The Dioscuri (sons of Zeus in Greek) is the name given to the twins Castor and Pollux (Polydeuces) who were the brothers of Clytemnestra and Helen of Sparta featured in The Illiad and The Odyssey. Their mother was Leda, Queen of Sparta and wife of Tyndareus. It is said the Leda was seduced by Zeus in the shape of a swan and gave birth to an egg that held her children. Some stories say she laid two eggs, one containing Helen and Pollux and the other containing Clytemnestra and Castor and that Helen and Pollux are the children of Zeus and Clytemnestra and Castor the children of Tyndareus born a more traditional way. Other sources say that Castor and Pollux were born of the egg and Clytemnestra and Helen are the daughters of Tyndareus. Whatever their paternity, the brothers were very close and went everywhere together.
Pollux was immortal and Castor was not. When Castor was killed in battle, Pollux asked Zeus to give his immortality to his brother. So Zeus put them both in the sky as the constellation Gemini.