Hydra is counted as one of the original 48 constellations listed by the 2nd century astronomer Ptolemy and represents a Water Snake, although in mythology one tale tells of a more exotic creature. Hydra is by area the largest of the 88 modern day constellations, measuring 1303 square degrees, as well as being the longest at over 100 degrees. It straddles the celestial equator from about 5° north to 30° south in declination. Hydra's northern end (the head) borders Cancer, Canis Minor and Monoceros, whilst the southern end (tail) borders Libra and Centaurus.

Sitting just above the snake, toward the tail, are two further constellations; Corvus (the Crow) and Crater (the Cup). Both these groups are also original constellations. A more modern group; Sextans; the sextant, resides behind the head. The head itself is denoted by an irregular loop of individually inconspicuous stars, which together are kind of conspicuous. From the head the water snake zigzags eastwards, almost to the southern horizon in places. Hydra is so long, the head rests in the SW, when the tail is only just clearing the SE horizon.

Hydra the Water Snake, with Corvus, Crater and Sextans - illustrated
(click for larger image)

In Greek mythology the constellation is associated with two worthy tales, (see Corvus for the tale relating to it, Crater and the Water snake). Perhaps the more famous myth depicts Hydra as the creature that Hercules slew in the second of his labours. Another of Hera's monstrous pets, the Hydra was a multi-headed creature, the middle one of which was immortal.

The creature lived in a swamp near the town of Lerna, ravaging the lands whenever it came forth. Hercules drew out the serpent by shooting flaming arrows into the swamp, before grappling with the beast. Hera saw the danger and sent a giant crab (Cancer) to aid her pet and nip at Hercules's foot, but he stamped on the crab and crushed it! Hydra was more difficult to overcome, no sooner could Hercules dispatch one head, two more grew in its place and he called for help to his charioteer, Iolaus; his nephew, who as each head was removed, cauterised the stump to prevent others growing in its place.

The Head of Hydra (click for larger image)

The tail of Hydra (click for larger image)

Despite its size Hydra contains only one fairly bright star, Alpha Hydrae or Alphard - the 'solitary one'. It is an orange giant of magnitude 1.98 some 177 light-years distant. It resides towards the head of Hydra. 

  • Beta Hydrae is a blue-white star of magnitude 4.3, 365 light-years from Earth.
  • Gamma Hydrae is a yellow giant of magnitude 3.0, 132 light-years from Earth. 
  • Epsilon Hydrae is the only bright binary star in Hydra, but is difficult to split in amateur telescopes; it has a period of 1000 years and is 135 light-years from Earth. In June of 2018 the proper name of Epsilon Hydrae given by the IAU became Nakshatra (Ashlesha) the lunar zodiacal constellation in Indian astronomy.
  • 27 Hydrae is a triple star with two components visible in binoculars and three visible in small amateur telescopes. The primary is a white star of magnitude 4.8, 244 light-years from Earth. The secondary, a binary star, appears in binoculars at magnitude 7.0 but is composed of a magnitude 7 and a magnitude 11 star; it is 202 light-years from Earth.
  • 54 Hydrae is a binary star 99 light-years from Earth, easily divisible in small amateur telescopes. The primary is a yellow star of magnitude 5.3 and the secondary is a magenta star of magnitude 7.4.
  • N Hydrae (N Hya) is a pair of stars of magnitudes 5.8 and 5.9. 
  • The other named star in Hydra; Sigma Hydrae (σ Hydrae) is called Minchir, from the Arabic for snake's nose. At magnitude 4.54, it is rather dim. 

Hydra is home to several variable stars.

  • R Hydrae is a fairly bright very red Mira type variable star over 2000 light-years from Earth; At its maximum of magnitude 3.5 it is easily visible to the naked eye, but at minimum (mag 10) a telescope is required. The period is 390 days.

