Constellations: Hydrus 'the Water Snake' Content from the guide to life, the universe and everything

Constellations: Hydrus 'the Water Snake'

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The shield of the Science, Mathematics and Engineering faculty of the h2g2 University.Constellations: Overview | Andromeda | Antlia | Apus | Aquarius | Aquila | Ara | Aries | Auriga | Boötes | Caelum
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Short form:Hyi
Represents:'Water Snake' or 'Male Hydra'
Area:243 sq deg
Co-ordinates1:Right Ascension 02h, Declination −70°

Hydrus is a southern constellation of modern origin. Though small in total area, it is slim and long so it shares its borders with several others: Eridanus; Tucana; Octans; Mensa; Dorado; Reticulum and Horologium. Octans 'the Octant' is the constellation furthest south and includes the south polar axis. The southernmost tip of Hydrus adjoins it within ten degrees of the polar axis. The most northerly border of Hydrus was drawn just below the brilliant 1st magnitude Achernar, (alpha Eridani), so if you can identify that, the triangular-shaped asterism below marks Hydrus' main stars. Hydrus contains some interesting stars, two of which have planets in attendance. It also boasts some superb NGC objects: a drop-dead gorgeous star cluster and a triple set of post-merger galaxies far, far away.


The water snake is one of a dozen constellations delineated by Dutch astronomers Pieter Dirkszoon Keyser and Frederick de Houtman during their voyage to the southern seas between 1595 and 1597 on board the Hollandia. It first appeared in Johann Bayer's Uranometria (star catalogue) of 1603. The French astronomer Nicolas Louis de Lacaille (1713 - 62) decided it should be known as l'Hydre Mâle (the male Hydra) in his 1756 catalogue Coelum Australe Stelliferum.

In 1922 the International Astronomical Union (IAU) redrew the borders of all the constellations and some were scrapped. Hydrus survived and remains one of the 88 internationally recognised constellations that we know today.

Julius Schiller

German cartographer and astronomer Julius Schiller (1580 - 1627) spent his adult life reworking the constellations, which he believed were Pagan in origin, creating new ones to honour Biblical figures. He used the stars in Hydrus and neighbouring Tucana (the Toucan) to represent the healing Archangel Raphael. Schiller's star atlas Coelum Stellatum Christianum was published in 1627, just before he died, but the new Christian-based star formations never caught on.


The scientific star names are Greek letters combined with the genitive name of the constellation. This is known as the 'Bayer designation' after the man who devised the system. Some stars have proper names as well - for example, alpha Centauri is Rigil Kentaurus - but there are no named stars in Hydrus. Other stars, like Gliese 3021, are known by their catalogue number. Recently discovered variable stars, like VW Hydri, are given upper case English letters.

Stars of Hydrus

Pi Hydri (you could also call it 'Pi Hyi') is a double star, a non-interacting pair of 6th magnitude giants: Pi1 is red and Pi2 is orange.

Eta Hydri is a double star comprising a blue-white dwarf, eta1, and a yellow giant, eta2, which has a confirmed planet in orbit.

Lambda Hydri is an orange giant straddling the border with Tucana. From our vantage point it looks like it is part of the Small Magellanic Cloud2 (SMC), an irregular dwarf galaxy which orbits the Milky Way. In reality lambda Hydri is much closer to us at only 200 light years3 distant. The SMC is about 210,000 light years away.

VW Hydri is an SU Ursae Majoris-type dwarf which underwent a super outburst in October 1984. Another outburst on 23 September, 1998, was recorded by X-ray observations in multiwavelength, including ultraviolet. VW Hydri is a binary system comprising a cataclysmic variable star (CV) and a 'normal' star, and the companion's perigee (closest orbital proximity) is causing the outbursts.

Unusually for such a small area, there is another cataclysmic variable white dwarf in Hydrus called WX Hydri. It underwent its most recent outburst on 28 August, 2008. CVs are sometimes discovered accidentally when stargazers are looking for other targets. They are easily monitored by amateurs with modest optical equipment and a whole community of watchers can be found at the website called CVnet.

