Up until this point in the story, Hera had done nothing to punish Callisto. The birth of the baby, however, was too much for the queen of the gods to endure. Hera leaped on Callisto and threw her to the ground. Callisto, pleading with Hera to spare her, could only watch with horror as her own arms and legs grew hairy and rough and her nails grew long and pointed. Soon the transformation was complete. In her jealous anger Hera had turned Callisto into a bear.
Callisto fled deep into the Pelasgian forest and lived alone in despair. Because her mind remained human, she fully understood what had happened to her. Yet her bear’s mouth would not allow her to talk, only growl, so she had no way of warning hunters that she was not to be feared.
Meanwhile, Zeus had given Arcas to one of his previous lovers, Maia, to raise. Maia was the eldest of the seven sisters who form the constellation of the Pleiades. In the past Zeus had seduced her too, and she had given birth to Hermes, messenger of the Olympian gods.
When Areas was 15 he went into the Pelasgian forest to hunt. Callisto, who was still living in the forest, recognized her son immediately. She followed Arcas because she was curious to see him and longed to be near him, but she knew she had to stay hidden. Suddenly. Arcas caught sight of the bear and pursued her.
Arcas was about to kill Callisto with his bow and arrow when Zeus intervened. He snatched up Callisto and set her into the heavens, so saving Arcas from the crime of killing his mother. The god transformed Callisto into a constellation, the Great Bear and Arcas into the brilliant star Arcturus. (According to another version of the myth, Arcas became king of Pelasgia as the heir of his grandfather Lycaon, and the land of Pelasgia was renamed Arcadia in his honor.)
Hera was not happy at seeing her one-time rival’s sufferings cut short, especially by the intervention of her own husband. She went to see her old nurse Tethys, the wife of the ocean, Oceanus. Tethys sympathized with Hera. She forbade the new constellation ever to plunge into the ocean to drink, which is why the Great Bear constellation never sets into the ocean, as other stars do, but always remains above the horizon.