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# definitions - Alpha_Centauri

## Alpha Centauri(n.)

1.brightest star in Centaurus; second nearest star to the sun

# definition (more)

definition of Wikipedia

# analogical dictionary

star[Classe]

astronomy[Domaine]

AstronomicalBody[Domaine]

constellation - star[Hyper.]

étoile (fr)[Classe...]

astronomy[Domaine]

AstronomicalBody[Domaine]

Alpha Centauri (n.)

# Alpha Centauri

Observation data Characteristics Epoch J2000.0      Equinox J2000.0 The position of Alpha Centauri A and Alpha Centauri B 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[3][4] U−B color index +0.23 B−V color index +0.69 Spectral type K1V[3][4] U−B color index +0.63 B−V color index +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 Mass 1.100[6] M☉ Radius 1.227[6] R☉ Luminosity 1.519[6] L☉ Surface gravity (log g) 4.30[7] Temperature 5790[6] K Metallicity 151%[6] Sun Rotation 22 days[8] Age 4.85×109[6] years Mass 0.907[6] M☉ Radius 0.865[6] R☉ Luminosity 0.500[6] L☉ Surface gravity (log g) 4.37[7] Temperature 5260[6] K Metallicity 160%[6] Sun Rotation 41 days[8] Age 4.85×109[6] years Orbit[9] 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) (secondary) 231.65 ± 0.076° 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 SIMBAD data ARICNS data
Location of Alpha Centauri in Centaurus

Alpha Centauri (α Centauri, α Cen; also known as Rigil Kentaurus, Rigil Kent , 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 the −1.46 magnitude Sirius and the -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 the 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 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 2.2° from the AB system. 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 the 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 the 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 the Sun's 25 days.

Alpha Centauri B is the companion star or secondary, slightly smaller and less luminous than the 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 0.123 M,[16] 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]

## Observation

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
The two bright stars are (left) Alpha Centauri and (right) Beta Centauri. The faint red star in the center of the red circle is Proxima Centauri. Taken with Canon 85mm f/1.8 lens with 11 frames stacked, each frame exposed 30 seconds.

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 Boötes showed an almost 0.5° 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]

As the stars of Alpha Centauri approach us, the measured proper motion and trigonometric parallax slowly increase.[26][46][46][63] Changes are also observed in the size of the semi-major axis 'a' of the orbital ellipse, increasing by 0.03 arcsec per century.[18][67] Also the apparent orbital period of Alpha Centauri AB is slightly shorter, caused by the reduced time required for the light to travel to Earth as the distance reduces (Doppler effect). This effect is gradually reduced (until the star system is closest to us, and is then reversed as the distance increases again).[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 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.[46] 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).[71]

The Alpha Centauri system will then begin to move away from the Solar System, 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 the fact that Alpha Centauri's orbit around the galactic centre is highly tilted with respect to the plane of our Milky Way galaxy).[46]

## Apparent movement

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.

Apparent motion of Alpha Centauri relative to Beta Centauri

## 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.[72] All the observational studies have so far failed to find any evidence for brown dwarfs or gas giant planets.[72][73]

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.[74] 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.[75] As of 2002, technologies did not allow for terrestrial planets like Earth to be detected around Alpha Centauri.[75] 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.[76]

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 Solar sail, or Nuclear Pulse Fusion technology may cut this down to a matter of decades.[77]

### Theoretical planets

Early computer-generated models of planetary formation predicted the existence of terrestrial planets around both Alpha Centauri A and B.[76][78][79], but most recent numerical investigations have shown that the gravitational pull of the companion star renders the accretion of planets very difficult [80][81] 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.[82][83][84][85]

Some astronomers speculated that any possible terrestrial planets in the Alpha Centauri system may be bone dry or lack significant atmospheres. 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[86] 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.[87] 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'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, 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.[86][88]

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.[89] However, the SIM mission was cancelled due to financial issues in 2010.[90]

## View from this system

Looking toward the Sun from Alpha Centauri in Celestia
Looking toward the sky around Orion from Alpha Centauri with Sirius near Betelgeuse 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 earthbound observers, except that Centaurus would be missing its brightest star. The 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 [91] 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.

