Castor (α Gem / α Geminorum / Alpha Geminorum) is the second brightest star in the constellation Gemini and one of the brightest stars in the nighttime sky. Although it has the Bayer designation "alpha", it is actually fainter than Beta Geminorum (Pollux).

Castor and Pollux are the two "heavenly twin" stars that give the constellation Gemini (meaning "twins" in Latin) its name.

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.

Data for the six stars that make up Castor:

• Castor Aa: spectral type: A1V (main sequence),
• Castor Ab: spectral type: M5V (main sequence),
• Castor Ba: spectral type: A2-5Vm (main sequence),
• Castor Bb: spectral type: M2V (main sequence),
• Castor Ca (YY Geminorum Aa): spectral type: M0.5 Ve,
• Castor Cb (YY Geminorum Ab): spectral type: M0.5 Ve.