Hydra is the largest of the 88 modern constellations, and was also one of the 48 constellations listed by Ptolemy. It should not be confused with Hydrus.

Notable features

Hydra is the largest modern constellation, at 1303 square degrees. Despite its size, Hydra contains only one reasonably bright star, Alphard (α Hya, 30 Hya), which is of apparent magnitude 1.98.

R Hydrae is a Mira variable star that ranges in magnitude between 3.5, when it is easily visible to the naked eye, to 10.9, when a telescope is required to see it.

There are several double stars of interest in Hydra. Epsilon Hydrae (ε Hya) is a binary star with components of magnitudes 3.3 and 6.8, separated by 2.7 arcseconds. N Hydrae (N Hya) is a pair of stars of magnitudes 5.8 and 5.9. Struve 1270 (Σ1270) consists of a pair of stars, magnitudes 6.4 and 7.4.

The constellation also contains the radio source Hydra A .

Mythology

The constellation Hydra resembles a twisting snake [1], and features as such in some Greek myths. In Greek mythology, a crow serves Apollo, and is sent to fetch water, but it rests lazily on the journey, and after finally obtaining the water in a cup, takes back a water snake as well, as an excuse. According to the myth, Apollo saw through the fraud, and angrily cast the crow, cup, and snake, into the sky. The origin of this story is likely to be the juxtaposition of this constellation with those of Crater (constellation), and Corvus (constellation), in the area of the sky known as the Sea.

The Hydra was also considered to be the Lernaean Hydra (as defeated by Herakles for one of his Twelve Labours) by the Greeks. Its position in the sky (below the ecliptic), together with the constellation Cancer (which lays near its head) may be the origin of parts of the myth.

See also

• Felis