Castor (α Geminorum) is the fainter star of the "heavenly twins." Pollux is its partner. Since Castor and Pollux are always considered together in ancient literature, see Pollux for the history and astrological significance of the two.
Astronomically, Castor was discovered to be a visual binary in 1719, with the magnitude of its components being 2.8 and 2.0. The separation of the components is about 6" and the period of revolution is around 350 years. Each of the components of Castor is itself a spectroscopic binary, making Castor a quadruple star system. Castor has a faint companion separated from it by about 72" but having the same parallax and proper motion; this companion is also a spectroscopic binary with a period slightly less than 1 day. Castor can thus be considered to be a sextuple star system, with six individual stars gravitationally bound together.