Constellations: Hydra 'the Sea Serpent' Content from the guide to life, the universe and everything

Constellations: Hydra 'the Sea Serpent'

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The sea serpent, in languor curved
About a rock, the world observed,
How all the beasts and birds
And fishes too, from near and far
Were pigeon-holed by genera
And tagged with Latin words.

–   'The Sea Serpent' by Claudius Jones, published in The Saturday Review of Literature 12 October, 1929.

The Constellation Hydra

Name:Hydra (Latin: 'sea serpent')
Short form:Hya
Area:1,303 sq deg
Co-ordinates1:Right Ascension 10h, Declination −20°

Hydra is the largest of the 88 internationally recognised constellations at over 1,300 sq deg in area, but it's hard to identify as the stars are so dim. It used to be even bigger — the smaller constellations Corvus 'the crow', Crater 'the cup' and Sextans 'the sextant' were created to make Hydra a more manageable size. In the original Hydra these three were the humps on its back. It would have looked like how some people imagine the Loch Ness monster rising above and dipping below the surface of the Scottish loch as it slithers along.

Hydra is south of the ecliptic2 and runs parallel to it, snaking its way between many neighbouring constellations. Clockwise from above, Hydra's borders are shared by Leo, Cancer, Canis Minor, Monoceros, Puppis, Pyxis, Antlia, Centaurus, Libra, Virgo, Corvus, Crater and Sextans.

There are three Messier objects in Hydra: an open star cluster, a globular cluster and the stunning, barred clockwise spiral galaxy known as the Southern Pinwheel. A fascinating feature is NGC 3242, a planetary nebula intriguingly called the 'Ghost of Jupiter'. The main attraction of this constellation has got to be the perfectly superimposed pair of overlapping galaxies catalogued NGC 3314, the study of which has given astronomers vital information about interstellar dust that 'fills the gap' between stars inside galaxies.


The Hydra was a mythical multi-headed3 monster which guarded the Golden Fleece sought by Jason and the Argonauts. After Jason slew the beast, the ruler who had been robbed, King Aeëtes, ordered his men to gather up the Hydra's teeth. When his army cornered Jason and his men, the king scattered the teeth and from where they fell up sprang the 'Children of the Hydra' — skeletons of people who had been slain by the monster. They were armed with swords and fought Jason and his men, killing all but Jason, who escaped back to the Argo. In this version, Hercules was one of the Argonauts, but he had disembarked before the battle to search for his friend Hylas who had been lost on the Isle of Bronze.

In an alternative version, the Hydra was a mythical multi-headed water monster which guarded the entrance to the Underworld. It had poisonous breath and its appearance was so hideous that some people who encountered it were frightened to death. One of the 12 labours that Hercules was given was to slay this fearsome beast. Every time he cut a head off, more grew back. So Hercules had to cheat and ask for help from his nephew, Iolaus, who had the brilliant idea of cauterising the neck stumps with a piece of burning wood as soon as his uncle had done the chopping.

After he had completed the task, Hercules dipped his arrows in the Hydra's toxic blood to make them more effective against the centaurs, which the Greeks were at war with at the time. This plan backfired, however, when Hercules accidentally injured his friend, Chiron — the centaur teacher of Jason, Asclepius and Achilles — with a stray arrow. Put out of his agony by Zeus, the immortal Chiron was elevated to the heavens as the constellation Centaurus, so the Hydra's blood claimed one more victim for the monster after its demise.

Yet another story concerns the god Apollo, who commanded his pet bird (a crow or raven) to fetch him some fresh water4 from Earth. The hungry bird spied a fig tree that was about to bring forth some succulent fruit, so it waited for them to ripen and had a feast. Knowing it needed an excuse as to why it had tarried, the bird plucked a serpent from the river, then filled Apollo's golden goblet with water. Apollo did not believe the bird's story that it had been attacked by the serpent, and in his fury hurled it, the serpent and the goblet from the celestial home of the gods. Their images came to rest in the night sky above the Earth, the bird (the constellation 'Corvus') still clutching the serpent (Hydra) in its claws, with the spilled goblet (the constellation 'Crater') by its side.


The scientific star names are simple to understand (if you know your Greek alphabet). For example: the 'alpha' star means it is the brightest star in that constellation. The next brightest is designated 'beta' etc. Combined with the genitive name of the constellation gives us the star's Bayer designation. Some stars have proper names as well; for example, alpha Hydrae is Alphard, others are known by their catalogue numbers.

That is how the system is supposed to work, but occasionally the stars are variable, or errors were made in the original categorising. Once tagged, the star order is not changed, to avoid confusion. In this constellation, we have the alpha star, with the second magnitude  Alphard the brightest, followed by gamma, zeta, nu, pi, epsilon, mu, theta, iota, upsilon, then delta, and finally in trails the beta star!

