Castor and Pollux
Castor and Pollux were twin brothers in Greek and Roman mythology. Together they are called the Dioscuri. They were the sons of Leda. Their twin sisters were Helen of Troy and Clytemnestra. Castor and Pollux were changed into the constellation called Gemini. Tyndareus, the king of Sparta, was the father of Castor (hence a mortal), while Zeus was the father of Pollux (a demigod).
The four children born to Leda were the two brothers Castor and Pollux, and two sisters, Helen and Clytemnestra; Castor and Clytemnestra were considered to be the children of King Tyndareus, Pollux and Helen were the offspring of Zeus. Castor and Pollux are universally named among the Argonauts, the crew of the Argo who sailed for Colchis with Jason. During the quest for the Golden Fleece, Pollux is noted for having bested the King of Bebryces during a boxing match.
One of the best known myths with Castor and Pollux is how they became 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 though 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 .