Alpha Centauri

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 and an established binary star system, Alpha Centauri AB (α Cen AB). To the unaided eye it appears a single star, whose total visual magnitude identifies it as the third brightest star in the night sky.

Alpha Centauri is the closest star system to our own solar system. Its distance is about 1.34 parsecs, or 4.37 light years.

Component designations

Alpha Centauri, or Rigil Kentaurus, is the single visual naked-eye star seen in the southern skies.

Telescopically, the brightest two stars make the Alpha Centauri AB system, often abbreviated as α Centauri AB or α Cen AB, and is a binary star in close orbit.

Alpha Centauri A (α Cen A) and Alpha Centauri B (α Cen B) are the individual stars, usually defined to identify them as the different component of the binary α Cen AB. An additional much more distant and fainter companion is called Proxima Centauri, Proxima or α Cen C. Proxima Centauri is a visual double, which is assumed to be associated with α Cen AB. Direct evidence that it has an elliptic orbit typical defining binary stars is yet to be found.

Together all three components make a visual triple star system, referred by double star observers as the triple star (or multiple star), α Cen AB-C.

This naming system allows specialist double star astronomers to define components and the meaning of the relationships between the different components. All component designations are held and controlled by the U.S. Naval Observatory, in a continuously updated catalogue called the Washington Double Star Catalogue or WDS and contains over 102,387 double stars or pairs using this system of designation.

Nature of the system

At −0.27v visual magnitude, Alpha Centauri to the naked-eye appears as 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, being only fractionally fainter than Arcturus at -0.04v magnitude. Alpha Centauri B at 1.33v magnitude is twenty-first in brightness.

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 colour, whose stellar classification is spectral type G2 V. From the determined mutual orbital parameters, α Cen A is about 10% more massive than our Sun, with a radius about 23% larger. The projected rotational velocity (v.sin i) of this star is 2.7±0.7 kms-1, resulting in an estimated rotational period of 22 days.

Alpha Centauri B is the companion star or secondary, slightly smaller and less luminous than our Sun. This main sequence star displays the spectral type of K1 V, being an observed deeper orangish-yellow colour than the primary star. By mass, α Cen B is about 90% of the Sun and 14% smaller in radius. The projected rotational velocity (v.sin i) is 1.1±0.8 kms-1, resulting in an estimated rotational period of 41 days. (An earlier estimate gave a similar rotation period of 36.8 days.) 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.

Alpha Centauri C, also known as Proxima Centauri, is of spectral class M5Ve or M5VIe, suggesting this is either a small main sequence star (Type V) or sub-dwarf (VI) with emission lines, whose B-V colour index is +1.81. Its mass is about 0.12 .

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. "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 α Cen AB does not differ significantly from either star, gravitationally this binary system is consider as if it were one object.


Resolution of the binary star α Cen AB is too close to be seen by the naked eye, as the angular separation varies between 2 and 22 arcsec, but through much of the orbit, both are easily resolved in binoculars or small 5 cm telescopes.

In the southern hemisphere, Alpha Centauri is one of the stars of The Pointers or The Southern Pointers with Beta Centauri or Hadar / Agena.

Both stars directly point towards the constellation Crux—the Southern Cross. The Pointers easily distinguish the true Southern Cross from the fainter asterism known as the False Cross. Beta Centauri lies some 4.5° west, mid-way between the Crux and α Centauri.

South of about -29° S latitude, α Centauri is circumpolar and never sets below the horizon. Both stars, including the Crux, are too far south to be visible for mid-latitude northern observers. Below about +29° N latitude to the equator during the northern summer, α Centauri lies close to the southern horizon. The star culminates each year at midnight on 24th April or 9 p.m. on the 8th June.

As seen from Earth, Proxima Centauri lies 2.2° southwest from Alpha Centauri AB. This is about four times the angular diameter of the Full Moon, and almost exactly half the distance between α and β 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. Some amateur and professional astronomers regularly monitor for outbursts using either optical or radio telescopes.

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. By 1752, Abbé Nicolas Louis de Lacaillé made astrometric positions using a meridian circle while Sir John Herschel in 1834 first made micrometrical observations. Since the early 20th Century, measures have been made with photographic plates.

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

Popularly known, 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. For this reason, we consider Alpha Centauri as the second star to have its distance measured.

R.T.A. Innes from South Africa in 1915 discovered Proxima Centauri 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 α Centauri AB, suggesting immediately it was part of the system and slightly closer to us than α 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).