Delta, beta, upsilon and tau Hydrae are binary star systems. Epsilon Hydrae, Ashlesha, is a multiple star system of (at least) four stars which has been known historically as 'a string of pearls which decorates the neck of the sea serpent'.

TW Hydrae is an orange dwarf star of the T Tauri type. Astronomers working at ALMA (the Atacama Large Millimetre Array) in Chile have been studying its protoplanetary disc:

[TW Hydrae] was so spectacular that even people here at the observatory were blown away by it. It looked like an artist's painting.
– Violette Impellizzeri, operations astronomer at ALMA

Star Table

StarDesignationName or
catalogue number
(light years5)
Spectral classification
and/or comments
α Hyaalpha HyaAlphard+1.9 var170Orange giant
γ Hyagamma HyaCauda Hydrae+2.9 var132Yellow giant
ζ Hyazeta HyaHydrobius+3.1 var151Yellow giant
ν Hyanu HyaPleura+3.1 var138Orange giant
π Hyapi HyaMarkeb+3.2 var101Orange giant
ε Hyaepsilon HyaAshlesha+3.3 var135Multiple star system
μ Hyamu Hya42 Hydrae+3.8248Orange giant
θ Hyatheta Hya22 Hydrae+3.9130Blue-white giant
ι Hyaiota Hya35 Hydrae+3.9 var276Orange giant
υ Hyaupsilon Hya39 Hydrae+4.1270Binary star system
δ Hyadelta HyaMautinah+4.1 var179Binary star system
β Hyabeta HyaHD 103192+4.2 var365Binary star system
η Hyaeta Hya7 Hydrae+4.3460Blue-white giant
ρ Hyarho Hya13 Hydrae+4.3333White giant
σ Hyasigma HyaMinchir+4.4350Orange giant
τ Hyatau HyaUkdah+4.5 var56Binary star system
κ Hyakappa Hya38 Hydrae+5.0500Blue-white giant
TW HyaTW HydraeHIP 53911+11.0 var176Orange dwarf

New General Catalogue (NGC)

The NGC catalogue was compiled by John Louis Emil Dreyer (director of the Armagh Observatory from 1882 to 1916).

Ghost of Jupiter and the Red FLIER Mystery

NGC 3242 is a planetary nebula called 'Ghost of Jupiter'. It used to be a giant star, before it ran out of fuel and exploded. What remains is the nebula, which is essentially an expanding cloud of gas and has nothing at all to do with planets, other than that is what the nebula looks like through a telescope. Sometimes interesting patterns are formed by the gas, and some features provide us with an intriguing mystery. At either side of this nebula is a prominent red FLIER — an acronym for Fast Low-Ionisation Emission Region. Astronomers do not know what caused these. One theory is that they are jets of compressed gas which were expelled by the star before the final collapse.

NGC 3242 is not unique in having red FLIERs; something similar is happening with NGC 6826, the planetary nebula called the 'Blinking Eye' in the constellation Cygnus 'the swan'. In that nebula, there is a clear 'bow-wave' between the FLIERs and nebula, indicating that either the FLIERs are moving towards the nebula or away from the centre more slowly than the expanding cloud of gas. We can assume that something similar is happening in the 'Ghost of Jupiter', but don't know for sure. It's probably a good thing that we don't fully understand the mechanics of the Universe, because while there are still mysteries to be solved life will continue to be interesting.

Overlapping Galaxies

When the order was given to find a pair of overlapping galaxies so interstellar dust could be studied, the discovery of NGC 3314 was a dream come true. In 1999, modern imagery was able to determine that the two galaxies were not colliding, or even related. The odds against finding two separate galaxies in our line of vision so the front one could be studied were pretty astronomical; astronomers got lucky.

We see the front galaxy (NGC 3314-A) as a face-on clockwise spiral, about 117 million light years away. Behind it, at a further 23 million light years distant, is a much bigger anti-clockwise spiral, almost edge-on from our perspective. The dark (star-less) areas of the foreground galaxy NGC 3314-A would normally have been indistinguishable from the black background of interstellar space. But because it is being illuminated from behind by NGC 3314-B astronomers are able to see the dark patches and measure them. This discovery provided a unique opportunity to gather important information about the composition of our own galaxy. The search for overlapping galaxies is one of the targets for amateur galaxy analysers at the Galaxy Zoo project.

NGC 5694

The globular cluster NGC 5694 was discovered on 22 May, 1784, by William Herschel, who catalogued it as H II.196. Its remoteness prevented further study, so he classed it as a 'faint nebula'. Astronomers Lampland and Tombaugh at Lowell Observatory, Arizona, US, declared that NGC 5694 was a globular cluster in 1932. Further study has ascertained that the cluster has a mind-boggling radius of 120 light years, and it is travelling towards us at a rate of 144km a second. Still over 113,200 light years distant, it is going to take a while yet to arrive in our galactic neighbourhood.

