Hydra is the largest of the 88 modern constellations, measuring 1303 square degrees, and also the longest at over 100 degrees.

Its southern end abuts Libra and Centaurus and its northern end borders Cancer. It was included among the 48 constellations listed by the 2nd-century astronomer Ptolemy.

It is commonly represented as a water snake. It should not be confused with the similarly named constellation of Hydrus.

Western mythology

The Greek constellation of Hydra is an adaptation of a Babylonian constellation: the MUL.APIN includes a “serpent” constellation that loosely corresponds to Hydra.

It is one of two Babylonian “serpent” constellations (the other being the origin of the Greek Serpens), a mythological hybrid of serpent, lion, and bird.

The shape of Hydra resembles a twisting snake and features as such in some Greek myths. One myth associates it with a water snake that a crow served Apollo in a cup when it was sent to fetch water; Apollo saw through the fraud, and angrily cast the crow, cup, and snake, into the sky.

It is also associated with the monster Hydra, with its many heads, killed by Hercules, represented in another constellation. According to legend, if one of the Hydra’s heads was cut off, two more would grow in its place.

However, Hercules’ nephew, Iolaus, seared the necks with a torch to prevent them from growing back and thus enabled Hercules to overcome the Hydra.

Mythology and equivalents in other cultures

In Hindu Mythology the star that equivalents Hydra is Ashlesha. In Chinese astronomy, the stars that correspond to Hydra are located within the Vermilion Bird and the Azure Dragon.

The head of Hydra was collectively known as “Min al Az’al,” meaning “belonging to the uninhabited spot” in Arabic.


Despite its size, Hydra contains only one moderately bright star, Alphard, designated Alpha Hydrae. It is an orange giant of magnitude 2.0, 177 light-years from Earth. Its traditional name means “the solitary one”.

Beta Hydrae is a blue-white star of magnitude 4.3, 365 light-years from Earth. Gamma Hydrae is a yellow giant of magnitude 3.0, 132 light-years from Earth.

Hydra has one bright binary star, Epsilon Hydrae, which is difficult to split in amateur telescopes; it has a period of 1000 years and is 135 light-years from Earth.

The primary is a yellow star of magnitude 3.4 and the secondary is a blue star of magnitude 6.7. However, there are several dimmer double stars and binary stars in Hydra. 27 Hydrae is a triple star with two components visible in binoculars and three visible in small amateur telescopes.

The primary is a white star of magnitude 4.8, 244 light-years from Earth. The secondary, a binary star, appears in binoculars at magnitude 7.0 but is composed of a magnitude 7 and a magnitude 11 star; it is 202 light-years from Earth. 54 Hydrae is a binary star 99 light-years from Earth, easily divisible in small amateur telescopes.

The primary is a yellow star of magnitude 5.3 and the secondary is a purple star of magnitude 7.4. 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 other main named star in Hydra is Sigma Hydrae, which also has the name of Minchir, from the Arabic for snake’s nose. At magnitude 4.54, it is rather dim.

The head of the snake corresponds to the Āshleshā Nakshatra, the lunar zodiacal constellation in Indian astronomy. The name of Nakshatra (Ashlesha) become the proper name of Epsilon Hydrae since 1 June 2018 by IAU.

Hydra is also home to several variable stars.

R Hydrae is a Mira variable star 2000 light-years from Earth; it is one of the brightest Mira variables at its maximum of magnitude 3.5. It has a minimum magnitude of 10 and a period of 390 days. V Hydrae is an unusually vivid red variable star 20,000 light-years from Earth.

It varies in magnitude from a minimum of 9.0 to a maximum of 6.6. Along with its notable color, V Hydrae is also home to at least two exoplanets.

U Hydrae is a semi-regular variable star with a deep red color, 528 light-years from Earth. It has a minimum magnitude of 6.6 and a maximum magnitude of 4.2; its period is 115 days.


*This article was originally published at en.wikipedia.org.