Alpha Centauri


Template: Infobox double star / Maintenance / single coordinate

Alpha Centauri [ alfa ʦɛntaʊ̯ʀi ] ( α Centauri, abbreviated as α Cen, but also Rigil Kentaurus, Rigilkent, Toliman or Bungula ) is an approximately 4.34 light years distant binary star system in the constellation Centaurus, which can be seen on the southern sky. It consists of the brighter yellow star Alpha Centauri A and Alpha Centauri B. the orange

Alpha Centauri is the closest star system to the Sun. As a double star Alpha Centauri is a total apparent brightness of -0.27 like the brightest star in this constellation and is perceived as a total dritthellstes star in the night sky. The lighter Alpha Centauri A alone only has an apparent magnitude of -0.01, making it the fourth brightest like star in the night sky.

It is disputed whether the remote Lj 0.22 from Alpha Centauri sun next star, the red dwarf Proxima Centauri (4.2 light years distance from the Sun ) is also on this system.

  • 9.1 starry sky
  • 9.2 The two suns

Location at the starry sky

Together with the 4.4 ° away Beta Centauri and the three brightest stars of the constellation of the Southern Cross, which lies to the west of them, they form the most significant cluster of stars of the first magnitude within a hand span the entire starry sky.

Alpha and Beta Centauri show as a line on the constellation of the Southern Cross. The "pointer" were so called by easily between the Southern Cross and the often so confused eastern asterism (star collection that is mistaken for a constellation ), the " False Cross" ( the constellation of sails of the ship or Vela ) to distinguish. The "wrong cross" includes the visible to the naked eye star ε Car, Turai, κ Vel Vel and δ.

Alpha and Beta Centauri are too far south, that one could see them from the northern mid- latitudes (eg Europe). As of 33 ° south latitude, the star is circumpolar and thus the star always remains above the horizon.

Alpha Centauri binary star system

The double star has an absolute magnitude of 4.1 mag. To the naked eye the two components A and B can not be separated from the earth. Only in a telescope with a 5 cm opening the individual stars are visible.

Once in 79.9 years, the two stars orbit on highly elliptical orbits with an eccentricity of 0.519, the distance from 11.5 to 36.3 AE is. The semi-major axis is about 23.9 AU. [A 1] In May 1995, the largest distance ( Apastron ) was reached. The closest approach ( periastron ) will be held in May 2035.

From the values ​​of the semi-axes and the orbital period, the total mass of the binary system to 2.08 solar masses can be calculated. [A 2]

The angular separation and the position angle change due to the relatively short orbital period within a few years significantly ( see table). During one revolution of the apparent distance between about 2 "and 22" varied.

Most of the currently determined distances of the three stars that are mentioned in the literature, based on the values ​​of the parallaxes of the Hipparcos star catalog (HIP ) of 1991.

Physical Properties

Alpha Centauri A and B are as common arisen pair of stars approximately 6.5 ± 0.3 billion years old. Both are ordinary main-sequence stars and are thus in a stable phase of hydrogen burning ( fusion of hydrogen into helium). Since Alpha Centauri A is more massive than Alpha Centauri B, it remains shorter in the main sequence before it develops into a red giant. This is Alpha Centauri A, in contrast to the smaller and thus more durable Alpha Centauri B for more than half his life behind him. Proxima Centauri, however, is only about 4.85 billion years old.

About Alpha Centauri A and B, which are together often referred to as α Cen AB, are detailed observations of surface vibrations from which the asteroseismology can draw conclusions about the internal structure of stars. Combine that with the traditional observation methods, we obtain more precise values ​​of the properties of the stars than would be possible with the individual methods.

Alpha Centauri A

Alpha Centauri A, the yellowish bright main star, is like the sun, a yellow dwarf of spectral type G2 V. This makes it one like the sun to the hotter G- stars (within the spectral class G extends the numerical designation from 0 ( hottest ) to 9 ( coolest ) Star ). The luminosity class V indicates that it belongs to the main sequence stars. It is characterized by an apparent brightness of 0.00 mag ( magnitude) (mag -1.46 ) after Sirius, Canopus ( -0.72 mag) and Arcturus ( -0.05 mag) before Wega (0.03 mag) of the fourth brightest star in the sky.

