Castor and Pollux
One of the best-known myths with Castor and Pollux is how they became a constellation. During a battle, Castor, being mortal, was killed. Heartbroken at the death of his brother, Pollux prayed to Zeus to make Castor immortal which meant Pollux would have to give up half of his immortality.  Eventually, Zeus agreed to the request, and so Castor and Pollux were transformed into the Gemini constellation.  To balance the cosmos, the Dioscuri would only be on Olympus one half of the year, and the other six months would be spent in the Underworld in the Elysium fields.