Although they were twins, Castor was mortal, while Polydeuces were immortal. They joined the Argonauts but on their journey clashed with two other Argonauts twins, Idas and Lynceus, over two beautiful women. Castor and Polydeuces carried the women off, but they were pursued. When Lynceus stabbed Castor, Polydeuces attacked and killed him. Zeus, meanwhile, stepped in and killed Idas with a thunderbolt. Polydeuces survived but Castor did not. Not wanting to live without his brother, Polydeuces begged Zeus to remove his immortality. Zeus granted his wish and placed the twins in the stars.
Fun Facts!
Gemini is the Twins, and for thousands of years the two brightest stars of this constellation, Castor and Pollux, have been thought to represent twins. In ancient Egyptian mythology, they were seen as a pair of sprouting plants, in Phoenician culture they were seen as a pair of kid goats. And the Romans believed they were Romulus and Remus, the twin boys who founded the city of Rome.
Castor and Polydeuces aren't the only twins in Gemini. What appears to be the star Castor is actually made up of three pairs, with each pair in orbit.
On their voyage, Polydeuces boxed Amycus, the giant son of Poseidon and the world’s biggest bully. When Amycus insisted someone fight him, Polydeuces offered and, even though Amycus had invented boxing and was immensely strong, Polydeuces was faster. He beat the bully.