Since Alpha Centauri A has the same spectral type and similar dimensions as the sun, he is considered the erdnächste " solar twin" ( which does not mean that they were created together). Its surface temperature is about 5800 K. With the 1.22 times the solar diameter to be larger than Alpha Centauri B. He has 1.1 solar masses and are 1.52 times as much radiation power from the sun. The chemical composition is very similar to that of the sun. The proportion of heavy elements ( elements with an atomic number greater than helium are known in astrophysics as metals), however, is almost 70 % higher ( of the metallicity [Fe / H] = 0.22 ± 0.05 A ). His habitable zone from 1.2 to 1.3 units astronomical (AE).

Alpha Centauri B

Alpha Centauri B is lit orange - yellow and belongs to the spectral type K1 with the luminosity class V. He points towards the lighter star Alpha Centauri A on only a brightness of 1.33 mag and is number 21 in the list of the brightest stars in the sky. He owns 0.93 solar masses and has a 0.86 -fold solar diameter. He, too, is like the sun composed. However, the proportion of heavy elements is a good 70 % higher ( the metallicity is [Fe / H ] B = 0.24 ± 0.05). It was determined a rotation period of 41 days. For comparison, the Sun rotates at about 25 days once around its own axis.

With a surface temperature of about 5300 K it is only slightly cooler than the Sun. However, because it reaches the lower temperature and the lower surface area 50 % of the solar radiation power. Thus, the brightness of the orange - yellow radiant K1 V star Alpha Centauri B is only a third of the larger star Alpha Centauri A. The habitable zone is located at a distance from 0.73 to 0.74 AE.

Although he is not as bright as Alpha Centauri A, Alpha Centauri B radiates in the X-ray region of the spectrum from more energy. Curve B of the light varies in short time intervals and it was observed, at least one flare.

Membership of Proxima Centauri star system to

The question of the membership of Proxima Centauri to Alpha Centauri is not clear safely. The majority of astronomers, however, assumes that Proxima Centauri is gravitationally bound to Alpha Centauri A and B.

The distance of Proxima Centauri to Alpha A and B is about 15,000 ± 700 AU or 0.21 Lj That's about 1,000 times the distance between Alpha Centauri A and Alpha Centauri B, or five hundred times the distance of Neptune from the sun. The angular distance of Proxima Centauri to Alpha A and B in the sky is about 2.2 degrees ( four full moon widths ).

Astrometric measurements as those of the Hipparcos satellite suggest that Proxima Centauri is in orbit around the binary star system with an orbital period on the order of 500,000 years (the figures vary from a few 100,000 years to a few millions of years ). Therefore, Proxima is sometimes referred to as Alpha Centauri C. Based on these data, the orbit with a minimum distance of 1000 AU and a maximum distance of 20,000 AU from the inner binary system would be extremely eccentric. Proxima Centauri would now close to its Apastron ( the farthest point in its orbit around Alpha Centauri A and B). It is even more precise measurements of the radial velocity is required to confirm this assumption.

According to estimates by R. Matthews and G. Gilmore are, starting from this small distance and the like airspeed, the chances that the observed arrangement is random, about one in a million.

Some radial velocity measurements, eg in the Gliese catalog, however, differ from those expected for a bound system values ​​so that it is not impossible that this is just a random star encounter. This assumption is also supported by simulations that resulted from the calculated binding energy of the system in only 44 percent of the options explored a bound system.

Studies from 1994 suggest that Proxima Centauri is a movement group together with the inner binary system and nine other star systems. Consequently, Proxima Centauri would not go around the couple Alpha Centauri in a bound movement, but its orbit is hyperbolic disturbed by the double star system. This means, Proxima Centauri would never perform a full orbit around Alpha Centauri A and B.