R Hydrae- Variable Mira type star (click for larger image)

  • V Hydrae is an unusually vivid red variable star 20,000 light-years from Earth. It varies in magnitude from a maximum of 6.6 to a minimum of 9.0. Along with its notable colour, V Hydrae is also home to at least two exoplanets.
  • U Hydrae is a semi-regular variable star with a deep red colour, 528 light-years from Earth. It has a minimum magnitude of 6.6 and a maximum magnitude of 4.2; its period is 115 days. 
  • GJ 357 is an M-type main sequence star located only 31 light-years from the Solar System. This star has three confirmed exoplanets in its orbit, one of which, GJ 357 d, is considered to be a "Super-Earth" within the habitable zone.

Deep Sky Objects

Hydra contains three Messier objects. M83, also known as the Southern Pinwheel Galaxy, is located on the border of Hydra and Centaurus, M68 is a globular cluster near M83, and M48 is an open star cluster in the western end of the serpent. 

M83 (NGC 5236) was actually the third galaxy discovered after M31 and M32 by Nicholas Lacaille in 1752. Aware of Lacaille's catalogue, it wasn't until 1781 that Messier finally tracked down this 8th magnitude southerly galaxy, noting 'nebula without star'. This face on spiral galaxy is sometimes called the 'Southern Pinwheel Galaxy' and is one of the most beautiful galaxies visible. It is easily observed in skies south of 40°N latitude, but is seen only 6 degrees or so above the horizon from UK shores, making it tricky observe and underappreciated. Large amateur telescopes; above 12 inches aperture, reveal its spiral arms, bar, and small, bright nucleus. In a medium-sized amateur instrument, around 8 inches in aperture, the spiral arms are only just visible under good conditions. Like our Milky Way, it is a grand design spiral galaxy 55,000 light-years in diameter and is 14.7 million Ly distant. M83 has been host to six supernovae, more than any Messier galaxy. It is best observed in late April. 

M83 -the southern pinwheel galaxy in Hydra (click for larger image)

There are several globular clusters in Hydra, only one of which is a Messier object, M68 (NGC 4590) which he observed in April 1780. The almost 500 million year, highly elliptical orbit, can take M68 over 100,000 light years out from the galactic centre. Currently it lies 36,000 light-years from Earth. M68 is visible in binoculars, but a medium sized amateur telescopes of 4"+ will start to resolve stars of this 8th magnitude object. Unfortunately from UK latitudes this can be difficult to achieve due to the poor seeing at such a low altitude (less than 8 degrees above the horizon). The cluster has a very low concentration of stars, one of the lowest for Messier globulars, all spread over a 120 light year diameter. The cluster is approximately 10 billion years old.  

The globular cluster M68 in Hydra (click for larger image)

Discovered in February of 1771 by Messier, the open cluster M48 (NGC 2548) lies in quite an isolated position, close by the western border of Hydra with Monoceros and around 14 degrees SE of Procyon in Canis Major. M48 was regarded as one of the missing Messier objects, since the position he gave corresponds to no object. It was known as NGC 2548 until 1959 when the error was pointed out, the cluster NGC 2548 residing 4 degrees south of the position given by Messier and matches the description he gave. Under very dark skies M48 can be glimpsed with the naked eye, but at mag 5.8 is easily spotted in binoculars. It is best appreciated in a short focal length telescope. M48 is a large cluster, six times the diameter of the Pleiades (almost size of a full moon) extending to some 38 light years. The core 80 or so member stars make up what has been described as a 'triangular' shape, but overall, estimates suggest as many as 320 stars located 2500 light years away belong to this 300 million year old cluster.  

The open cluster M48 in Hydra (click for larger image)

NGC 3242 is a very fine planetary nebula of magnitude 7.5, 1400 light-years from Earth. Discovered in 1785 by William Herschel, it is also known as the "Ghost of Jupiter" because of its striking resemblance to the planet as seen through a small telescope. Its blue-green disk is visible in small telescopes, whilst larger instruments will reveal the halo and central star. 

