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).(31). Interesting objects are: the nebula H. IV. 27 Hydrae, a planetary nebula, gaseous and whose light is about equal to an 8th magnitude star; e 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.