Hydra is a constellation in the southern hemisphere. It is the largest constellation in the sky. Its name is Latin for "water snake," referring to the constellation's shape, which resembles a twisting snake. Hydra was created by the Greek astronomer Ptolemy in the 2nd century. The Japanese call the constellation Nuriko, while the Chinese know it as Liu, or Willow.
In Greek mythology, Hydra is identified with the water snake that Apollo's crow (represented by the constellation Corvus
) brought to the god as an excuse for returning very late after being sent to fetch some water. Apollo was angered by the fraud and cast the bird, the cup and the water snake into the sky, where they became the constellations Corvus
and Hydra respectively. Hydra is also associated with the Lemaean Hydra, a water beast with seven heads, killed by Heracles as one of his Twelve Labours.
The constellation Hydra occupies an area of 1303 square degrees and contains nine stars with known planets. It can be seen at latitudes between +54° and -83° and is best visible at 9 p.m. during the month of April.
Despite being so large, the constellation Hydra has few prominent features. The brightest star, 
alpha Hydrae or Alphard ("the solitary one") is of second magnitude. It is a binary star with a giant rich in barium for its primary component. It was also once known as "the backbone of the Serpent."
Two other notable stars in the constellation are both magnitude 3 yellow giants: 
gamma Hydrae is also known as Dhanab al Shuja ("the Snake's tail"), and 
zeta Hydrae is sometimes called Hydrobius ("water-dweller").
The constellation is home to three Messier objects. Messier 83 (NGC 5236), or the Southern Pinwheel Galaxy, lies near the border between Hydra and Centaurus
, approximately 15 million light-years away. It is an intermediate spiral galaxy and one of the closest and most luminous barred spiral galaxies in the night sky. It was the site of six known supernovae.
Messier 60 (NGC 4590) is a globular cluster about 33,000 light-years distant. It contains at least 42 known variable stars and about 250 giants, and is approaching us at 112 kilometres per second.
Messier 48 (NGC 2548) is an open star cluster lying near the border with the constellation Monoceros
. In good conditions, the cluster is visible to the naked eye.
Hydra belongs to the Hercules family of constellations, along with Hercules
, Corona Australis
, Triangulum Australe
Constellations directly bordering Hydra are Antlia
, Canis Minor