The constellation Hydra (Hydra-Hya)

The name, the story, the myth


The two small constellations on the back of the Hydra, the Cup and the Crow, lead to distinguish different parts in the water snake, something that already the ancient mythographers have frequently done. The astronomer John Flamsteed (1646-1719) proposed a division in four from head to tail or from West to East: Hydra, Hydra et Crater, Hydra et Corvus and Continuation Hydra. The figure in its entirety is considered a female water snake, as opposed to the more modern southern water snake, Hydrus, which is the male created as a complement of the Hydra.


It is an ancient constellation. From Mesopotamia there is evidence, dated around 1200 B.C., of its identification with the primitive water snake Tiamat, killed by Marduk during the Great War of the Gods, an association also found for the Dragon, as well as for the sea monster Ceto.

The most known legend about the water snake identifies it with the Hydra of Lerna of the docic labors of Heracles (Hercules). Lerna was a fertile coastal region adjacent to the city of Argos. It was terrorized by the monstrous Hydra, which lived in a swamp of unknown depth. The creature, with the body of a dog and nine heads (according to the most common version), each one spitting and blowing poisonous fumes. It also had the ability to grow back two or even three heads as soon as one was cut off or crushed. To defeat her, Heracles followed the instructions dictated by Athena.

He pulled the monster out of its lair by shooting glowing arrows and confronted it by holding his breath. He began to chop off its heads, but with each beheading, more heads immediately returned. The trusty friend Iolaus rushed to help and began to cauterize the cuts on the necks of the Hydra as they were cut off using hot embers, taking away their ability to regenerate themselves. Heracles thus managed to find the golden and immortal head of the Hydra among the others that were still shaking, and cut it off with a clean, sure and precise blow. Then he buried it under a heavy rock, gutted the body of the monster and dipped its arrows in its blood, becoming deadly for anyone who was hit by it.


It is a really big constellation, since it covers an area of 1303 square degrees: just think that it starts from the parts of the Cancer constellation, rolls up and unrolls below Leo and Virgo, and ends between the plates of Libra. If you know, for example, how big is Leo or even more Virgo, then you can vaguely imagine how vast is the constellation of Hydra. I was saying that maybe we have it there in front of our eyes without knowing it: thanks to the map of Stellarium there should be no more problems to find it in the sky!

Although it is poor in particularly bright stars, it has a fair group of nearby stars, another family of big stars (including a star Gargantua) and many very interesting deep sky objects. This time let's start with the latter, a couple of which are really suggestive.

Deep Sky Objects

Let's start this rich overview of photos (almost all taken by HST: I remember that clicking on them you can see a higher resolution version) with the beautiful Southern Pinwheel Galaxy (the southern pinwheel), known and catalogued as M83

the beautiful galaxy M83

Let's then move on to a globular cluster, M68, also really splendid (look at it in high resolution!)

the Globular Cluster M68

followed by an open star cluster, catalogued by Messier as M48

the open star cluster M48

Moving on to the objects of the New General Catalogue, we encounter NGC 3109, an irregular galaxy


the irregular galaxy NGC 3109

and then we come across the barred spiral galaxy NGC 3621.

the barred spiral galaxy NGC 3621

But the main course is still to come: in the meantime we see a very strange planetary nebula (NGC 3242) called The Ghost of Jupiter, which takes its name from the fact that the telescope looks like the planet, but under very ghostly appearances.

the nebula called "The Ghost of Jupiter"

We have finally arrived at an absolutely exceptional photograph for the singular coincidence that testifies: we are seeing NGC 3314, a more unique than rare example of perfect alignment between two galaxies, actually very far apart. The nearest one, NGC 3314a, is in fact about 117 million light years away from us, while the second one, NGC 3314b, is about 140 million al: it is the 23 million al between the two galaxies that make the difference: the two objects in fact do not interact at all with each other...

the "double" galaxy NGC 3314

Several stars close to us

As I said at the beginning, the Hydra contains 8 stars below the threshold that I imposed myself in these articles, 60 light years, a distance from which our Sun becomes a star invisible to the naked eye (its magnitude drops below the sixth), in the science fiction hypothesis that tomorrow man can go to that distance, near one of these nearest stars, to verify how much we know only according to the formulas: thanks to our spaceship Celestia, we are able to anticipate what the spectacle will be in the eyes of the lucky galactic navigators (human or machine) observing the position of the Sun on the celestial vault.


In this table I have grouped these stars indicating their increasing distance in light years, the name of the star (with the link, clicking on which opens the image taken with Celestia) and lastly its spectral class.

