Constellations: Overview | Andromeda | Antlia | Apus | Aquarius | Aquila | Ara | Aries | Auriga | Boötes | Caelum
Camelopardalis | Cancer | Canes Venatici | Canis Major | Canis Minor | Capricornus | Carina | Cassiopeia | Centaurus
Cepheus | Cetus | Chamæleon | Circinus | Columba | Coma Berenices | Corona Australis | Corona Borealis | Corvus
Crater | Crux | Cygnus | Delphinus | Dorado | Draco | Equuleus | Eridanus | Fornax | Gemini | Grus | Hercules | Horologium
Hydra | Hydrus | Indus | Lacerta | Leo | Leo Minor | Lepus | Libra | Lupus | Lynx | Lyra | Mensa | Microscopium | Monoceros
Musca | Norma | Octans | Ophiuchus | Orion | Pavo | Pegasus | Perseus | Phoenix | Pictor | Pisces | Piscis Austrinus
Puppis | Pyxis | Reticulum | Sagitta | Sagittarius | Scorpius | Sculptor | Scutum | Serpens | Sextans | Taurus
Telescopium | Triangulum | Triangulum Australe | Tucana | Ursa Major | Ursa Minor | Vela | Virgo | Volans | Vulpecula

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

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.

Mythology

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.

Stars

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

In 2007 a superjovian world, HD 70573 b, was discovered, but it is unconfirmed as of 2018.

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.

NGC 5694

The globular cluster NGC 5694 was discovered on 22 May, 1784, by Sir 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.

Romantic Nebula and Star Combination

Abell 33 is a planetary nebula located about 2,500 light years away. It's almost-perfectly round, and, serendipitously, a bright star catalogued HD 83535 appears on the edge from our vantage point - two completely separate astronomical objects coming together to create a 'Diamond Ring Nebula'.

Galaxies

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.

NGC 4993

NGC 4993 is an elliptical galaxy discovered by Sir William Herschel in 1789. In October 2017 it was announced that a gravitational wave, later named GW170817, had been detected. Two neutron stars which had been orbiting each other closer and closer had finally merged, a process known as a kilonova. Such events are responsible for heavy metals such as gold and platinum in the Universe.

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.

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 non-terrestrial worlds, are 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'.

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.