The Alpha Centauri system moves obliquely to our solar system and reduce the distance with a radial velocity of about 22 km / s Proxima Centauri approaches, however, only 16 km / s of the sun.

In a thousand years, Alpha Centauri moves about one degree of arc (equivalent to two full Moon widths ) in the sky on. In about 4000 years, Alpha Centauri will have visually approximated so closely to Beta Centauri, that they will form an apparent double star. [A 3] In reality Beta Centauri with 520 Lj times further away than Alpha Centauri around 120 by the sun and its proper motion is only about 1% of the proper motion of Alpha Centauri.

In about 28,000 years, the Alpha Centauri system will have reached a distance of 3 light years to the solar system its closest approach, and then increase the distance again. Here, the star system is up to -1.28 may be bright and on the border between the constellations water snake ( Hydra ) and sailing of the ship appear. Only Sirius will be a little brighter in the sky.

Then the star will slowly disappear under the stars of the Milky Way. Then the once so dominant star in the inconspicuous constellation telescope is covered by the freiäugige visibility. This unusual position is explained by Alpha Centauri own independent galactic movement, which has a high inclination with respect to the Milky Way.

Observed Planet

The European Southern Observatory announced on October 16, 2012 with the observation of Alpha Centauri B accompanying planet Alpha Centauri B-flat. According to the journal article it lies at a distance of about 6 million km (0.04 AU) ten times closer than Mercury to its star that it orbits in each of 3,236 days, and thus clearly outside the habitable zone. With a minimum mass of about 1.1 Earth masses, he would be one of the smallest found so far in a Sun-like star planets (as of October 2013) and at the same time the Earth closest outside the solar system. The detection was performed using the HARPS spectrograph, which Alpha Centauri B has been observed in a period of four years. The radial velocity change produced by the planet Alpha Centauri B-flat at Alpha Centauri B is only 0.51 m / s

A recent study has not confirmed the discovery of the planet (but not definitely disproved ).

Possibility of planet formation

Current computer models for planet formation calculated that terrestrial planets could form close to Alpha Centauri A as well as on Alpha Centauri B. These results are supported by the discovery of the planet in a binary star system, such as gamma Cephei, the high metallicity of the Alpha Centauri system and the existence of numerous satellites around Jupiter and Saturn.

However, to ensure safe are gas giants like Jupiter and Saturn, which can not form because of gravitational perturbations in a binary star system. Therefore, it is not surprising that to date, no abnormalities were found in the radial velocity, which indicate such. Due to the lack of a gas giant go some astronomers believe that any existing terrestrial planet in the Alpha Centauri system could be dry. This is based on the assumption that gas giants like Jupiter and Saturn are crucial to ensuring that comets are directed into the interior of a star system and bring by the impact of water on the planet. It may be that this effect despite the absence of the gas planets on the assumption that Alpha Centauri A would assume the role of Jupiter for Alpha Centauri B, or vice versa. It is also conceivable that Proxima Centauri in the periastron distract a lot of comets from the Oort cloud of the system, and thus could provide possible terrestrial planets around the stars A and B with water. Since no Oort cloud yet been demonstrated, there is also the possibility that it was completely destroyed during the formation of the system.

Up to what distance stable orbits for planets in a binary system are possible, is not fully understood. For Alpha Centauri A, the estimates vary from 1.2 AE to half the perihelion distance of 6.5 AU. Otherwise, they could be ripped out of its original orbit at its very source or later due to gravitational disturbances by Alpha Centauri B.

In order to detect Earth-like planets in the habitable zone of sun-like stars using the method of measurement of the radial velocity, very accurate measurements of the order of centimeters per second are required. Here, the " wobble " is (English wobbling ) of the central star caused by the gravity of planets is measured. Alpha Centauri seems well suited for these measurements because it shows a very low activity ( oscillation of the star eruptions in the chromosphere ). It is likely that some years data must be collected in order to detect a possible planets.

