Hydra, in astronomy, southern constellation lying S of Corvus, Crater, Virgo, Leo, and Cancer. It is a long, slender, winding constellation traditionally depicted as a snake and known also as the Water Monster or Sea Serpent. The only bright star in the constellation is Alphard (Alpha Hydrae); it is a double star, but the two components can be detected only with a telescope. Hydra reaches its highest point in the evening sky in the spring.
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