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.[92]

### 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[clarification needed] 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.[86]

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.[86] 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[93] (often shortened to Rigil Kent,[94] former Rigjl Kentaurus;[95][96] Riguel Kentaurus[97] in Portuguese), derived from the Arabic phrase Rijl Qantūris[94] (or Rijl al-Qantūris,[98] (Arabic: رجل القنطورس) 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").[94] During the 19th century, the northern amateur popularist Elijah H. Burritt called the star Bungula,[99] possibly coined from "β" and the Latin ungula ("hoof").[94] 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.[100] Consequently, α Centauri itself is known as 南門二 (Nán Mén Èr, English: the Second Star of Southern Gate.)[101]

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.

## References

1. ^
2. ^
3. ^ a b Hoffleit+ (1991). "The Stars of Centaurus". Yale University Observatory. Retrieved 2009-03-10.
4. ^ a b c 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.
6. Kervella, Pierre; Thevenin, Frederic (March 15, 2003). "A Family Portrait of the Alpha Centauri System". ESO. Archived from the original on 16 June 2008. Retrieved 2008-06-06.
7. ^ a b Gilli G.; Israelian G.; Ecuvillon A.; Santos NC.; Mayor M. (2006). "Abundances of Refractory Elements in the Atmospheres of Stars with Extrasolar Planets". Astronomy and Astrophysics 449 (2): 723–36. arXiv:astro-ph/0512219. Bibcode 2005astro.ph.12219G. DOI:10.1051/0004-6361:20053850.
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): 280–85. 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.
11. ^ Mason, B.D.; Wycoff, G.L. I. Hartkopf, W.I.. (2008). "Washington Visual Double Star Catalog, 2006.5 (WDS)". U.S. Naval Observatory.
12. ^ Burnham, Robert (1978). Burnham's Celestial Handbook. Courier Dover. p. 549. ISBN 0-486-23567-X.
13. Research Consortium on Nearby Stars (2007-09-17). "The One Hundred Nearest Star Systems". RECONS. Georgia State University. Archived from the original on 12 November 2007. Retrieved 2007-11-06.
14. ^ Guinan, E.; Messina, S. (1995). "IAU Circular 6259, Alpha Centauri B". Central Bureau for Astronomical Telegrams.
15. ^ Robrade, J.; Schmitt, J. H. M. M.; Favata, F. (2005). "X-rays from α Centauri – The darkening of the solar twin". Astronomy and Astrophysics 442 (1): 315–321. arXiv:astro-ph/0508260. Bibcode 2005A&A...442..315R. DOI:10.1051/0004-6361:20053314.
16. ^  —some of the data is located under "Measurements".
17. ^ Kervella, Pierre; Thevenin, Frederic (2003-03-15). "A Family Portrait of the Alpha Centauri System: VLT Interferometer Studies the Nearest Stars". ESO. Archived from the original on 7 June 2007. Retrieved 2007-07-09.
18. Heintz, W. D. (1978). Double Stars. D. Reidel. p. 19. ISBN 90-277-0885-1.
19. ^ Worley, C.E.; Douglass, G.G. (1996). Washington Visual Double Star Catalog, 1996.0 (WDS). United States Naval Observatory.
20. ^ Van Zyl, Johannes Ebenhaezer (1996). Unveiling the Universe: An Introduction to Astronomy. Springer. ISBN 3-540-76023-7.
21. Hartung, E.J.; Frew, David Malin, David (1994). "Astronomical Objects for Southern Telescopes". Cambridge University Press.
