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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. It had the unusual property that whenever one of its heads was severed, two new heads immediately sprouted from the stump; Heracles overcame this by immediately cauterizing the stump with a torch.

The term "Hydra" is often used today when referring to a problem that seems to share this sort of characteristic, for example when attempts to suppress a particular piece of information result in it being disseminated even more widely.

Hydra is also a Greek island.

Hydra is also the name of a distributed operating system developed at Carnegie Mellon University.