Alpha Centauri

Alpha Centauri A^[1]/B^[2]

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
Characteristics
Spectral type	G2V[3][4]
U−B color index	+0.23
B−V color index	+0.69
Characteristics
Spectral type	K1V[3][4]
U−B color index	+0.63
B−V color index	+0.90
Astrometry
Radial velocity (Rv)	−25.1 ± 0.3[5][6] km/s
Proper motion (μ)	RA: −3642[3] mas/yr Dec.: 699[3] mas/yr
Parallax (π)	747.1 ± 1.2[7] mas
Distance	4.366 ± 0.007 ly (1.339 ± 0.002 pc)
Absolute magnitude (MV)	4.38 / 5.71
Details
Alpha Centauri A
Mass	1.100[8] M☉
Radius	1.227[8] R☉
Luminosity	1.519[8] L☉
Surface gravity (log g)	4.30[9] cgs
Temperature	5790[8] K
Metallicity	151%[8] Sun
Rotation	~22.5 ± 5.9 days[10]
Age	6 ± 1 Gyr
Alpha Centauri B
Mass	0.907[8] M☉
Radius	0.865[8] R☉
Luminosity	0.500[8] L☉
Surface gravity (log g)	4.37[9] cgs
Temperature	5260[8] K
Metallicity	160%[8] Sun
Rotation	47 days
Orbit[11]
Companion	Alpha Centauri AB
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) (secondary)	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
SIMBAD	data
Exoplanet Archive	data
ARICNS	data
Extrasolar Planets Encyclopaedia	data

Location of Alpha Centauri in Centaurus (right-click on starmap to enlarge)

Alpha Centauri (α Cen); also known as Rigil Kent (/ˈraɪdʒəl ˈkɛnt/) or Toliman, is the brightest star in the southern constellation of Centaurus and the third brightest star in the night sky.^[12][13] The Alpha Centauri system is located 1.34 parsecs or 4.37 light years from the Sun, making it the closest star system to our Solar System.^[7] 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 makes it the third brightest star (other than the Sun) seen from Earth after the −1.46 magnitude Sirius and the −0.72 magnitude Canopus.

Its 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 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.

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 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 400 times the size of Neptune's orbit.

The Alpha Centauri system has been reported to contain one planet, the Earth-sized Alpha Centauri Bb, which would be the closest known exoplanet to Earth,^[14][15][16] but its existence has been questioned.^[17][18]

Nature and components

"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 −0.27v visual magnitude,^[19] it is fainter only than Sirius and Canopus. The next brightest star in the night sky is Arcturus. Actually a multiple star system, its two main stars are 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.^[20] 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.

Component sizes and colors. Shows the relative sizes and colors of stars in the Alpha Centauri system and compares them with those of 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 color,^[21] whose stellar classification is spectral type G2 V.^[22] From the determined mutual orbital parameters, Alpha Centauri A is about 10% more massive than the Sun, with a radius about 23% larger.^[8] 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,^[10] 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 −0.01 magnitude,^[22] being fractionally fainter than Arcturus at −0.04v magnitude.

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,^[4][22] making it more an orange color than the primary star.^[21] Alpha Centauri B is about 90% the mass of the Sun and 14% smaller in radius.^[8] 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.)^[23] 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.^[24] Alpha Centauri B at 1.33v magnitude 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 M5 Ve^[22] or M5 VIe, suggesting this is either a small main-sequence star (Type V) or subdwarf (VI) with emission lines. Its B−V color index is +1.90 and its mass is about 0.123 solar masses (M_☉),^[25] or 129 Jupiter masses.^[26]

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.^[27] "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.^[28]

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).^[29] 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,^[30] 5.0 ± 0.5 Gyr,^[31] 5.2–7.1 Gyr,^[32] 6.4 Gyr,^[33] and 6.52 ± 0.3 Gyr.^[34] 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.^[29]

Observation

View of Alpha Centauri from the Digitized Sky Survey 2

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,^[35] but through much of the orbit, both are easily resolved in binoculars or small 5 cm (2 in) telescopes.^[36]

In the southern hemisphere, Alpha Centauri forms the outer star of The Pointers or The Southern Pointers,^[36] so called because the line through Beta Centauri (Hadar/Agena),^[37] some 4.5° west,^[36] points directly to the constellation Crux — the Southern Cross.^[36] The Pointers easily distinguish the true Southern Cross from the fainter asterism known as the False Cross.^[38]

