Hydra (the sea monster) is the largest of the 88 modern constellations, and was also one of the 48 constellations listed by Ptolemy. It should not be confused with Hydrus.
Despite its size, Hydra contains only one reasonably bright star (Alphard, α Hya). It also contains the radio source Hydra A.
In Greek mythology, the Hydra is the many-headed serpent which Heracles defeated. See Lernaean Hydra
Hydra is one of the Saronic Islands[?] in Greece.
Hydra is also the name of a distributed operating system developed at Carnegie Mellon University.
Hydra is the name of a genus of simple fresh-water polyp, of the class Hydrozoa[?].