Conditions for life

Based on the similarity of the two stars, as for the age, the type of star, the spectral type and the stability of the orbits, it is suspected that this star system could offer one of the best known conditions for extraterrestrial life.

A planet around Alpha Centauri A would have a spacing of about 1.2 to 1.3 AE have to have Earth-like temperatures. This would, based on the solar system, roughly corresponding to an orbit between Earth and Mars. For the less bright, cooler Alpha Centauri B, this distance would have about 0.73 to 0.74 AE (about the distance from Venus to the Sun ) amount.

Alpha Centauri A and B were high on the top 100 list of the objectives planned by the NASA Terrestrial Planet Finder. This list includes the most promising stars to the Earth-like planets are suspected. However, the construction of this space telescope has been postponed due to budget cuts indefinitely.

The sky over Alpha Centauri

Starry sky

From the Alpha Centauri system as seen from the sky presents an observer from similar from the earth. Most constellations like Ursa Major and Orion look almost unchanged. In the constellation Centaurus, of course, missing the brightest star. In contrast, the sun appears as a bright star like 0.5 in the constellation Cassiopeia. The \ / \ / of Cassiopeia transformed into a / \ / \ / and the sun forms instead of Segin ( ε Cas ) the new eastern end of the constellation. The sun is antipodal ( in the opposite direction ) to the seen from the Earth position of Alpha Centauri, so at the coordinates RA 2393502h 39m 35s and DE Expression error: Missing operand for 60 ° 50 '.

Closer standing bright stars like Sirius, Altair and Procyon are to be seen in clearly shifted positions. Sirius now belongs to the constellation Orion and is 2 degrees west of Betelgeuse, which he does not have the same brightness of -1.46 likes having as from Earth seen, but only -1.2 mag. Even the more distant star Fomalhaut and Vega appear slightly offset. Proxima Centauri, despite its small distance of 13,500 AU (one quarter light year) only an inconspicuous star with a brightness of 4.5 mag. This illustrates how faint is the red dwarf.

The nearest major neighbor of the star Alpha Centauri system are to the Sun ( distance 4.34 light years ) with a distance of 6.47 light years Barnard's Star, 9.5 light years, Sirius and 9.7 Lj Epsilon Indi. Barnard's star is also from the sun at a distance of 5.96 light years is the second nearest star.

The two suns

An observer on a hypothetical planet around Alpha Centauri A or B sees each other's star as a very bright object. An Earth-sized planet orbiting at a distance of 1.25 AU (roughly mid-way between Earth and Mars orbit ) Alpha Centauri A orbits ( and this would require 1.34 years), receives from him about the amount of light that the earth receives from the Sun. Alpha Centauri B is displayed depending on the position in its orbit from 5.7 to 8.6 mag " darker" ( -21 to -18.2 mag). The 190 - to 2700 - times fainter than Alpha Centauri A, but still about the same factor brighter than the full Moon.

At Alpha Centauri B would have an Earth-sized planet at a distance of 0.7 AU (corresponding to a circumnavigation duration of about 0.6 years ) orbits the star in order to get the same amount of radiation as the earth from the sun. Alpha Centauri A then radiates depending on the position in the orbit of about 4.6 to 7.3 mag ( -22.1 to -19.4 mag) fainter than the primary star. This is 70 - to 840 - times fainter than Alpha Centauri B, but still 520 - to 6300 - times brighter than the full Moon.

In both cases, one has the impression that the "second sun " in the course of a planet orbiting the sky at the year of observation. Assuming a low orbital inclination of the orbit of Alpha Centauri A towards Alpha Centauri B, the stars are in the course of a " year " once close together; six months later, is to be deemed a midnight sun of the secondary star. After a further six months, this cycle is terminated. For a hypothetical Earth-like planets around a star the second of the two sun is not bright enough to the climate or the photosynthesis of plants to be affecting (even if he can come close as the Saturn the Sun). However, the more distant star makes sure that he lit the night sky for half a year so far that it looks more dark blue instead of black as pitch. You could wander around easily and easy to read even without additional light.


