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. To the unaided eye it appears as a single star, whose total visual magnitude of -0.27 would identify it as the third brightest star in the night sky behind -0.72 magnitude Canopus. Alpha Centauri is a binary star system, designated Alpha Centauri AB (α Cen AB), whose individual components are named Alpha Centauri A (α Cen A ) and Alpha Centauri B (α Cen B ). Both stars lie 1.34 parsecs or 4.37 light years away from our Sun.

A third star, known as Proxima Centauri, Proxima or Alpha Centauri C (α Cen C ), is likely gravitationally associated with the closer binary, Alpha Centauri AB. Proxima is now placed at the slightly closer 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. True separation of Proxima from Alpha Centauri AB is about 0.06 parsecs, 0.2 light years or 13,000 Astronomical Units (AU), equivalent to 400 times the size of Neptune's orbit.

"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. With the aid of a telescope, Alpha Centauri can be resolved into a binary star system in close orbit. This is known as the Alpha Centauri AB system, often abbreviated as α Centauri AB or α Cen AB.

Alpha Centauri A (α Cen A ) and Alpha Centauri B (α Cen B ) are the individual stars of the binary system, usually defined to identify them as the different component of the binary α Cen AB. As viewed from Earth, there is likely an additional companion located 2.2° away from the AB star system, whose distance is much greater than the observed separation between stars A and B. This companion is Proxima Centauri, Proxima, or α Cen C. 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, and they are assumed to be gravitationally associated with each other. Direct evidence that Proxima Centauri has an elliptical orbit typical of binary stars has yet to be determined.

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.


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 millenniacitation needed, though the possibility of space sail, or Nuclear Pulse Fusion technology may cut this down to a matter of decades.


Proxima Centauri b

In August 2016, the European Southern Observatory announced the discovery of a planet slightly larger than the Earth orbiting Proxima Centauri. Proxima Centauri b was found using the radial velocity method, where periodic Doppler shifts of spectral lines of the host star suggest an orbiting object. From these readings, the radial velocity of the parent star relative to the Earth is varying with an amplitude of about 2 metres (6.6 ft) per second. The planet lies in the habitable zone of Proxima Centauri, but it is possible that the planet is tidally locked to the star, resulting in temperature extremes that would be difficult for life to overcome.

Alpha Centauri Bc

On 25 March 2015, a scientific paper by Demory and colleagues published transit results for Alpha Centauri B using the Hubble Space Telescope for a total of 40 hours. They evidenced a transit event possibly corresponding to a planetary body with a radius around 0.92 R⊕. This planet would most likely orbit Alpha Centauri B with an orbital period of 20.4 days or less, with only a 5 percent chance of it having a longer orbit. The median average of the likely orbits is 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. It likely has lakes of molten lava and would be far too close to Alpha Centauri B to harbour life.

Alpha Centauri D

In images taken on 7 July 2014 (343.5 GHz) and 2 May 2015 (445 GHz), researchers discovered a source in the far infrared located within 5.5 arcseconds of α Cen AB. Based on its proper motion, it was at first thought to be a part of the Alpha Centauri system. Further analysis, however, found that the object must be closer to the Solar System, and that it may be gravitationally bound to the Sun. The researchers suggest that the object may be an extreme trans-Neptunian object (ETNO) beyond 100 AU (15 billion kilometres), a super-Earth at around 300 AU (45 billion kilometres), or a very cool brown dwarf at around 20,000 AU (3.0 trillion kilometres).

Most used system in SF and in Space Opera settings:

