Who is the woman that became the Great Bear?

The woman is named Callisto. She was a hunter's servant, turned into a bear by the goddess Hera. Callisto also had a son named Arcas who didn't know that his mother had been turned into a bear. So, he was about to kill her when Zeus, Hera's husband, put them both into the sky as the greater and lesser bears, Ursa Major and Ursa Minor. That is the legend of how Callisto became a bear, and then became Ursa Major.