22. ^ a b c Norton, A.P., Ed. I. Ridpath (1986). Norton's 2000.0 :Star Atlas and Reference Handbook. Longman Scientific and Technical. pp. 39–40.
23. ^ Mitton, Jacquelin (1993). The Penguin Dictionary of Astronomy. Penguin Books. p. 148.
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.
25. ^ "'The '"Constellations : Part 2 Culmination Times"'". Southern Astronomical Delights. Retrieved 2008-08-06.
26. Matthews, R.A.J.; Gilmore, Gerard (1993). "Is Proxima really in orbit about α Cen A/B?". Monthly Notices of the Royal Astronomical Society 261: L5. Bibcode 1993MNRAS.261L...5M.
27. ^ Page, A.A. (1982). "Mount Tamborine Observatory". International Amateur-Professional Photoelectric Photometry Communication 10: 26. Bibcode 1982IAPPP..10...26P.
28. ^ Aitken, R.G., "The Binary Stars", Dover, 1961, p. 1.
29. Hartkopf, W.; Mason, D. M. (2008). "Sixth Catalog of Orbits of Visual Binaries". U.S. Naval Observatory.
30. ^ Herschel, J.F.W. (1847). Results of Astronomical Observations made during the years 1834,5,6,7,8 at the Cape of Good Hope; being the completion of a telescopic survey of the whole surface of the visible heavens, commenced in 1825.. Smith, Elder and Co, London.
31. ^ a b c Kamper, K.W.; Wesselink, A. J. (1978). "Alpha and Proxima Centauri". Astronomical Journal 83: 1653. Bibcode 1978AJ.....83.1653K. DOI:10.1086/112378.
32. ^ Aitken, R.G., "The Binary Stars", Dover, 1961, pp. 236–237.
33. ^
34. ^ a b Pannekoek, A., "A Short History of Astronomy", Dover, 1989, pp. 345–6
35. ^ "The Hipparcos Catalogue – R.A. 14h-19h, HIP: 68301-93276". ESA. Retrieved 2008-08-06.
36. ^ "Hipparcos Data Vol.8. (1997)". ESA. Archived from the original on 6 August 2008. Retrieved 2008-08-06.
37. ^ "The 150 Stars in the Hipparcos Catalogue Closest to the Sun (1997)". ESA. Retrieved 2008-08-06.
38. ^ "Contents of the Hipparcos Catalogue (1997)". ESA. Retrieved 2008-08-06.
39. ^ a b Aitken, R.G., "The Binary Stars", Dover, 1961, p. 236.
40. ^ $[(11.2+35.6)/2]^3/79.91^2=2.0$, see formula
41. ^ Kim, Y-C. J. (1999). "Standard Stellar Models; alpha Cen A and B". Journal of the Korean Astronomical Society 32: 119. Bibcode 1999JKAS...32..119K.
42. ^ Kim, Y-C. J. (1999). "Standard Stellar Models; alpha Cen A and B". Journal of the Korean Astronomical Society 32: 119. Bibcode 1999JKAS...32..119K.
43. ^ Andrew James (2008-03-11). "ALPHA CENTAURI : 6". Homepage.mac.com. Retrieved 2010-08-12.
44. ^ a b c Aitken, R.G., "The Binary Stars", Dover, 1961, p. 235.
45. ^ Anosova, J (1994). "Dynamics of nearby multiple stars. The α system". Astronomy and Astrophysics 292: 115. Bibcode 1994A&A...292..115A.
46. Matthews, R.A.J. (1994). "The Close Approach of Stars in the Solar Neighbourhood". Quarterly Journal of the Royal Astronomical Society 35: 1–8. Bibcode 1994QJRAS..35....1M.
47. ^ Wetheimer, J.G.; Gregory Laughlin (2008). "Are Proxima and Alpha Centauri Gravitationally Bound?". The Astronomical Journal 132 (5): 1995–1997. arXiv:astro-ph/0607401. Bibcode 2006AJ....132.1995W. DOI:10.1086/507771.
48. ^ Ségransan, D.; Kervella, P.; Forveille, T.; Queloz, D. (2003). "First radius measurements of very low mass stars with the VLTI". Astronomy and Astrophysics Letters 397 (3): L5–L8. arXiv:astro-ph/0211647. Bibcode 2003A&A...397L...5S. DOI:10.1051/0004-6361:20021714.
49. ^
50. ^