South of about 29° S latitude, Alpha Centauri is circumpolar and never sets below the horizon.^[39] 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 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.^[37] The star culminates each year at midnight on 24 April or 9 p.m. on 8 June.^[37][40]

As seen from Earth, Proxima Centauri lies 2.2° southwest from Alpha Centauri AB.^[41] 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.^[22] Some amateur and professional astronomers regularly monitor for outbursts using either optical or radio telescopes.^[42]

Observational history

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."^[43]

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.^[44] By 1752, French astronomer Abbé Nicolas Louis de Lacaille made astrometric positional measurements using a meridian circle. Later, John Herschel made the first micrometrical observations in 1834.^[45] Since the early 20th century, measures have been made with photographic plates.^[46]

By 1926, South African astronomer William Stephen Finsen calculated the approximate orbit elements close to those now accepted for this system.^[47] All future positions are now sufficiently accurate for visual observers to determine the relative places of the stars from a binary star ephemeris.^[48] Others, like the Belgian astronomer D. Pourbaix (2002), have regularly refined the precision of any new published orbital elements.^[49]

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 the 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. Scottish astronomer Thomas 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.^[50] For this reason, Alpha Centauri is considered as the second star to have its distance measured because it was not formally recognized first.^[50] Alpha Centauri is inside the G-cloud, and the nearest known system to it is Luhman 16 at 3.6 light years.^[51]

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).^{[52][53][54][55]}

Distance

Alpha Centauri distance estimates

Non-trigonometric distance estimates are marked in italic. The most precise estimate is marked in bold.

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 ^[11] 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,^[49] 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).^[49][67] This is a consequence of the binary's moderate orbital eccentricity e = 0.5179.^[49] From the orbital elements, the total mass of both stars is about 2.0 M_☉^[68]—or twice that of the Sun.^[67] The average individual stellar masses are 1.09 M_☉ and 0.90 M_☉, respectively,^[69] though slightly higher masses have been quoted in recent years, such as 1.14 M_☉ and 0.92 M_☉,^[22] or totalling 2.06 M_☉. Alpha Centauri A and B have absolute magnitudes of +4.38 and +5.71, respectively.^[22][46] Stellar evolution theory implies both stars are slightly older than the Sun^[8] at 5 to 6 billion years, as derived by both mass and their spectral characteristics.^[41][69]

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.^[49] Next closest approach will be in February 2016, at 4.0 arcsec through 300°.^[49][70] Observed maximum separation of these stars is about 22 arcsec, while the minimum distance is 1.7 arcsec.^[71] Widest separation occurred during February 1976 and the next will be in January 2056.^[49]

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.^[49]

Companion: Proxima Centauri

The much fainter red dwarf named Proxima Centauri, or simply Proxima, is about 15,000 AU away from Alpha Centauri AB.^[27][41][46] 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.^[41] However, it is also possible that Proxima is not gravitationally bound and thus moving along a hyperbolic trajectory^[72] with respect to Alpha Centauri AB.^[27] 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.^[41] Theoretically, Proxima could leave the system after several million years.^[73] It is not yet certain whether Proxima and Alpha Centauri are truly gravitationally bound.^[74]

Proxima is a red dwarf of spectral class M5.5V with an absolute magnitude of +15.53, 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.^[75]

Kinematics

Stars closest to the Sun, including Alpha Centauri (25 April 2014).^[76]

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.^[77] 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.^[78]

Edmond Halley in 1718 found that some stars had significantly moved from their ancient astrometric positions.^[79] For example, the bright star Arcturus (α Boo) in the constellation of Boötes showed an almost 0.5° difference in 1800 years,^[80] as did the brightest star, Sirius, in Canis Major (α CMa).^[81] Halley's positional comparison was Ptolemy's catalogue of stars contained in the Almagest^[82] whose original data included portions from an earlier catalog by Hipparchos during the 1st century BCE.^[83][84][85] Halley's proper motions were mostly for northern stars, so the southern star Alpha Centauri was not determined until the early 19th century.^[71]

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.^[56][86] He soon realised this system displayed an unusually high proper motion,^[87] and therefore its observed true velocity through space should be much larger.^[88][71] In this case, the apparent stellar motion was found using Abbé Nicolas Louis de Lacaille's astrometric observations of 1751–1752,^[89] 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.^[90][91] 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.^[73] Using spectroscopy the mean radial velocity has been determined to be 25.1 ± 0.3 km/s towards the Solar System.^[5][6]

As the stars of Alpha Centauri approach us, the measured proper motion and trigonometric parallax slowly increase.^{[41][73][73][90]} Changes are also observed in the size of the semi-major axis of the orbital ellipse, increasing by 0.03 arcsec per century.^[27] 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.^[27] 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.^[92][93][94] Some slight differences of about 0.5% in the measured proper motions are caused by Alpha Centauri AB's orbital motion.^[90]

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.^[73] 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).^[95]

The Alpha Centauri system will then begin to move away from the Solar System, showing a positive radial velocity.^[73] 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).^[73]

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.