"Alpha Centauri " is a name by the Bayer classification. Alpha ( α ) is the first letter of the Greek alphabet, and Centauri ( the genitive to Latin Centaurus, the Centaur ) shows the membership of the constellation Centaurus.

The proper name Rigil Kentaurus (. Often abbreviated as Rigil Kent, ) formerly Rigjl Kentaurus and Riguel Kentaurus ( in Portuguese ) is from the Arabic phrase Rijl Qantūris (or Rijl al - Qantūris; رجل قنطورس, DMG riǧl qanṭūris ) derived meaning " foot of the centaur. "

The name is also used Toliman ( also wrong Tolimann ) comes either from the Arabic ( الظلمان, DMG AZ- Zulman ) or the Hebrew language. In Arabic it means " bouquets " and in Hebrew as much as " the Heretofore and Hereafter " and / or " offspring of the vine ".

The name nowadays rarely used Bungula was probably formed by " β " and from Latin ungula ( " foot" ) and as well as Rigil called the front leg of the centaur.

In the Chinese language is Alpha Centauri Nánmén'èr (南门 二), " Second Star of the South Gate ", called ( as mentioned form Alpha and Beta Centauri collectively, the " Southern Pointers" to the constellation of the Southern Cross ).

Most of the double star is named after the Bayer designation Alpha Centauri.


Even the ancient Greeks knew Alpha Centauri. However, due to the continuing precession he wandered among the European horizon and was eventually forgotten.

The Inca used in Kenko two cylindrically shaped, close to each other standing stones, which peaked about 20 centimeters and as a targeted stones in Stargazing, especially the Pleiades and Alpha Centauri served.

The explorer Amerigo Vespucci mapped by the first half of his last voyage ( 1501-1502 ) Alpha Centauri, Beta Centauri, and the constellation of the Southern Cross.

According to the renowned double star observer Robert Aitken (1961) and as now reported in the sixth catalog of double stars in 2008, the Jesuit priest Jean Richaud discovered in December 1689 in Pondicherry (India), the duplicity of Alpha Centauri, while with a passing near comets a telescope observed.

The apparent proper motion of Alpha Centauri was observed due to the astrometric observations of the French astronomer Abbé de La Caille 1751-1752.

Thomas James Henderson, a Scottish astronomer, calculated at the Cape Observatory first the distance to Alpha Centauri. He measured between April 1832 and May 1833, the annual trigonometric parallax of both stars. He noted the high proper motion of the star and concluded that Alpha Centauri would be a very nearby star. After the parallax measured from 0.745 arc seconds, he came to the conclusion that Alpha Centauri slightly less than 1 parsec ( 3.26 light years ) was removed. The value was 33.7 % too low, but even relatively precisely at this time. However, he has not yet published the results, since he seriously doubted due to the high values ​​. It was not until 1839, when Friedrich Wilhelm Bessel released his own precise measurements of the parallax of 61 Cygni in 1838, he published his results. Alpha Centauri is therefore officially the second star whose distance was calculated.

In 1870 there was the first flag of South Australia. It contained the Southern Cross, there were the two stars Alpha Centauri and Beta Centauri as landmarks. Even in the current flag of Australia the Southern Cross is still included.

1926, William Stephen Finsen, the parameters of the orbital elements of Alpha Centauri A and B. The future positions were now in ephemerides (tables, the positions of the moving astronomical objects list ) are calculated. Other astronomers as D. Pourbaix in 2002 have little to correct the orbit and the orbital elements. The eighty -year orbital period for α Centauri AB is therefore quite accurate.


Since Alpha Centauri is the closest star system to the Sun, it is often theme in science fiction - such as in the movie Avatar - or in video games such as Sid Meier 's Alpha Centauri. This interstellar travel, the human exploration and the discovery and colonization of possible planets play a role.