Star Table

StarDesignationName or
catalogue number
Brightness (m)Distance
(light years)
Spectral classification
and/or comments
βbeta HydriHD 2151+2.824Yellow sub-giant
αalpha HydriHD 12311+2.971Yellow-white dwarf
γgamma HydriHD 24512+3.2200Red giant
δdelta HydriHD 15008+4.1130White dwarf
εepsilon HydriHD 16978+4.2150Blue-white giant
η2eta2 HydriHD 11977+4.7217Yellow giant/planetary system
λlambda HydriHD 4815+5.1200Orange giant
π1pi1 HydriHD 14141+5.5700Red giant
π2pi2 HydriHD 14287+5.6460Orange giant
Gliese 3021GJ 3021HD 1237+6.6 var57Yellow dwarf/planetary system
η1eta1 HydriHD 11733+6.7400Blue-white dwarf
VWVW HydriVW HydriCV212Dwarf nova/cataclysmic variable
WXWX HydriWX HydriCV350Dwarf nova/cataclysmic variable

The Ben Ben Asterism

The stars alpha, gamma and beta Hydri form a triangular asterism when connected in the imagination. This was viewed as auspicious by cultures such as the ancient Egyptians, who designed and built pyramids from that basic geometric shape. They knew the triangle shape as a Ben Ben. The corresponding star alignment in the night sky earned the asterism the name 'Ben Ben'. It was believed that this was the resting place of the Bennu (phoenix) before it burst into flames and rose to its own place in the heavens. The constellation Phoenix touches a corner with Hydrus, but they do not share a border; they are separated by Tucana and the extreme southerly portion of Eridanus.

New General Catalogue (NGC)

The NGC was compiled by John Louis Emil Dreyer (the director of the Armagh Observatory from 1882 to 1916). Technological advances have enabled astronomers to enhance the original finds of the pioneers like Herschel and in some cases correct their data. One object, NGC 1511, was discovered in Hydrus and listed as a galaxy. The revised NGC/IC data informs us that this 'object' is actually a triple set of galaxies whose co-ordinates are so similar that they practically appear as one from our vantage point. Their closeness to each other means that they are interacting, although this isn't yet a cosmic trainwreck4. They will eventually form one huge galaxy when they've finished their intergalactic dance. NGC 1511 has already gobbled up an ancient galaxy which strayed too close, according to the Royal Astronomical Society of Australia. They state that 'the peculiar optical ridge to the east of NGC 1511 is probably the stellar remnant of a galaxy completely disrupted by interactions with NGC 1511'.

NGC 602 is a young (in galactic terms) star cluster just five million years old. This stellar nursery is estimated to be 200,000 light years from us and yet in the Hubble Space Telescope image it's possible to see galaxies which could be several hundred million light years distant. This link at the HubbleSite gives you an option to zoom in on distant galaxies for closer inspection.

NGC Table

CatalogueNameTypeBrightness (m)Remarks
NGC 1511ESO 55-4Galaxy+11Sa
NGC 1511AESO 55-5Galaxy+13.4SBa
NGC 1511BESO 55-6Galaxy+14.5SBcd
NGC 602N90Nebulous cluster+7.5Discovered in 1826
by James Dunlop

Extrasolar Planets in Hydrus

There have been three extrasolar planetary systems found in Hydrus up to 2010. Figures given in the table below are the length of the planet's orbital period around its parent star, which we know of as a year. The mass of the extrasolar planet is compared to that of Jupiter, our Solar System's largest planet, known by astronomers as the 'Jovian scale'.

Gliese 3021 b (sometimes catalogued as GJ 3021 b) is a gas giant orbiting a yellow dwarf, a star similar in make-up to our own Sun. Astronomers have labelled this planet a 'Sulfurous Cloud Jovian' - in other words the atmosphere is poisonous.

Eta2 Hydri b (aka HD 11977 b) is a 'blue and cloudless' gas giant orbiting yellow giant star eta2 Hydri. Very few planets have been found orbiting giant stars.

