The Brighter Stars of Cygnus
ZeusThe Swan is the god Zeus in disguise flying off to seduce a woman. Exactly which woman is in some question.
NemesisIn some stories the target of Zeus' affections is the nymph Nemesis, and the outcome of Zeus' conquest is an egg, which was given to Queen Leda of Sparta. From the egg hatched the beautiful child who was to become Helen of Troy.
Queen LedaIn other versions of the story, it was Queen Leda herself, who was seduced by Zeus, but nevertheless slept with her husband King Tyndareus on the very seem evening. There was still an egg produced as the result of the coupling - or even two eggs. After all Zeus had taken the form of a bird. From the egg - or eggs, came the Twins Castor and Polydeuces (known to the Romans as Pollux). We see the Twins in the sky as the Gemini.
Helen and ClytemnestraSome say that there were two girls born from the eggs as well, Helen and her sister Clytemnestra.
Helen and Polydeuces were the children of Zeus and immortal. Castor and Clytemnestra were the children of Leda's mortal husband Tyndareus and were therefore mortal.