In Greek mythology, Callisto or Kallisto was a nymph of Lycaon. Transformed into a bear and set among the stars, she was the bear-mother of the Arcadians, through her son Arcas.
As a follower of Artemis, Callisto, who Hesiod said was the daughter of Lycaon, king of Arcadia, took a vow to remain a virgin, as did all the nymphs of Artemis. But to have her, Zeus disguised himself, Ovid says, as Artemis herself, in order to lure her into his embrace. Callisto was then turned into a bear, as Hesiod had told it:
...but afterwards, when she was already with child, was seen bathing and so discovered. Upon this, the goddess was enraged and changed her into a beast. Thus she became a bear and gave birth to a son called Arcas.
Either Artemis "slew Kallisto with a shot of her silver bow," perhaps urged by the wrath of Hera, or later Arcas, the eponym of Arcadia, nearly killed his bear-mother when she had wandered into the forbidden precinct of Zeus. In every case, Zeus placed them both in the sky as the constellations Ursa Major, called Arktos, the "Bear", and Ursa Minor.
According to Ovid, it was Zeus who took the form of Artemis so that he might evade his wife Hera's detection, forcing himself upon Callisto while she was separated from Artemis and the other nymphs. Her pregnant condition was discovered some months later while bathing with Artemis and her fellow nymphs. Upon this, Artemis was enraged and expelled Callisto from the group, and subsequently she gave birth to Arcas. Hera then took the opportunity to avenge her wounded pride and transformed the nymph into a bear. Sixteen years later Callisto, still a bear, encountered her son Arcas hunting in the forest. Just as Arcas was about to kill his own mother with his javelin, Zeus averted the tragedy by placing mother and son amongst the stars as Ursa Major and Minor, respectively. Hera, enraged that her attempt at revenge had been frustrated, appealed to Oceanus that the two might never meet his waters, thus providing a poetic explanation for their circumpolar positions.