Callisto was a beautiful nymph in the retinue of Artermis
(Diana), the chaste goddess, and thus also sworn to chastity. The god
Zeus (Jupiter) fell in love with Callisto and disguised
himself as Diana herself in order to lure the nymph into his embrace and
seduce her. Callisto’s resulting pregnancy is soon discovered and she is
then turned into a bear. Different versions of the tale attribute the transformation
to different figures, among these Diana, who does this in retribution for
the transgression, Hera, who is furious at Jupiter
for getting yet another paramour, and Jupiter himself, who acts to protect
Callisto and her child. She gives birth to a son, Arcas, and they are both
turned into constellations known as the Great Bear and the Little Bear.
See: François Boucher. Jupiter and Callisto.