Callisto and Constellation of Ursa Major

Zeus was the Olympian God and he was the king of all the Greek gods. 

Before the Olympians became the Greek Gods, the Titans were considered as the Gods of the Greeks.

It was believed that Zeus along with the other Olympians fought and defeated the Titans.

Zeus’s wife was Hera. However Zeus was frequently unfaithful to Hera and Zeus had many lovers. Hera was jealous of Zeus and frequently tried to injure or hurt Zeus’ lovers or the children born to Zeus and his other lovers.

One of Zeus’s children through other lovers was Artemis, who was the Goddess of the hunt. 

Artemis had a twin brother called Apollo. Apollo was the God of religious law and he helped humans by telling them his knowledge of the future and he also told the humans about the will of his father - Zeus.

Callisto was a nymph and she was most probably the daughter of Lycaon of Arcadia. Callisto had joined Artemis in the hunt. There were many women who joined Artemis in the hunt. All of them promised to remain unwed. 

However, Callisto was loved by Zeus.

In all the versions of the story Callisto was turned into a bear. 

In some versions Callisto was turned into a bear by Zeus himself to hide Callisto from Hera.

In some other cases Hera or Artemis changed Callisto into a bear, to teach Zeus a lesson or simply because they were angry with Callisto.

Eventually it so happened that during a hunt Artemis killed Callisto whom she mistook to be a real bear. 

It could be that this was the work of the jealous Hera who did not like any of Zeus’s lovers. 

Arkas was the child growing in the womb of Callisto when she was killed by Artemis and Arkas was the child of Callisto and Zeus. 

Zeus gave Arkas to Maia, a Titaness, to raise. 

After that Zeus placed Callisto among the stars as the constellation Ursa Major or the great Bear.

Adapted from Greek mythology