  • Gordon R. Dickson's? Childe Cycle? (1959) has the planets Cassida? and Newton? in orbit around Alpha Centauri A and B, respectively.
  • In Larry Niven's Known Space (1964 onward), Wunderland is a habitable planet circling Alpha Centauri, and was the earliest extra-solar colony in human history. Later it is occupied for a long time by the Kzinti?, an intelligent catlike species, after their first contact with humans, which resulted in several interstellar wars.
  • In Stewart Cowley's? Terran Trade Authority? setting (1978–1980), Alpha Centauri is the home system of the Alphans?, the first alien race Terrans made contact with and allies in the war with Proxima Centauri.
  • In Isaac Asimov's Foundation and Earth? (1986), Foundation councillor Golan Trevize and his traveling companions find the last survivors of a radioactive Earth on a largely marine planet, Alpha?, which is in orbit around the largest star of the Alpha Centauri system. The name of the settlement is New Earth.
  • In The Domination? series (1988 onward) by S. M. Stirling, Samothrace? is a planet in the Centauri system inhabited by descendants of those who managed to escape Earth after an oppressive militaristic nation known as The Domination of the Draka? won control of the planet.
  • In the Starfire? series of novels (2002 onward) by David Weber and Steve White, Alpha Centauri is the most important system in the Terran Federation? due to the large number of warp-point junctions in the system and its proximity (one warp transit) to Earth. It is the headquarters and principal shipyard of the Terran Federation Navy. Because of the nature of warp junction travel, it was believed to be secure from any attack because of the Terran Federation's immense strategic depth, however in the novel In Death Ground, the Arachnids? discovered a closed warp point into the Alpha Centauri system, allowing the system to be threatened and seriously attacked.
  • In C.J. Cherryh's Alliance-Union future history, the Alpha Centauri system is the site of the space settlement Beta Station, established in 2039 but mysteriously abandoned around 2160. Because it is off the main lines of trade and apparently has no special resources of note, the system is not recolonized for at least the next 300 years (i.e. when Cyteen takes place).
  • In Doctor Who?, Alpha Centauri is home to a slug-like, six-tentacled species. They are timid, pacifistic, prone to panic and change their skin colours in response to their moods, but are still loyal and dutiful members of the Galactic Federation?. If they have individual names, they've never been mentioned: it's always been 'The Delegate from Alpha Centauri'.
  • In Star Trek?, an Earth-like planet orbiting Alpha Centauri was one the five founding members of the United Federation of Planets. The Star Fleet Technical Manual shows the flag and seal of the "Alpha Centauri Konkordium of Planets?," presumably the government of the star system in the original series time frame.
  • In the original Marvel comics for the Transformers?, the planet Cybertron? was originally in orbit of Alpha Centauri. Due to the Great Wars, however, the planet was knocked out of orbit and set adrift.
  • The computer games Frontier?: Elite II (1993) and Frontier: First Encounters (1995), Alpha Centauri is notable as being the location of the discovery of the first extrasolar planet with flowing surface water. The planet (named Eden) was later determined as being uninhabitable, though a small research station is present on the surface. The system is an unpopular destination for players as the only starport in Alpha Centauri is usually 900 to 1,000 A.U. away from where ships tend to exit hyperspace from. This usually means wasting an (in-game) month trying to reach the port without many prospects for combat or trade.
  • Sid Meier's Alpha Centauri? (1999), a spiritual sequel to Civilization, involves seven ideological factions competing to colonize the Earth-like planet Chiron? that orbits Alpha Centauri. An expansion pack, called Alien Crossfire, has also been released, also set in the Alpha Centauri system. A GURPS book (GURPS Alpha Centauri?, 2002) was published to allow role-playing in this universe.
  • Michael Ely has written a trilogy of novels based on Sid Meier's Alpha Centauri? (1999) computer game: Centauri Dawn (2000), Dragon Sun (2001), and Twilight of the Mind (2002). The game also spawned a graphic novel, Alpha Centauri: Power of the Mindworms (2000), written by Steve Darnell and illustrated by Rafael Kayanan.
  • In Earth & Beyond? (2002), Alpha Centauri is featured as a Terran colonized star system with two habitable planets: Zweihander?, a Super-Earth orbiting Alpha Centauri A and Witburg?, a planet about the size of Mars orbiting Alpha Centauri B.
  • In the GDW's 2300 AD?, Tirane? is the first planet of Alpha Centauri A, a garden habitable world, and it houses many colonies of several Terran nations. Tirane is the first habitable world discovered by mankind, the importance of Tirane has grown up until to be converted into one of the core worlds. Limbes? is the third planet orbiting Alpha Centauri B, a post-garden world, sterilized by greenhouse effect. The ESA nations have a research station in orbit around the world. Despite almost a century of study, no surviving life forms have been detected, but fossil evidence indicates a rich biosphere just under 1000 million years ago. Sheol?, the first planet of Alpha Centauri B, is a hothouse with fairly large mineral deposits, a combined Mexican/Argentine research station is maintained in orbit around the world. Moiroi? (and its associated satellites: Clotho?, Lachesis?, and Atropos?), is the sole planet of Proxima Centauri, a few nations have scientific or mining stations in the moons.
  • Escape Velocity game. System of planet Landfall.
  • A system in the Space: Above and Beyond TV-series is called Proxima.

Wikipedia, Sid Meier's Alpha Centauri, IES,