It is the largest constellation in the sky. It represents, according to the most popular myth, the snake of the swamp of Lerna, born of Typhon and Echidna, brought up by Hera herself, to task Heracles with one of the twelve mythic labors.
The Hydra was a monster with many heads, as many as one hundred according to some authors. With his mephitic breath he could kill anyone who came near him, ravaged crops and stole cattle.
Heracles tried to kill him cutting off his heads one at a time, which immediately re-grew. He asked the help of his nephew Iolaus, who set fire to the nearby forest and burnt the flesh with the embers, so that the head could not re-grow. In his blood Heracles dipped his arrows, making them poisonous.
The snake was placed in the sky in memory of the victory of Heracles, and so was the crab sent to aid Hydra by Hera, who immortalized it in the constellation of Cancer.
The head of the Hydra is formed by some stars of magnitude 3, just east of Procyon, the brightest star in the constellation Canis Minor. Its heart is marked by the red star Alphard ( a Hydrae ).