Although a few legends tell of Callisto welcoming Zeus…

““Although a few legends tell of Callisto welcoming Zeus with open arms, most of the versions have Zeus resorting to trickery. In these versions, knowing that Callisto was completely devoted to both Artemis and her vow of chastity, Zeus appeared to the nymph as the goddess Artemis herself while Callisto lay resting under a tree. Once Callisto’s guard was down, Zeus abandoned his disguise and used force against her. To make matters worse, Callisto ended up pregnant from the encounter. Fearing Artemis’ legendary wrath, Callisto tried to conceal her condition but finally was no longer able to one morning when all the nymphs bathed together in a forest glade. Furious that Callisto betrayed her vow (even though by most accounts Callisto hadn’t done so willingly), Artemis turned her into a bear, which she then hunted down and killed. In other versions, Callisto was still allowed to give birth to her son, Arcas, who in turn encountered his mother in her bear form and killed her. In yet other versions, Artemis was on the verge of killing Callisto when Zeus interfered and placed her in the sky where she can be seen as Ursa Major. (Interestingly enough, Riordan’s Artemis takes credit for placing Callisto in the sky herself.)””

— Rick Riordan