The binary system

With the orbital period of 79.91 years, the AB components of this binary star can approach each other to 11.2 astronomical units, equivalent to 1.67 billion kilometres or about the mean distance between the Sun and Saturn, or recede as far as 35.6 AU or 5.9 billion kilometres—approximately the distance from the Sun to Pluto. This is unlike most of the planetary orbits in the Solar System, whose moderate orbital eccentricity is e = 0.5179. From the orbital elements, the total mass of both stars is about 2.0 ΣM - or twice that of the Sun. An average individual stellar mass is 1.09 M and 0.90 M, respectively, though quoted in recent years are some slightly higher masses. I.e. 1.14 M and 0.92 M, or totalling 2.06 ΣM. Alpha Centauri A and B have absolute magnitudes of +4.38 and +4.71, respectively. Stellar evolution theory implies both stars are slightly older than the Sun at 5 to 6 billion years, as derived by both mass and their spectral characteristics.

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 are now 8.29 arcsec through 237° (2008) reducing to 7.53 arcsec through 241° (2009). Closest approach will next be in February 2016, at 4.0 arcsec through 300°. (See External Reference. ) Observed maximum separation of these stars is about 22 arcsec, while the minimum distance is a little less than 2 arcsec. Widest separation occurred during February 1976 and the next will be in January 2056.

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. Both stars are presently now decreasing in apparent distance.

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.α This is equivalent to 0.21 ly or 1.94 trillion kilometres —about 5% the distance between the Sun and α Cen AB. Proxima may be gravitationally bound to α Cen AB, orbiting it with a period between 100,000 and 500,000 years. However, it is also possible that Proxima is not gravitationally bound and thus is moving along a hyperbolic trajectory around α Cen AB. 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. Theoretically, Proxima could leave the system after several million years. It is not yet certain whether Proxima and Alpha are truly gravitationally bound

Proxima is a 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 Mʘ (rounded to 0.12 Mʘ) or about one-eighth that of the Sun.

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. 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.

Edmond Halley in 1718 found that some stars had significantly moved from their contemporary astrometric positions. For example, the bright star Arcturus (α Boo) in the constellation of Boŏtes showed an almost ½° difference in 1800 years., as did the brightest star Sirius in Canis Major (α CMa). Halley's positional comparison was Ptolemy's catalogue of stars known today as the Almagest whose original data was plagiarised from Hipparchos during the 1st Century B.C. Many of Halley's proper motions were mostly for northern stars, so the southern star Alpha Centauri was not determined until the early 19th Century.

Scottish born observer Thomas James Henderson in the 1830s at the Royal Observatory at the Cape of Good Hope discovered the true distance of Alpha Centauri. He soon realised this system displayed an unusually high proper motion, and therefore its observed true velocity through space should be much larger. In this case, the apparent stellar motion was found using Abbé Nicolas Louis de Lacaille astrometric observations of 1751-52, 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-1 (mas/yr) or -3.678 arcsec per year in right ascension and +481.84 mas.yr-1 or 0.48184 arcsec per year in declination. As proper motions are cumulative, the motion of Alpha Centauri is about 6.1 arcmin/century (367.8 arcsec/ century), 61.3 arcmin/millennia or 1.02 ° /millennia. These motions each century is about one-fifth and twice, respectively, the diameter of the full moon. Spectroscopy has determined the mean approaching radial velocity of α Cen AB as -25.1±0.3 kms-1.

A more precise calculation involves taking into account the slight changes in the stellar distance by the star's own motion. Alpha Centauri is presently slowly increasing the measured proper motion and trigonometric parallax as the stars approach us. 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 star currently approach us. Also the orbital period of α Cen AB is also slightly shorter by some 0.006 years per century, caused by the change of light time as the distance reduces. Consequentially, the observed position angle of the stars are subject to changes in the orbital elements over time, as first determined by equations by W. H. van den Bos in 1926. Some slight differences of about 0.5% in the measured proper motions are caused by α Cen AB's orbital motion.

Based on these observed proper motions and radial velocities, Alpha Centauri will continue into the future 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 A.D. 29,700, in the present-day constellation of Hydra, α Centauri will be exactly 1.00 pc. or 3.26 ly. away. Then it will reach the stationary radial velocity (RVel) of 0.0 kms-1and the maximum apparent magnitude of -0.86v - similar to present day Canopus. Soon after this close approach, the system will then begin to move away from us, showing a positive radial velocity. In A.D. 43,300, α Centauri will pass near 2nd magnitude Alphard / Alpha Hydrae (α Hya). Then the apparent magnitude will be +1.03v at the distance of 1.64 pc. or 5.36 ly.

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

Possibility of planets

Discoveries 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 α Cen A or α Cen B individually, or be on large orbits around the binary α Cen 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. All the observational studies have so far failed to find any suggestions of either; brown dwarfs, gas giants (planets) or small extrasolar terrestrial planets.

