Callisto
This is where Calista came from. It means "most beautiful." Ok, so Callisto is this priestess friend of Artemis. Zeus takes a liking to her (she's hot) and assumes Artemis's form to seduce her. Then someone (either Zeus, so he wouldn't get caught in an affair, or Hera, because she caught Zeus in an affair) turned her into a bear, which Artemis shot while she was hunting. Zeus turned her into a constellation (Ursa Major), which I'll admit is a nice way to say you're sorry.