Relative size of stars in Alpha Centauri system
Alpha Centauri is a triple star system. It consists of two main stars, Alpha Centauri A and Alpha Centauri B (which form a binary star together), and a dimmer red dwarf named Proxima Centauri. The larger member of the binary star, Alpha Centauri A, is similar to the Sun, but a little larger and brighter. Like the sun, its spectral type is G2 V. The smaller of the two, Alpha Centauri B, is dimmer, with a spectral type of K1 V. The two orbit one another elliptically (e=0.52), approaching as close as 11.2 astronomical units and receding to 35.6 AU with a period of just under 80 years.
Computer models of planetary formation suggest that terrestrial planets would be able to form close to both Alpha Centauri A and B, but that gas giants planets similar to our Jupiter and Saturn would not be able to form because gravitational effects of the binary stars would keep these planets from forming. Some have speculated that any terrestrial planets in the Alpha Centauri system may be dry because it is believed that Jupiter and Saturn were crucial at directing comets into the inner solar system and providing the inner planets with a source of water.
The red dwarf Proxima Centauri is only about 13,000 astronomical units away from Alpha Centauri and may be in orbit about it, with a period on the order of 500,000 years or more. For this reason, Proxima is sometimes referred to as Alpha Centauri C. However, it is not clear if it really is in orbit, although the association is unlikely to be entirely accidental as it shares approximately the same motion through space as the larger star system.
Viewed from Alpha Centauri, the sky would appear very much as it does to us with most of the constellations such as Ursa Major and Orion being unchanged. However, Centaurus would be missing its brightest star and Cassiopeia would be home to a bright star in the form of Sol, our Sun.
See also: List of nearest stars