Alpha Centauri, brightest star in the constellation Centaurus and 3d-brightest star in the sky; also known as Rigil Kent or Rigil Kentaurus; 1992 position R.A. 14^h39.1^m, Dec. -60°49'. It is a yellow main-sequence star of the same spectral class (G2 V) as the sun and of about the same size and mass; its apparent magnitude is -0.26. Actually, Alpha Centauri is a triple-star system, the components being designated A, B, and C. Alpha Centauri C is also called Proxima Centauri because it is the closest star to the earth (other than the sun), at a distance of 4.28 light-years, components A and B are currently 4.34 light-years away. Proxima Centauri orbits about the common center of mass of the system with a period of more than 250,000 years, so that in about 125,000 years it will be more distant than A and B.