HYDRA  , in

astronomy, a constellation of the southern hemisphere, mentioned by Eudoxus (4th century B.C.) and Aratus (3rd century B.c.), and catalogued by Ptolemy (27 stars), Tycho Brahe (19) and Hevelius (31) . Interesting objects are: the nebula H . I V . 27 Hydrae, a planetary nebula, gaseous and whose light is about equal to an 8th magnitude star; Hydrae, a beautiful triple star, composed of two yellow stars of the 4th and 6th magnitudes, and a blue star of the 7th magnitude; R . Hydrae, a long period (425 days) variable, the range in magnitude being from 4 to 9.7; and U . Hydrae, an irregularly variable, the range in magnitude being 4.5 to 6 .