Hydra constellation. Optical image of stars in and around part of the constellation Hydra, the water snake. North is at top. The brightest star in Hydra, Alphard (Alpha Hydrae), is just above and right of centre. The bright blue star at top centre is Regulus (Alpha Leonis) in the constellation Leo. Hydra is the largest constellation in the sky. The head of the snake is at upper right and its body stretches far off frame to the left. Between Alphard and Regulus is the constellation Sextans (the sextant). The constellations Antlia (the air pump) and Pyxis (the compass) are at lower centre and lower right respectively.