Son of Leda
; brother of Clytemnestra
; half-brother of Pollux
, children by the same mother of Zeus
, who seduced Leda in the form of a swan. Nine months later, she laid two eggs, each of which contained two children, separated by gender, one within each wholly-mortal (Castor and Clytemnestra) and the other with god's blood (Pollux and Helen of Troy
Part of the Dioscuri or Hero Twins, the other of which was Pollux (or Polydeuces).
In his life Castor was a skilled horseman, but as a mortal upon his death he was consigned to Hades and the Elysian Fields while his brother was admitted to Olympus as befitting a divinity. However, Pollux interceded, saying that he could not bear eternal separation from his twin brother. Zeus couldn't bear to send his son to Hades but couldn't let Castor into Olympus, so instead he put them, together forever, in the sky.
The star Castor is the head of one of the two figures in the constellation Gemini - it is white, of the second magnitude and fittingly less bright than the head of his brother Pollux.