Planets

Until the 1990s, technologies did not exist that could detect planets outside the Solar System.^[96] Since then, exoplanet-detection capabilities have steadily improved to the point where Earth-mass planets can be detected.

Alpha Centauri Bb

On 16 October 2012, researchers, mainly from the Observatory of Geneva and from the Centre for Astrophysics of the University of Porto, announced that an Earth-mass (M_⊕) planet had been likely detected in orbit around Alpha Centauri B using the radial velocity technique.^[97][98] Over three years of observations had been needed for the difficult analysis,^[14] finding the planet orbiting very close to the host star at just 0.04 AU and completing one orbit every 3.236 days.^[15] It is not in Alpha Centauri B's habitable zone, with a surface temperature estimated to be 1200 °C (about 1500 K),^[99][100] far too hot for liquid water and also above the melting temperatures of many silicate magmas. For comparison, the surface temperature of Venus, the hottest planet in the Solar System, is 462 °C (735 K). Alpha Centauri Bb has a minimum mass of 1.13 M_⊕.^[15]

Other possibly detected planets

On 25 March 2015, a scientific paper by Demory et al. published transit results for Alpha Centauri B using the Hubble Space Telescope for a total of 40 hours.^[102] Although the team could rule out, with 96.6% confidence, transit events for Alpha Centauri Bb (which does not preclude its existence, merely the fact that it is in the same plane as the Sun and Alpha Centauri B), they evidenced a transit event possibly corresponding to a planetary body with different orbital parameters. 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.^[103]

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.^[104] All the observational studies have so far failed to find any evidence for brown dwarfs or gas giants.^[104][105]

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.^[106] 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.^[96] 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.^[107]

Radial velocity measurements of Alpha Centauri B with HARPS spectrograph ruled out planets of more than 4 M_⊕ to the distance of the habitable zone of the star (orbital period P = 200 days).^[15]

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.^[108]

Theoretical planets

Early computer-generated models of planetary formation predicted the existence of terrestrial planets around both Alpha Centauri A and B,^{[107][109][110]} but most recent numerical investigations have shown that the gravitational pull of the companion star renders the accretion of planets very difficult.^[106][111] 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.^{[96][112][113][114]}

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.^[115] 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.^[116] 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.^[73] 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.^[73]

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.^[115][117]

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.^[118] The SIM mission, however, was cancelled due to financial issues in 2010.^[119]

View from this system

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 an observer on Earth, 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 02^h 39^m 35^s +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 ^[120] 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 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.^[121]

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 magnitudes are much dimmer than the 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 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 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.^[115]

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.^[115] 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,^[122] short for Rigil/Rigel Kentaurus,^{[123][nb 3]} the romanization of the Arabic name رجل القنطورس Rijl Qanṭūris,^[122] from the phrase Rijl al-Qanṭūris "the foot of the Centaur".^[124] 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".^[122] During the 19th century, the northern amateur popularist Elijah H. Burritt used the now-obscure name Bungula,^[125] possibly coined from "β" and the Latin ungula ("hoof").^[122] 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.^[36]

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.^[126]

To the Australian aboriginal Boorong people of northwestern Victoria, Alpha and Beta Centauri are Bermbermgle,^[127] two brothers noted for their courage and destructiveness, who speared and killed Tchingal "The Emu" (the Coalsack Nebula).^[128] The form in Wotjobaluk is Bram-bram-bult.^[127]

See also

• Alpha Centauri in fiction
• List of brightest stars
• List of nearest stars
• Project Longshot

Notes

References

External links

Wikimedia Commons has media related to Alpha Centauri.

• 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

• "A Family Portrait of the Alpha Centauri System". SpaceRef.com. Retrieved 21 March 2003. 
• Alpha Centauri System
• O Sistema Alpha Centauri (Portuguese)
• Alpha Centauri – Associação de Astronomia (Portuguese)
• Thompson, Andrea (2008-03-07). "Nearest Star System Might Harbor Earth Twin". SPACE.com. Archived from the original on 2 June 2008. Retrieved 2008-07-17. 

Coordinates: 14^h 39^m 36.4951^s, −60° 50′ 02.308″