Hydra is a constellation in the southern celestial hemisphere, commonly represented as a water snake. The largest of the 88 modern constellations, it stretches more than 100 degrees across the sky. Its southern end abuts Libra and Centaurus, while its northern end borders Cancer. It was included among the 48 constellations listed by the 2nd-century astronomer Ptolemy. Despite its size, Hydra contains only one moderately bright star, Alphard, designated Alpha Hydrae, which is an orange giant of magnitude 2.0, 177 light-years from Earth. Other stars include Beta Hydrae, a blue-white star of magnitude 4.3, as well as Gamma Hydrae, a yellow giant of magnitude 3.0. Hydra contains three Messier objects: M83, also known as the Southern Pinwheel Galaxy, located on the border with Centaurus; M68, a globular cluster near M83; and M48, an open cluster at the western end of the constellation.

This illustration was produced around 1823 and comes from Urania's Mirror, a set of 32 astronomical star chart cards. Along with Hydra, the chart also depicts the constellations of Noctua (obsolete), Corvus, Crater, Sextans Uraniae (now Sextans), Felis (obsolete), Lupus, Centaurus, Antlia Pneumatica (now Antlia), Argo Navis (obsolete) and Pyxis Nautica (now Pyxis).Lithograph credit: Sidney Hall; restored by Adam Cuerden