Castor was born in Sparta, the son of Tyndareus and Leda. The circumstances of his birth were extraordinary. Zeus, the king of the gods who ruled from Mount Olympus with his thunderbolt, had desired Leda and came to her in the form of a swan. On the same night, Tyndareus also lay with his wife. From this double union, two eggs were produced, or in some versions four children were born from a single egg. From one egg came Castor and Clytemnestra, the children of Tyndareus, both mortal. From the other came Polydeuces and Helen, the children of Zeus, both destined for immortality. Castor and Polydeuces grew up as twins, identical in appearance, inseparable in action, and so alike that no one could tell them apart. They were the Dioscuri, the sons of Zeus, though only one of them truly was.

The brothers were famous throughout Greece for three things: horsemanship, boxing, and hunting. Castor was the horseman, the finest rider in Laconia, whose skill at managing spirited horses was unmatched. Polydeuces was the boxer, whose fists were so powerful that he once killed a man with a single blow in a formal contest. Together they were invincible. They sailed with Jason and the Argonauts on the quest for the Golden Fleece, and when the Argo was caught in a terrible storm, it was the Dioscuri who appeared as twin stars above the mast, calming the waves and saving the ship. They hunted the Calydonian Boar alongside Meleager, Atalanta, and the greatest heroes of the age. They rescued their sister Helen when she was abducted by Theseus as a child, marching on Athens with an army and carrying her back to Sparta while Theseus was away in the underworld.

But the defining episode of Castor's life was the cattle raid that killed him. The Dioscuri had gone to Arcadia to steal cattle from their cousins Idas and Lynceus, the sons of Aphareus. Idas was a man of enormous strength, and Lynceus had eyesight so keen that he could see through solid earth and stone. The twins drove the cattle away, but Idas and Lynceus pursued them. A quarrel broke out over the division of the herd. Idas, in a rage, drew his spear and struck Castor through the chest. The mortal twin fell, his blood staining the Arcadian earth, and Polydeuces was left alone.

The death of Castor was more than Polydeuces could bear. He killed Lynceus in revenge, and then he turned to his father Zeus and begged for the one thing that could end his grief. He asked to share his immortality with his brother, to give up half his divine life so that Castor might live again. Zeus, moved by the love between the twins, granted the prayer. He offered them two choices: they could both live forever on Olympus, but apart from one another, or they could share a single immortality, alternating between the realm of the gods and the house of Hades, the underworld where the dead dwell. Polydeuces chose the second option without hesitation. From that day forward, the twins would spend one day in the bright halls of Olympus and the next in the gloomy dark of the underworld, never separated, never alone, their lives woven together as completely as their birth.

In death, the Dioscuri became greater than they had been in life. They were worshipped throughout Greece as the Anakes, the Lords, and their cult was especially powerful in Sparta, where young men sacrificed to them before battle. They were the protectors of sailors, and their appearance as twin lights above the masthead - what the Romans called St. Elmo's fire - was the most welcome omen a ship could receive. Pindar, the Theban poet who composed victory odes for athletic champions, sang of them as the saviours of ships and the models of brotherly love. And in the heavens, they were placed side by side as the constellation Gemini, the Twins, two bright stars that circle the zodiac together, never rising or setting more than a few hours apart, a celestial image of the bond that death could not break.