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Alpha Centauri, the third brightest star in the sky, is a yellowish-white star of magnitude -0.27 which lies at the foot of the Centaur. At only 4.3 light years distant, it is also the closest star system to our own. Alpha Centauri is in fact a triple star system consisting of two rather close, Sun-like stars (α Cen A and B), accompanied by a very distant, faint red dwarf (α Cen C or Proxima Centauri).


History and Mythology

Alpha Centauri has the traditional name Rigil Kentaurus (often shortened to Rigil Kent), meaning "Foot of the Centaur" in Arabic. An alternative name is Toliman, whose etymology may be Arabic (meaning "the Ostriches"), or Hebrew (meaning "The Heretofore and the Hereafter" and/or "Shoot of the Vine"). The first magnitude star Hadar (Beta Centauri) lies about 4 degrees west of Alpha Centauri. In the southern hemisphere, Alpha and Beta Centauri are known as "The Pointers", as both stars directly point towards the constellation Crux, the Southern Cross.

The Arabs called the α and β Cen pair Hada and Wazn, meaning "Ground and Weight". They saw the pair skimming along the southern horizon, and thought they could not lift themselves into the sky. Alpha Centauri was well known in ancient Egypt. Temples were built to celebrate the time when the star rose with the Sun at the autumn equinox. The ancient Chinese designated the Alpha and Beta Centauri as "Nan Men", which means the "The South Gate of the Sky"; as mentioned before, Alpha and Beta Centauri together form the "Southern Pointers" to the Southern Cross.

Alpha Centauri A and B

Although Alpha Centauri has been known for millennia, it was first discovered to be a double star in 1689 by Father Richaud in India. Its two bright components, α Cen A and B, orbit one another with a period of 79.9 years, during which time their angular separation varies from 2 to 22". Their separation was at maximum in 1976, will be at minimum in 2016, then at maximum again in 2056. The orbit has an eccentricity of about 0.52, so the true distance between α Cen A and B varies from 11.2 AU (roughly the distance from the Sun to Saturn) to 35.6 AU (roughly the distance from the Sun to Pluto).
Alpha Centauri A is a main-sequence, hydrogen-fusing star very similar to the Sun. It has a class G2 V spectrum, a surface temperature of 5800 K, and an absolute magnitude of +4.4. It is about 1.23 times the Sun's diameter, 1.1 times as massive as the Sun, and about 1.5 times as luminous. Sunlike stars increase in luminosity as they age, so Alpha Centauri has been estimated to be older than our Sun, probably 6 - 7 billion years as opposed to the Sun's 4.6 billion. Given α Cen A's mass and the greater age, it may be close to running out of hydrogen fuel.

Alpha Centauri B is a class K1 V star with a somewhat cooler surface temperature of 5300 K, making an obvious color contrast. It has an absolute magnitude of +5.7, a luminosity 45% of the Sun's, a mass 90% of the Sun's, and is 14% smaller in radius.

Proxima Centauri

The red dwarf C component is often called Proxima Centauri because it is about a seventh of a light year closer to us than the two brighter components (4.22 vs 4.36 ly). It is therefore the nearest individual star to our Solar System. In our sky, dim Proxima appears at 13th magnitude, about 2 degrees south of α Cen A and B; it was not discovered until 1915 by Robert Thorburn Ayton Innes.



Sizes of the Alpha Centauri triple system's components relative to our Sun.
Proxima Centauri is a red dwarf star of spectral class M5 Ve. It is extremely faint and small; its luminosity is merely 0.0008 that of the Sun. Its mass is only one tenth the Sun's, and it is smaller in size than the planet Saturn. Like many red dwarfs, Proxima is a "flare star" that can roughly double in brightness sporadically from hour to hour. Its designated variable star name is V645 Centauri.

At around 15,000 AU from the brighter A and B components, it may be gravitationally bound to the system. But if so, it is in a huge orbit with a period of several hundred thousand years, and may leave the system after some millions of years.

Possible Planets

Since Alpha Centauri A is very similar to our own Sun, many speculate whether it might possess planets that harbor life. Alpha Centauri A and B are 1.7 - 1.8 times richer in "metals", i.e. elements heavier than hydrogen, than our own Sun. Most stars known to have planets have similarly high metallicities. Recent simulation results conclude that multiple planets could have formed in close orbits around both heavy-element-rich stars.

In a binary system, a planet must orbit closer to its "home" star than about 1/5th of the closest approach of the other star, or its orbit will be disrupted by the gravitational pull of that second star. The distance where an Earth-type planet would be "comfortable" with liquid water is centered around 1.25 AU for α Cen A, and around 0.74 AU for α Cen B. Hence, both α Cen A and B could have one or two "rocky" planets in orbital zones where liquid water is possible. Both stars are therefore prime targets in the search for Earth-like planets around nearby stars.

Hubble Space Telescope observations of Proxima Centauri before 1994 found astrometric perturbations, possibly due to the gravitational pull a planet with 80% of Jupiter's mass, at a distance of about 0.17 AU from Proxima. In 1996, the Hubble Space Telescope might have directly observed a brown-dwarf companion to Proxima, at an apparent separation of about 0.5 AU, implying an orbital period of around a year. However, later observations found no evidence to support the existence of any companions larger than 0.8 times Jupiter's mass in orbits with periods between 1.0 - 2.7 years.

In October 2012, scientists announced the discovery of a Earth-mass planet orbiting Alpha Centauri B. At a distance of 6 million km, with an orbital period of just 3.2 days, Alpha Centauri Bb would have been the closest extrasolar planet yet discovered. The research team, led by Xavier Dumusque of Geneva Observatory, spotted Alpha Centauri Bb using an instrument called the High Accuracy Radial velocity Planet Searcher, or HARPS. The planet's detection required heroic efforts to extract its signal from the background noise of the star's radial-velocity measurements.

However, in 2013, a new analysis of the data by Artie Hatzes (Thuringian State Observatory, Germany) failed to confirm the planet's existence. By late 2015, the original discovers confirmed the “death sentence” for α Cen Bb, illustrating the need for more data collection before confirmation.

In any case, Alpha Centauri is frequently featured in both computer games and science fiction because of its status as our nearest galactic neighbor. Most recently, James Cameron's epic 2009 movie Avatar was set on a fictional moon orbiting a gas giant in the Alpha Centauri system.

If our own Sun were viewed from the Alpha Centauri system, it would appear as a bright yellow star of magnitude +0.46 in the constellation Cassiopeia, almost as bright as Capella appears in our sky.