The Hydra constellation is the largest constellation in the night sky and the largest of the 88 constellations officially recognized by the International Astronomical Union . It fills the space between the zodiacal constellations along the ecliptic on one side, and the Milky Way on the other.

This constellation meanders southeast from a small circle of stars two-thirds of the way up in the west-southwest sky to the east-southeast horizon. It is the largest constellation in the sky, covering 1,303 square degrees, hence it is partially visible from both hemispheres at certain times of the year.

It is a long, thin constellation, with its head slightly north of the celestial equator, but whose tail dips below a declination of 30°S. The angular distance between the head and the tail of the snake is 85°. And, despite its large size, Hydra is a dim constellation with only one star brighter than 3rd magnitude.

the brightest object

The brightest star in this constellation is Alfard; it has an apparent magnitude of nearly +2.0, making it the brightest star in Hydra. The star Alfard is an orange giant of spectral type K3II-III with a surface temperature of 4,000ºK, not unlike the well-known Aldebaran (the brightest star in the constellation Taurus). It has a visual luminosity 400 times greater than that of the Sun, although being so far from us, exactly 177 light years, we see it fainter in the sky than Aldebaran itself. Alfard’s name comes from an Arabic phrase meaning ‘the lonely one on the snake’.

These are the brightest and most popular stars in Hydra: Alfard, Gamma Hydra, Zeta Hydra, Aslesha, Ukdah, Zhang or Minchir. Although all of them are quite faint, from a dark sky, most of the stars in the constellation could be tracked with the naked eye.

As a curiosity, among the 157 bright galaxies of Hydra is the unusual galaxy ESO 510-G13. This +13.4 magnitude galaxy is located near the southeastern end of the constellation. This edge-on spiral galaxy is about 240 million kilometers from us.

There are three Messier objects in Hydra:

Hydra is home to some bright deep-sky objects, including the open cluster Messier 48, which is visible to the naked eye in good atmospheric conditions and lies near Hydra’s westernmost boundary with Monoceros, the galaxy M83, known as the Pinwheel. Austral -located 15.04 million light years away- and the globular cluster M68 discovered by Charles Messier in 1780 and which is located about 33,000 light years away from Earth.

classical mythology

The Hydra constellation is associated with the legend of the second labor of Hercules , which could be an echo of a Babylonian legend in which the hero Gilgamesh slew a seven-headed monster. The Hydra was a freshwater serpent born from Echidna (a monstrous nymph, half woman, half serpent, who belonged to the lineage of the Forcides) and Typhoon (a Titan, brother of Cronus and divinity of hurricanes and volcanic eruptions) . They are the parents of monsters like Hydra. In Greek mythology, the Hydra is often identified as the Lernaean Hydra, a multi-headed creature slain by Hercules; in the sky, however, he is depicted with a single head. Hercules had, as one of his twelve labors, the task of killing this many-headed, poisonous-breathing monster that lived in a swamp. It was not an easy task as he had to hold his breath during the fight to avoid breathing in the monster’s breath.

Referencia: Hydra the multi-headed serpent : deep-sky delights Magda Streicher Monthly Notes of the Astronomical Society of South AfricaVol. 72, No. 1_2
Published Online: 1 Feb 2013 https://hdl.handle.net/10520/EJC132997

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