- (astronomy) The brightest star in the southern constellation of Centaurus. Alpha Centauri is actually a binary star with Alpha Centauri C or Proxima Centauri as a third component. Alpha Centauri A is the brightest star of the system and is somewhat brighter than our sun. Alpha Centauri B is somewhat fainter than Alpha Centauri A but much brighter than Proxima Centauri.
From α (brightest) + Centauri ("of the constellation Centaurus")