Which ancient Greek mythological god or goddess turned Callisto into a bear?

In ancient Greek mythology, Callisto was a nymph, a minor female deity generally regarded as a personification of nature, and typically depicted as a beautiful maiden. She was the daughter of King Lycaon of Arcadia. The name ‘Kalliste’ in Greek means ‘most beautiful’.

Callisto was also one of the followers of Artemis, the Greek goddess of the hunt, wilderness, wild animals, the Moon and chastity.

According to the mythological story, Zeus, king of the gods on Mount Olympus was attracted to Callisto. He transformed himself into the figure of Artemis to lure Callisto and then seduce her. She became pregnant and gave birth to a son named Arcas.

Hera was Zeus’s sister and wife, goddess of women, marriage, family and childbirth in Ancient Greek religion and mythology. One of her defining characteristics was her jealous and vengeful nature against Zeus’ numerous lovers and illegitimate offspring. Hera learned of the birth and expelled Callisto from her group and turned her into a bear.

Years later when Callisto’s son Arcas was hunting and he was about to mistakenly kill his mother, Callisto was set among the stars as Ursa Major (‘Latin for ‘the Great Bear’). As a result, Callisto was then the bear-mother of the Arcadians and her son ascended to the throne replacing King Lycaon.

Ursa Major is a constellation in the northern sky, one of the original 48 constellations listed by Ptolemy in the 2nd century CE.