nán Mén Èr, English: the Second Star of Southern Gate.)[99]

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.[21]

Use in modern fiction

Alpha Centauri's relative proximity makes it in some ways the logical choice as "first port of call". Speculative fiction about interstellar travel often predicts eventual human exploration, and even the discovery and colonization of planetary systems. These themes are common to many works of science fiction and video games.

See also


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  4. ^ a b Datin, Kellie; Dewarf, LE.; Guinan, EF.; Carton, JM. (January, 2009). "FUSE Observations of alpha Centauri B". American Astronomical Society (American Astronomical Society) 213: 200. Bibcode 2009AAS...21340609D. 
  5. ^ Söderhjelm, Staffan (January 1999), "Visual binary orbits and masses POST HIPPARCOS", Astronomy and Astrophysics 341: 121–140, Bibcode 1999A&A...341..121S  See Table 3.
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  8. ^ a b c d Bazot, M.; et al. (2007). "Asteroseismology of α Centauri A. Evidence of rotational splitting". Astronomy and Astrophysics 470 (1): 295–302. Bibcode 2007A&A...470..295B. doi:10.1051/0004-6361:20065694. 
  9. ^ a b Pourbaix, D.; et al. (2002). "Constraining the difference in convective blueshift between the components of alpha Centauri with precise radial velocities". Astronomy and Astrophysics 386 (1): 208–85. arXiv:astro-ph/0202400. Bibcode 2002A&A...386..280P. doi:10.1051/0004-6361:20020287. 
  10. ^ Söderhjelm, Staffan (1999). "Visual binary orbits and masses post Hipparcos". Astronomy and Astrophysics 341 (1): 121–40. Bibcode 1999A&A...341..121S. 
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  16. ^ "SIMBAD query result: V* V645 Cen – Flare Star". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 2008-08-11. —some of the data is located under "Measurements".
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  24. ^ 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.
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  40. ^ [(11.2 + 35.6) / 2]3 / 79.912 = 2.0, see formula
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  64. ^ 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.
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  67. ^ The semi-major axis size is calculated from the changing radial velocity (v) in km/s, the distance of the Sun to α Centauri AB is therefore v/(4.74 AU/yr). Using the trigonometric parallax π in arcsec, the changes in a are found using Δa = −1.0227×10−6 × a× v × π /yr . Period changes (Tp) are calculated by Tp = P × (1 − v/c), where c is the speed of light in km/s .
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  70. ^ 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.
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  89. ^ The coordinates of the Sun would be diametrically opposite Alpha Centauri AB, at α=02h 39m 36.4951s, δ=+60° 50′ 02.308″
  90. ^ Computed; using in solar terms: 1.1 M and 0.92M, luminosities 1.57 and 0.51 L*/L, Sun magnitude −26.73v), 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).
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Coordinates: Sky map 14h 39m 36.4951s, −60° 50′ 02.308″

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