Callisto was a very beautiful Nymph and the favorite companion of Artemis, the goddess of the hunt. Her name derived from the Greek word "kallos", meaning beauty, loveliness.
As a companion of Artemis, Callisto had to take a vow to remain a virgin.
One day, while Callisto was wandering around the woods, she was spotted by Zeus, the King of the Gods, who desired her deeply, so he approached the Nymph by taking the form of Artemis and seduced her.
Callisto was fully aware that Artemis would have none but maidens in her company, so she had to remain in the woods alone until she bore to Zeus a son which she named Arcas.
When Hera, the wife of Zeus, got informed of the birth, she decided to punish the beautiful Nymph by taking all beauty away from her and turning her into a bear.
Soon afterwards, Artemis killed Callisto with arrows, but Zeus took pity on her. So he sent his messenger Hermes to save Callisto's child, Arcas, and transformed Callisto in the constellation of Ursa Major, also known as the Great Bear. Callisto's son Arcas would later pon become the founder of Arcadia in Greece