Cancer was a creature sent by Hera to destroy Heracles, one of Zeus' illegitimate children whom she hated the most. This giant crab latched on to Heracles foot as he was battling the Hydra. He already had his hands full as everytime he cut off one of the Hydra's heads, two more grew in it's place. Before finishing off the Hydra, he turned his attention to the crab and crushed it. As a reward for the crab's obedience and sacrifice, Hera placed it's image among the stars.