Their intriguing tale exists in various versions, each subtly different from the other. Yet the central plot details remain the same: Kastor and Polydeukes, Latinised to Castor and Pollux, were twin brothers. Zeus, disguised as a swan, seduced their mother, Leda, the Spartan queen who was married to king Tyndareus. As the story goes, because a swan had seduced Leda, the four children, Castor and Pollux, and their twin sisters, Helen and Clytemnestra, were born from an egg. But, not all of them could call the king of gods, Zeus, their father: Pollux was born immortal, while Castor was mortal. 

When Castor was killed in battle, Pollux pleaded with Zeus to take his immortality so he could be with his human brother. But Zeus offered a different solution: the twins could share immortality, meaning they would switch between the gods on mount Olympus and among the mortals in the underworld. 

So according to the myth, Pollux gave half of his immortality to his brother. Some believe Castor and Pollux were placed among the stars as a reward for their superhuman love. As the constellation of Gemini, they are often referred to as the evening and the morning star, the dusk and dawn, and the sun and moon. Indeed, can day exist without night or the sun without the moon? They are all necessarily dependent on each other.