Castor and Polydeuces were twin brothers. They were the sons of Leda, the Queen of Sparta though they had different fathers. The twins were born from an egg after Zeus seduced their mother, disguised as a swan. King Tyndareus was the father of Castor, making him mortal, while Polydeuces was the immortal son of Zeus. They went of many adventures together, including one in which they saved their sister Helen of Troy. The two boys were very close but sadly, Castor was killed on one of these adventures.

Polydeuces, heartbroken begged his father Zeus to let him share his immortality with his brother. Zeus granted him his request. The twins alternated as gods on Mount Olympus( stars) and with Hades in the under world.