Based on computer simulations, other planetary astronomers consider that any potential terrestrial planets that did once orbit near the stars' habitable zones are now likely no longer located there. The loss, several billion years ago, of these small bodies probably happened during the system's formation. All may have since been ejected by significant disruptions caused by strong gravitational or perturbation effects generated between the two main stellar components.

However, computer simulations show that a planet can form within a distance of 1.1 AU of Alpha Centauri B and the orbit of that planet may remain stable for at least 200 million years.

Alpha Centauri is envisioned as the first target for unmanned interstellar exploration. Crossing the huge distance between the Sun and α Centauri using current spacecraft technologies would take several centuries. This may be the only means of obtaining direct evidence that such planets exist if ground or orbit based observatories are unable to detect them.

Theoretical planets

Some computer models of planetary formation predict the existence of terrestrial planets around both Alpha Centauri A and B. Other models also suggested that formation of gas giant planets similar to Jupiter and Saturn remain unlikely because of the significant gravitational and angular momentum effects of this binary system. 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.

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 likely very 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. We could discount this, if for example, α Centauri B happened to have giant gas planets orbiting α Centauri A (or conversely, α Cen A for α Cen B). 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. As yet, we have no direct evidence of the existence of such an Oort Cloud around α Centauri AB, and theoretically this may have been totally destroyed during the system's formation.

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, being about the distance that Venus is from the Sun.

To find evidence of such planets, currently both Proxima Centauri and α Centauri AB are 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 is possible with this new instrument.

View from this system

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 familiar // shape of Cassiopeia instead become ///.

From Alpha Centauri, most of the familiar constellations like Ursa Major and Orion would appear almost unchanged. Bright stars relatively close to us, such as Sirius, Procyon and Altair, would have markedly different sky positions. Sirius, for example, would become part of Orion, some 2° west of Betelgeuse, and shining a little dimmer as we know it, at -1.2 magnitude. Other similar close bright stars like Arcturus, Fomalhaut and Vega, would be displaced little from their familiar positions in the sky. As the closest star would be the low luminosity red dwarf Proxima Centauri at 0.25 ly. in distance, shining as an inconspicuous 4.5 magnitude star. Its slow and gradual movement against the background stars would easily detected over several decades.

From Proxima itself, α Centauri AB would appear like two close brilliantly 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 α Cen A and B would be −6.5 and −5.2, respectively.

View from a hypothetical planet

Any hypothetical planet orbiting around either α Centauri A or α Centauri B would see an intensely bright star in the sky with a small discernible disk. For example, an Earth-like planet about 1.25 Astronomical Unit (A.U.) from α Cen A (with an orbital period of about one year three months or 1.3(4) a would get Sun-like illumination from its primary. α Cen B would appear 5.7 to 8.6 magnitudes dimmer than the Sun at visual magnitudes −21.0 to −18.2, respectively, or 190 to 2700 times dimmer than α Cen A, but still 170 to 2300 times brighter than the full moon. Conversely, some similar Earth-like planet at 0.71 A.U. from α Cen B would receive significant illumination from α Cen A, which would shine 4.65 to 7.3 magnitudes dimmer than the Sun at visual magnitudes of −22.1 to −19.4, respectively. Similarly, α Cen B would be 70 to 840 times dimmer or some 520 to 6300 times brighter than the full moon. During this hypothetical planet's year of 0.6(3) a, would see the intensely bright companion star circle an ecliptical path around the sky, but its illumination would not significantly effect climate nor influence plant photosynthesis.

Assuming this hypothetical planet had a low orbital inclination with respect to the mutual orbit of α Cen A and B, then the secondary star would start beside the primary at 'stellar' conjunction. Half the period later (about forty years), 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 dark blue - similar to the sky during totality at any total solar eclipse. People could easily walk around and clearly see the surrounding terrain. Also reading a book would be quite possible without any artificial light. 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 (often shortened to Rigil Kent., former Rigjl Kentaurus; Riguel Kentaurus in Portuguese), derived from the Arabic phrase Rijl Qantūris (or Rijl al-Qantūris, 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 ("the Ostriches"). During the 19th century, the northern amateur popularist Elijah H. Burritt called the star Bungula, possibly coined from "β" and the Latin ungula ("hoof"). This latter name is rarely used today. In Chinese, Alpha Centauri is Nánmén'èr (南門二), "Second Star of the Southern Gate". Together, Alpha and Beta Centauri form the "Southern Pointers", as they point towards Crux, the asterism of the Southern Cross.

Use in modern fiction

Alpha Centauri's relative proximity makes it in some ways likely the logical choice as "first port of call". Speculative fiction about interstellar travel 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


External links

Hypothetical planets or exploration

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