NGC Table

(light years)
NGC 5236M83Barred CW spiral galaxy+7.615 millionSouthern Pinwheel Galaxy
NGC 4590M68Globular cluster+7.833,000+100,000 stars
NGC 5694UnnamedGlobular cluster+10.1113,200Radius: 120 light years
NGC 2548M48Open star cluster+5.51,50080+ stars
NGC 3242Ghost of JupiterPlanetary nebula+10.31,400Has red FLIERs
NGC 3314-AUnnamedCW spiral galaxy+12.5117 millionOverlapping NGC 3314-B
NGC 3314-BUnnamedACW spiral galaxy+12.5140 millionOverlapped by NGC 3314-A


Southern Pinwheel

M83, the Southern Pinwheel Galaxy, is awesome. In April 2008 it was announced that new stars had been discovered in the otherwise barren outskirts of the galaxy.

Abell 1060

Abell 1060 is the name of the Hydra Cluster of Galaxies, which spans about ten million light years and contains over 100 bright galaxies. Galaxy clusters are gravitationally bound to each other but not necessarily interacting.

Hydra A

Hydra A is a cluster of galaxies that is a mind-boggling 840 million light years distant. Over 200 galaxies have been counted in this cluster. Near the middle is an X-ray radio source that could be a supermassive black hole. There is a remarkable gas cloud that is millions of light years in length, with an outer temperature of 42 million degrees.

Hickson 40

The Hickson Compact Group 40 (Hickson 40) is a 'group' of galaxies; that is, two or more but not enough to qualify as a 'cluster'. Hickson 40 contains three spiral galaxies, one elliptical and one lenticular. They are so close they are interacting with each other, and will eventually merge into one ginormous galaxy. Hickson 40 is 300 million light years distant from our galaxy.

Extrasolar Planets in Hydra

Several extrasolar planetary systems had been found in the constellation Hydra; the first was discovered in 2000. One has two planets and another has three. The nomenclature that has been decided upon for extrasolar planet discoveries is to use a lower-case letter after the parent star catalogue number (or name), eg 'HD 74156 b'. This stays with the planet no matter whether more discoveries are subsequently made within the same solar system, and despite the position of the new planet relative to the star.

Therefore, the first-discovered planet of HD 74156 is HD 74156 b, with HD 74156 c and HD 74156 d being detected later. The planets HD 122430 b, HD 82943 b and HD 74156 d are in their star system's habitable zone but, as they are gas giants, not considered candidates in the search for extra-terrestrial life. However, should the planets have any rocky moons with enough gravity to retain an atmosphere, it would make them a distinct possibility.

The 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 with that of Jupiter, our Solar System's largest planet, known to astronomers as the 'Jovian scale'.

Extrasolar Planets Table

Star name or
catalogue number
catalogue number
Planet mass
(Jovian scale)
Orbital period
(Earth days)
Year of discoveryComments
HD 122430HD 122430 b3.73452003Superjovian; habitable zone
HD 70573HD 70573 b68502007Superjovian; eccentric orbit
HD 72659HD 72659 b3.33,6302002Superjovian; eccentric orbit
HD 82943HD 82943 b1.84352000Gas giant; habitable zone
HD 82943HD 82943 c1.82202001Gas giant; eccentric orbit
HD 74156HD 74156 b1.9522001Gas giant; eccentric orbit
HD 74156HD 74156 c82,4762001Superjovian; eccentric orbit
HD 74156HD 74156 d0.43462007Gas giant; habitable zone
WASP-15WASP-15 b0.543.752008Hot gas giant
GJ 433GJ 433 b0.01972009Hot terrestrial
GJ 433GJ 433 c0.143,6932012Gas giant
HD 86264HD 86264 b71,4752009Superjovian
HD 90156HD 90156 b0.05749.772009Hot Neptune
HD 86226HD 86226 b1.51,5342010Gas giant
WASP-25WASP-25 b0.583.762010Hot gas giant
GJ 3634GJ 3634 b0.022.652011Hot super-Earth
WASP-84WASP-84 b0.698.522013Hot gas giant

Down To Earth

  • There is a Hydra Island in Greece.

  • Hydra is a programming language.

  • The 'teeth of the Hydra' are mentioned in the song 'Get It On', written by Marc Bolan and performed by T Rex.

  • There is a bronze statue of Hercules slaying the Hydra in the Louvre Museum, Paris, France.

1Current IAU guidelines use a plus sign (+) for northern constellations and a minus sign (−) for southern ones.2The apparent path of the Sun across the sky.3Peisander of Camirus described it as multi-headed in his poetry; Pausanias in his 160 AD Description of Greece said Peisander exaggerated the number to make it appear more fearsome, and it really had only one head.4On a diet of nectar and ambrosia, water must have seemed like a refreshing change!5A light year is the distance light travels in one year, roughly 5.88 trillion miles or 9.46 trillion km.

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