King Athamas of Boeotia married the nymph Nephele at Hera's command. She was a nymph made from a cloud in Hera's image to trap the centaur, Ixion. The couple had two children; a son named Phrixus and a daughter named Helle. After a while, Athamas grew tired of Nephele and left her.
Athamas then remarried, and his second wife was Ino, the daughter of Cadmus. She also gave him children -- two sons named Learches and Melicertes. Nephele was angry and asked Hera to assist in avenging Athamas' desertion. While this was going on, Zeus asked Ino and Athamas to care for Dionysus, as he wanted to keep him a secret from Hera. Hera eventually punished them for this by making them go mad. Ino was terribly jealous of Athamas and Nephele's children and plotted their deaths.
Ino tricked Athamas into attempting to sacrifice Phrixus, but he was saved by Heracles who sent a golden ram to carry him to Colchis. Helle grabbed hold of the winged ram, but fell to her death between the Mediterranean and Black Sea when she lost her hold. The strait, to this day, is named after her -- the Hellespont (Dardanelles). Upon reaching Colchis, Phrixus sacrificed the golden ram to Zeus, who in turn placed the ram's image among the stars in honor of it's heroism.