While Callisto spent her days and nights with Artemis' other followers, she caught the eye of Zeus. Knowing that the maiden had taken a vow of chastity, Zeus resorted to deception to get at Callisto. He came to her disguised as Artemis, and the young huntress let down her guard. Seizing the opportunity Zeus raped her.
Callisto became pregnant, and tried desperately to conceal her condition form the goddess. After all, she had, in a way, broken her vow to the goddess and she feared her anger. Callisto had been successful for a time, but then a day came when all of the young women who followed Artemis disrobed to bathe together in a spring. By now Callisto was beginning to show, and once she was naked her secret was revealed. Artemis was furious and she banished the young woman from her fold. Callisto wandered off to have her child alone.
Hera decided that this was the time to exact her revenge. She gripped Callisto's hair and threw her to the ground where the new mother was transformed into a bear. The hunter became the hunted. The child that Callisto had by Zeus was spirited away by Hermes to be raised by his mother, Maia. He was named Arcas, meaning "bear," and he grew up to be a fine hunter himself. Some sources have the bear captured and taken to Callisto's own father, Lycaon.
According to some sources Artemis herself killed the bear that was once Callisto, but it is usually accepted that when Arcas was out hunting as a young man he encountered the bear. Callisto recognized the handsome youth as the son she could not raise herself. Forgetting her present form, she tried to come near him, but her loving mother's arms were now strong, furry paws, and her once soothing voice was now a rumbling growl. The bear scared Arcas, and he took aim at her with his spear. Zeus took pity on his former victim and intervened. He placed Callisto in the sky as the constellation Ursa Major, or "great bear," and then took Arcas and placed him in the sky near his mother as Ursa Minor, the "little bear."
Hera was not pleased with this arrangement, especially since Callisto was another of her husband's infidelities. She went to her nurse, Tethys, the wife of Oceanus, and beseeched her to punish Callisto and Arcas. Tethys decided to deprive the pair of water, and so the great bear and the little bear are cursed to circle in the skies, never to dip below the horizon for a refreshing bath or a cool drink. Here the peoples of ancient Greece explained why the two constellations are circumpolar, visible all year round.