Callisto was a very beautiful Nymph and the favorite companion of Artemis, the goddess of the hunt. Callisto's name is derived from the Greek word "kallos", which means beauty, loveliness.
As a companion of Artemis, Callisto had to take a vow to remain a virgin.
One day, while Callisto was wandering through the woods, she was discovered by Zeus, the king of the gods, who deeply desired her, so he approached the nymph by taking the form of Artemis and seducing her.
Callisto was aware that Artemis would only have virgins in her company, so she had to remain alone in the forest until she bore Zeus a son, whom she named Arcas.
When Hera, Zeus' wife, learned of the birth, she decided to punish the beautiful nymph by taking away all her beauty and turning her into a bear.
Soon after, Artemis killed Callisto with arrows, but Zeus took pity on Callisto. So he sent his messenger Hermes to save Callisto's child, Arcas, and transformed Callisto into the constellation Ursa Major, also known as the Great Bear. Callisto's son Arcas would later become the founder of Arcadia in Greece