Starting from the nearest one, GJ 3877, we see that the Sun appears in a zone of sky where stars of our Aries (Hamal and Sheratan), Centaur (α Cen), stars of Perseus and above the well known Pleiades coexist: therefore the Sun is in a boreal zone where the real intruder is α Cen, because of its proximity, for the usual fantastic game of three-dimensionality of the stars in space.

From Gliese 433 the Sun appears weaker and this time among stars of Andromeda (and in fact on the left you can see the Galaxy M31), of Pegasus, with the usual intruder.

To demonstrate the extension of the constellation of Hydra, the Sun, seen from HIP 56452, appears in a zone of sky still containing stars of Andromeda and Pegasus, but this time we find a couple of illustrious intruders (Sirius and Raccoon) and other famous stars like Altair and Vega.

From the last star of which I realized the photo, HIP 47592, the Sun appears more or less in the same zone of Gliese 433 with the Andromeda Galaxy nearby: prospectively, it's obvious!

A very big star and other not bad stars

comparison between the stars of Hydra and other notes

Looking at the diagram I made to compare the largest stars of Hydra with other stars that we gradually met in the series of articles, this time a real stellar monster, a red supergiant variable (W Hya) 550 times the beauty of our Sun, just mentioned up there on the upper left, followed by a train of smaller stars, almost all of spectral class K: 44 Hya is a monster of 140 times the diameter of our Sun, not bad indeed.

Then we find 28, φ2, α, ω, μ, E, a and finally F Hya, with values to scale between 64 and 33 times our Sun: I wanted to reach F Hya first of all because it's an umpteenth ordinary star, as big as the blazoned Aldebaran and then because it's a yellow giant, therefore an elder sister of our Sun, exactly of the same spectral class G2.

the supergiant W Hya from the distance of 10 UA

Don't worry, I haven't forgotten my friends Whyani and the stellar monster around which their home planet revolves: despite being 82.4 UA from their star and in a planet covered with clouds, this red supergiant makes you feel its disturbing presence heavily, giving a high temperature and a reddish colored sky. From the usual distance of 10 UA this star appears as a threatening red disk with a diameter of 23°.

The appearance of Hydra over time

We arrived at the usual appointment with the retrospective of how the Hydra was depicted in past centuries and how it is seen in modern times (by the programmers of Stellarium). It should be the Hydra of Lerna, the monster fought and killed by Heracles in one of his twelve mythical labors: it should have nine heads, of which the one represented is the central one, the only immortal, at least until the arrival of the mythical hero.

Let's start with Uranometria, where this monster appears with open jaws and a body wrapped around itself like a corkscrew.

the Hydra according to Uranometry

According to Hevelius instead this big snake is a little more linear, with a spiral near the head, drawn definitely ugly and threatening

the Hydra according to Hevelius

Stellarium again represents it with various coils, but this time it seems to be more a serpent, dangerous and threatening, but still a serpent, with forked tongue.

the Hydra according to Stellarium

Names of the stars: a couple of families

One thing that you notice every time you talk about an extended constellation with many stars, is that the ancient sailors had a limited repertoire for the names to associate to the stars: it was much easier to create a family of stars with the same name and in which each representative was marked by a (Roman) progressive number.

  • Alfard (α Hya): the solitary
  • Dhanab (γ Hya): found on the internet
  • Minazal I, II, III, IV and V, η, ε, ρ and ζ Hya): uninhabited place
  • Ukdah I, II, III and IV (τ1, τ2, 33 and ι Hya): the node
  • Al sharasif (κ Hya): the ribs
  • Sherasiph (ν Hya): found on the internet
  • Sataghni (π Hya): found on the internet
  • Minkalshuja (σ Hya): the nose of the snake

Visibility of the constellation

As said many times it is a very long constellation and therefore it is a bit more complicated to identify the period in which it is visible, at the usual convenient time, 9 p.m..

From mid-December until late April, Hydra is located on the eastern horizon, between E and SE. Culmination in the south begins in early April and ends in July. Finally, it lies low on the horizon from the beginning of July until mid-August.

It's very strange: when the final part of the tail is rising, the head is already culminating South and when the head is about to set, the tail is culminating South. Practically (at 9 p.m.) the Hydra begins to show itself when its head comes out in late April and can be seen night after night, uninterruptedly until mid-August, when the final part of the tail sets.

For not being a circumpolar constellation, it is a good record, due only to its longitudinal extension.



Audio Video The constellation Hydra (Hydra-Hya)
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