She was a follower of goddess Artemis, and therefore, she must have taken a vow of chastity. However, Zeus saw her and fell in love with her; to lure her, he turned into Artemis and eventually took advantage of her. Callisto bore a son, Arcas. After this, she was turned into a bear, either by Zeus while trying to hide his misdeeds, by Hera out of jealousy, or by Artemis out of anger that she broke her vow of chastity.
Not content with Callisto's fate as a bear, Hera continued to work against her to get Artemis to think she was a normal bear and slay her. Zeus came to the rescue turning her into the constellation Arctos, the Great Bear, also known as Ursa Major. At Zeus' direction, Hermes saved Arcas from the womb and took him to be raised by Maia. She was joined in the skies with her son, who became the nearby constellation Arctophylax, the Little Bear, also known as Ursa Minor. Continuing to hold a grudge, Hera persuaded Tethys and Oceanus to forbid Callisto from entering their realm, the ocean. As a result, Callisto must perpetually circle the Northern Star and never set over the horizon.