51. ^ Berry, A., "A History of Astronomy", Dover, 1989, pp. 357–358
52. ^ Pannekoek, A., "A Short History of Astronomy", Dover, 1989
53. ^ Holberg, JB (2007). Sirius: Brightest Diamond in the Night Sky. Praxis Publishing. pp. 41–42. ISBN 0-387-48941-X.
54. ^ Tung, Brian. "Star Catalogue of Ptolemy". The Astronomy Corner: Reference (2006).
55. ^ Newton R.R., "The Crime of Claudius Ptolemy", T. Baltimore: Johns Hopkins University Press, (1977)
56. ^ Pannekoek, A., "A Short History of Astronomy", Dover, 1989, p. 157
57. ^ Grasshoff, G. (1990). The History of Ptolemy's Star Catalogue. Springer. pp. 319–394. ISBN 0-387-97181-5.
58. ^ Astronomical Society of South Africa. "Henderson, Thomas [FRS (2008)"].
59. ^ Henderson, H. (1839). "On the parallax of α Centauri". Monthly Notices of the Royal Astronomical Society 4: 168. Bibcode 1839MNRAS...4..168H.
60. ^ Pannekoek, A., "A Short History of Astronomy", Dover, 1989, p. 333
61. ^ Maclear, M. (1851). "Determination of Parallax of α1and α2 Centauri". Astronomische Nachrichten 32 (16): 243. Bibcode 1851AN.....32..243.. DOI:10.1002/asna.18510321606.
62. ^ N.L., de La Caillé; Raven-Hart, R. (trans.& ed.) (1976). Travels at the Cape, 1751–1753: an annotated translation of Journal historique du voyage fait au Cap de Bonne-Espérance. Cape Town. ISBN 0-86961-068-6.
63. ^ a b c European Space Agency: The Hipparcos and Tycho Catalogues Search facility(2008)
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.
65. ^ Nordström, B.; et al. (2004). "The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of ~14000 F and G dwarfs". Astronomy and Astrophysics 418 (3): 989–1019. arXiv:astro-ph/0405198. Bibcode 2004A&A...418..989N. DOI:10.1051/0004-6361:20035959.
66. ^ HD 128620/1, database entry, The Geneva-Copenhagen Survey of Solar neighbourhood, J. Holmberg et al., 2007, CDS ID V/117A. Accessed on line 19 November 2008
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 .
68. ^ van den Bos, W. H. (1926). "A Table of Orbits of Visual Binary Stars (aka. First Orbit Catalogue of Binary Stars)". Bulletin of the Astronomical Institutes of the Netherlands 3: 149. Bibcode 1926BAN.....3..149V.
69. ^ van den Bos, W. H. (1926). "Table of Visual Binary Stars". Union Observatory Circular 2: 356.
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.
71. ^ Sky and Telescope, April 1998 (p60), based on computations from HIPPARCOS data.
72. ^ a b "Why Haven't Planets Been Detected around Alpha Centauri". Universe Today. Archived from the original on 21 April 2008. Retrieved 2008-04-19.
73. ^ Tim Stephens (7 March 2008). "Nearby star should harbor detectable, Earth-like planets". News & Events. UC Santa Cruz. Archived from the original on 17 April 2008. Retrieved 2008-04-19.
74. ^ Thebault, P., Marzazi, F., Scholl, H. (2009). "Planet formation in the habitable zone of alpha centauri B". Monthly Notices of the Royal Astronomical Society 393: L21–L25. arXiv:0811.0673. Bibcode 2009MNRAS.393L..21T. DOI:10.1111/j.1745-3933.2008.00590.x.
75. ^ a b Quintana, E. V.; Lissauer, J. J.; Chambers, J. E.; Duncan, M. J.; (2002). "Terrestrial Planet Formation in the Alpha Centauri System". Astrophysical Journal 2 (2): 982. Bibcode 2002ApJ...576..982Q. DOI:10.1086/341808.
