Zeus and the queen of Sparta bore a pair of twin brothers --Pollux and Castor.  After they caught a herd of cattle along with their cousins, the cousins fatally wounded Castor with a blow from a spear, to try to take the cattle for their own.  On Castor's death, Pollux asked to share the fate of his brother and this touched Zeus, who set them up as a constellation, enabling them to live alternatively in Olympus and the underworld.  They lived one day, died another, and were always together.