Kallisto was the daughter of Lycaon, King of Arcadia and also was one of Artemis' hunting attendants. As a companion of Artemis, she took a vow of chastity. Zeus appeared to her disguised as Artemis, or in some stories Apollo, gained her confidence, then took advantage of her. As a result of this encounter she conceived a son, Arcas.
Enraged, Hera or Artemis (some accounts say both) changed her into a bear. Arcas almost killed the bear, but Zeus stopped him just in time. Out of pity, Zeus placed Callisto the bear into the heavens, thus the origin of Callisto the Bear as a constellation. Some stories say that he placed both Arcas and Callisto into the heavens as bears, forming the Ursa Minor and Ursa Major constellations.