Alpha Centauri (also known as Rigil Kentaurus) is the nearest bright star to the sun, at a distance of about 4.4 light years. It is also the 4th brightest in the sky. Though it appears to the eye (and in the telescope) as a single point of light, one of the 'Pointers' to the Southern Cross, it is a multiple system of three stars. A fourth star Proxima Centauri appears to be associated with it but is 2 degrees distant on the sky, twice the width of this image.

Alpha Centauri is the closest star system beyond our own solar system, being 4.39 light-years distant (about 25.8 trillion miles or 277,600 AU) (Proxima Centauri, often regarded as part of the system, is 4.26 light-years distant). This makes it a logical choice as "first port of call" for speculative fiction about space travel, which often assumes eventual human exploration of the system or even colonization of possible planets within it.

In Star Trek, this system is the closest stellar neighbor to the Sol system, and the site of the Alpha Centauri colony. (ENT: "Future Tense"). The Alpha Centauri colony is believed to have been a founding member of the United Federation of Planets, along with Earth, Andoria, Tellar, and Vulcan.

Humans had established an outpost in the Alpha Centauri system by the early 22nd century. Among its residents during this time was warp drive developer Zefram Cochrane, who moved to the system from Earth sometime following his historic test flight of the Phoenix in 2063. He left the system in 2119 for an unknown destination. He was presumed dead in space. (TOS: "Metamorphosis")

After the Dominion invaded and conquered Betazed in 2374, many believed Alpha Centauri could be their next target. (DS9: " In the Pale Moonlight")

In reality, Alpha Centauri is the brightest star in the southern constellation of Centaurus. Although it appears as a single point to the naked eye, Alpha Centauri is actually a system of three stars, one of which is the fourth brightest star in the night sky. Alpha Centauri is famous in the Southern Hemisphere as the outermost 'pointer' to the Southern Cross, but it is too far south to be visible in most of the northern hemisphere. The two brightest components of the system are too close to be resolved as separate stars by the naked eye and so are perceived as a single source of light with a total visual magnitude of about -0.27 (brighter than the third brightest star in the night sky, Arcturus). Alpha is one of very few bright stars that are a similar colour to the Sun. This is not because Sun-like stars are rare, rather it is because they are intrinsically faint.

Rigil Kentaurus ("Foot of the Centaur" in Arabic) is the fourth brightest star in the night sky as well as the brightest star in Constellation Centaurus. Like Sol, it is a yellow-orange main sequence dwarf star of spectral and luminosity type G2 V. It has about 1.105 ± 0.007 times Sol's mass (Guedes et al, 2008) and 1.23 its diameter (ESO; and Demarque et al, 1986), and is about 52 to 60 percent brighter than Sol. Since Alpha Centauri A is very similar to our own Sun, however, many speculate whether it might contain planets that harbor life. According to Weigert and Holman (1997), the distance from the star where an Earth-type planet would be "comfortable" with liquid water is centered around 1.25 AUs (1.2 to 1.3 AUs) -- about midway between the orbits of the Earth and Mars in the Solar System -- with an orbital period of 1.34 years using calculations based on Hart (1979), but more recent calculations based on Kasting et al (1993) allow for a wider "habitable zone."

In a binary system, a planet must not be located too far away from its "home" star or its orbit will be unstable. If that distance exceeds about one fifth of the closest approach of the other star, then the gravitational pull of that second star can disrupt the orbit of the planet. Recent numerical integrations, however, suggest that stable planetary orbits exist: within three AUs (four AUs for retrograde orbits) of either Alpha Centauri A or B in the plane of the binary's orbit; only as far as 0.23 AU for 90-degree inclined orbits; and beyond 70 AUs for planets circling both stars (Weigert and Holman, 1997). Hence, under optimal conditions, either Alpha Centauri A and B could hold four inner rocky planets like the Solar System: Mercury (0.4 AU), Venus (0.7 AU), Earth (1 AU) and Mars (1.5 AUs).

The much dimmer companion star is a main sequence, reddish-orange dwarf (K0-1 V). It appears to have only 93.4 ± 0.7 percent of Sol's mass (Guedes et al, 2008), about 86.5 percent of its diameter, and 45 to 52 percent of its luminosity (ESO; and Johnson and Wright, 1983, page 681). Viewed from a planet at Earth's orbital distance around Alpha Centauri A, this companion B star would provide more light than the full Moon does on Earth as its brightest night sky object, but the additional light at a distance greater than Saturn's orbital distance in the Solar System would not be significant for the growth of Earth-type life.

On February 25, 2008, a team of astronomers released a paper on simulation results which support the conclusions of previous studies that multiple-planet systems could have formed in close orbits around both heavy-element rich, Alpha Centauri A and B. Focusing on Star B, their simulations suggest that at least one planet in the one to two Earth-mass range could have formed within orbital distances of 0.5 to 1.5 AUs; of particularly note, the simulations frequently generated a Earth-like planet in or near Star B's habitable zone (where liquid water could exist on the planet's surface). Additional simulation work presented in the paper also indicates that long-term telescopic observations may detect wobbles from such planets using the radial velocity method. Star B, a orange-red dwarf with a relatively calm chromosphere and acoustic p-wave mode oscillations, is an easier target for detecting wobbles from terrestrial planets, possibly within only three years of "high cadence" observations for a 1.8 Earth-mass planet