76. ^ a b Javiera M. Guedes, Eugenio J. Rivera, Erica Davis, Gregory Laughlin, Elisa V. Quintana, Debra A. Fischer (2008). "Formation and Detectability of Terrestrial Planets Around Alpha Centauri B". Astrophysical Journal 679 (2): 1582–1587. arXiv:0802.3482. Bibcode 2008ApJ...679.1582G. DOI:10.1086/587799.
77. ^
78. ^ Javiera Guedes, Terrestrial Planet Formation Around Alpha Cen B
79. ^ see Lissauer and Quintana in references below
80. ^ Thebault, P., Marzazi, F., Scholl, H. (2009). "Planet formation in the habitable zone of alpha centauri B". Monthly Notices of the Royal Astronomical Society 393: L21–L25. arXiv:0811.0673. Bibcode 2009MNRAS.393L..21T. DOI:10.1111/j.1745-3933.2008.00590.x.
81. ^ M. Barbieri, F. Marzari, H. Scholl (2002). "Formation of terrestrial planets in close binary systems: The case of α Centauri A". Astronomy & Astrophysics 396 (1): 219 – 224. arXiv:astro-ph/0209118. Bibcode 2002A&A...396..219B. DOI:10.1051/0004-6361:20021357.
82. ^ P.A. Wiegert and M.J. Holman (1997). "The stability of planets in the Alpha Centauri system". The Astronomical Journal 113: 1445 – 1450. arXiv:astro-ph/9609106. Bibcode 1997AJ....113.1445W. DOI:10.1086/118360.
83. ^ Lissauer, J. J., E. V. Quintana, J. E. Chambers, M. J. Duncan, and F. C. Adams. (2004). "Terrestrial Planet Formation in Binary Star Systems". Revista Mexicana de Astronomia y Astrofisica (Serie de Conferencias) 22: 99–103. Bibcode 2004RMxAC..22...99L.
84. ^ Quintana, E. V.; Lissauer, J. J.; Chambers, J. E.; Duncan, M. J.; (2002). "Terrestrial Planet Formation in the Alpha Centauri System". Astrophysical Journal 2 (2): 982–996. Bibcode 2002ApJ...576..982Q. DOI:10.1086/341808.
85. ^ Quintana, E. V.; Lissauer, J. J.; (2007). "Terrestrial Planet Formation in Binary Star Systems". Planets in Binary Star Systems.
86. ^ a b c d Croswell, K. (1991). "Does Alpha Centauri Have Intelligent Life?". Astronomy 19: 28–37.
87. ^ "Proxima Centauri and Habitability". Centauri-dreams.org. 2006-07-05. Retrieved 2010-08-12.
88. ^ "If Alpha Centauri Has Earth-like Planets, Can We Detect Them?". Universe Today. Archived from the original on 14 March 2008. Retrieved 2008-03-10.
89. ^ "Planet Hunting by Numbers", (Press Release), NASA, Stars and Galaxies, Jet Propulsion Laboratory, 18 October 2006. Retrieved 24 April 2007.
90. ^ Mullen, Leslie (2 June 2011). "Rage Against the Dying of the Light". Astrobiology Magazine. Archived from the original on 4 June 2011. Retrieved 2011-06-07.
91. ^ The coordinates of the Sun would be diametrically opposite Alpha Centauri AB, at α=02h 39m 36.4951s, δ=+60° 50′ 02.308″
92. ^ 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).
93. ^ Bailey, F., "The Catalogues of Ptolemy, Ulugh Beigh, Tycho Brahe, Halley, and Hevelius", Memoirs of Royal Astronomical Society, vol. XIII, London, 1843.
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95. ^ Hyde T., "Ulugh Beighi Tabulae Stellarum Fixarum", Tabulae Long. ac Lat. Stellarum Fixarum ex Observatione Ulugh Beighi, Oxford, 1665, p. 142.
96. ^ Hyde T., "In Ulugh Beighi Tabulae Stellarum Fixarum Commentarii", op. cit., p. 67.
97. ^ da Silva Oliveira, R., "Crux Australis: o Cruzeiro do Sul", Artigos: Planetario Movel Inflavel AsterDomus.
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100. ^ (Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
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