HD 10180, a yellow dwarf star like our own Sun, had five planets detected by the HARPS (High Accuracy Radial velocity Planet Searcher) programme at the European Southern Observatory. The announcement was made on 24 August, 2010. Two further planets were discovered in the system towards the end of 2010, making it the most-populated planetary system to date.

Extrasolar Planets Table

Star name or
catalogue number
catalogue number
Planet mass
(Jovian scale)
Orbital period
(Earth days)
Year of discoveryComments
Gliese 3021
(GJ 3021)
Gliese 3021 b
(GJ 3021 b)
3.31342000Gas giant
Eccentric orbit
Eta2 Hydri
(HD 11977)
Eta2 Hydri b
(HD 11977 b)
6.57112005Gas giant
HD 20003HD 20003 b0.037811.852011Hot Super-Earth
HD 20003HD 20003 c0.042233.822011Hot Super-Earth

The HD 10180 Planetary System

catalogue number
Planet mass
(Jovian scale)
Earth mass
Orbital period
(Earth days)
AUYear of discoveryPosition from starComments
HD 10180 b0.00421.31.170.022010#1Rocky; tidally-locked
HD 10180 c0.0413.15.760.062010#2Hot gas giant
HD 10180 d0.03711.7516.360.132010#3Hot gas giant
HD 10180 e0.0792549.750.272010#4Hot gas giant
HD 10180 f0.075241220.492010#5Hot gas giant
HD 10180 g0.06721.46011.422010#6Gas giant
HD 10180 h0.202642,2223.42010#7Gas giant

The Mystery of IC 1717

IC 1717 is referenced as belonging to Hydrus. John Louis Emil Dreyer's original description is 'eF, eS, mE 25°, stell N' which means: eF (excessively faint), eS (excessively small), mE (much extended) 25° (possibly the extension angle), stell N (stellar nucleus). Definitely, something was there when Dreyer observed and catalogued it, but there's nothing there now. Its spacial co-ordinates are given as Right Ascension: 01h:32m:30s Declination: 67° 32' 12", making it a close neighbour of yellow giant star eta2 Hydri, which just happens to have a planetary system, as discussed above. We don't know for sure what IC 1717 was, all we know is Dreyer thought it worthy of recording for posterity. We can discount things like faulty observation because he was so famously meticulous, nor would it have been a supernova because that would certainly have obliterated the nearby planet, and comets and asteroids have their own designations. If IC 1717 was the remains of a planet (we'll nickname it 'Icy-double-seventeen' for the sake of discussion) then it's possible what Dreyer saw was the ghostly trail of the final orbit of the doomed planet. Were there inhabitants who warred with their fellows and ended up destroying their world? Maybe the global catastrophe came about due to scientists tinkering with nature. There are many legitimate questions with as many or more guesses and hypotheses, and the only answers lie in sifting through what it isn't (or wasn't). What is left remains a possibility, but we'll never know for sure, unless the inhabitants of Planet Icy-double-seventeen launched a rocket containing records of their history and culture (and what led to their ultimate destruction) into the black void for another civilisation to discover, study and (hopefully) learn from their misfortune.

Hydrus in Science Fiction

  • In the 1980 book Serpent's Reach written by sci-fi author CJ Cherryh, alpha Hydri is the star at the centre of a solar system which contains a planet called Cerdin. This is the home world of the Majat, an insectoid alien race living in the Alliance-Union universe of the near-future.
  • The yellow sub-giant beta Hydri features prominently in the literary world of science fiction:

    • Time For The Stars - Robert Heinlein
    • A Canticle For Leibowitz - Walter Miller Jr
    • Uller Uprising - H Beam Piper
    • Daughters Of Earth - Judith Merrill
    and beta Hydri is a heavily-populated planet, according to the computer game Frontier.
1Current IAU guidelines use a plus sign (+) for northern constellations and a minus sign (−) for southern ones.2Some SMC objects have their own catalogue with N numbers, as well as regular designations.3A light year is the distance light travels in one year, roughly 5.88 trillion miles or 9.46 trillion km.4A 'cosmic trainwreck' of two (or more) colliding galaxies can take around a billion years.

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