NGC 3242-the 'Ghost of Jupiter nebula in Hydra - general location
(click for larger image)

The Ghost of Jupiter -NGC 3242 in detail -Pic credit-NASA (click for larger image)

NGC 5694 is an exceedingly distant globular cluster of magnitude 10.2, over 100,000 light-years from the sun. Also called "Tombaugh's Globular Cluster", it was first discovered in 1784 by William Herschel, but its status as a globular cluster was not verified until 1932, when Clyde Tombaugh looked at photographic plates taken of the region near Pi Hydrae in May 1931.

Hyrda is home to many other galaxies, too numerous to mention in detail, but one interesting galaxy is; NGC 4993, an elliptical galaxy which was the source of detected gravitational waves from the merger of two neutron stars.


Corvus and Crater 

These two small, adjacent constellations can be traced relatively easily given clear seeing, resting on the tail of Hydra, although they are by no means bright. Both are counted in the original 48 star groups listed by Ptolemy and are linked in a moral tale that harks back to the time of Eratosthenes in the third century BC. It is said that Apollo was about to make a sacrifice to Zeus and sent a crow to fetch water from a sacred running spring. With a bowl in its claws, the crow departed, but en-route came across a fig tree laden with unripe fruit. Ignoring the task, the crow waited for the fruit to ripen, by which time Apollo had been forced to find a source of water himself. After gorging on the delicious fruit, the crow picked up a water-snake in his claws to use as an alibi and returned with it to Apollo, blaming the serpent for blocking the spring. Apollo saw through this and that the crow had lied, condemning the bird to a life of thirst and in memory of this incident Apollo placed the crow, the cup, and the water-snake together in the sky.


Corvus the Crow and Crater the Cup - illustrated (click for larger image)

Corvus, along with adjacent Crater, sit just above the tail of Hydra. The four brightest stars in Corvus; Gamma, Delta, Epsilon and Beta Corvi, form a distinctive, if not terribly bright, quadrilateral in the night sky. The shape of this box-like asterism has been likened to that of a type of sail known as a spanker, and because the 1st magnitude star in the nearby constellation of Virgo is Spica, Corvus is sometimes called Spica’s spanker, especially by mariners.

Corvus - the Crow (click for larger image)

The brightest stars in Corvus are:-

  • Gamma Corvi or Gienah - “the right wing of the crow” is a giant star located approximately 165 light years from Earth. It is the brightest star in Corvus at mag 2.58. 
  • The second-brightest star, Beta Corvi, Kraz is a yellow-white G-type bright giant that is located about 140 light years from Earth.
  • Delta Corvi or Algorab - ‘the Crow’ is a class A0 star located approximately 87 light years away
  • Epsilon Corvi or Minkar meaning the nostril of the crow.” is a K2 III class star that is approximately 303 light-years from Earth.
  • Alpha Corvi, or Alchiba - ‘tent’ is only the fifth brightest star in the constellation. It is a class F0 dwarf or subdwarf some 48 light years distant.

Corvus contains no Messier objects, but it is not devoid of interest to the amateur astronomer with a number of objects visible in smaller/medium telescopes. The center of Corvus is home to the 10th magnitude planetary nebula, NGC 4361. The nebula itself resembles a small elliptical galaxy. 

A galaxy grouping sprawls across Corvus and Crater and may contain over 25 members. The best known member is NGC 4038 and 4039; the Antennae galaxy, located just north of 31 Crateris. At a distance of 45 million light years, two interacting galaxies appear heart shaped from our view point, with huge stellar tidal tails that trail from the galaxy ends. Both originally spiral galaxies, extensive star formation due to gas cloud interaction is underway. The system is a source of multiple ultra-luminous X-rays, the nature of which is unknown, but may be from rare types of x-ray emitting binary stars. Appearing as a peculiar warped wispy arc in small and modest telescopes, this 10th magnitude object is a favourite for imaging observers.

NGC 4038 & 4039 - the Antennae galaxies (click for larger image)

Crater the Cup is fainter than Corvus, but given clear, dark skies, the outline is reasonably conspicuous to the eye, shaped like a goblet or chalice. There are no stars brighter than mag + 3.4 and like Corvus it contains no Messier Objects. It does contain a number of 11th and 12th mag galaxies, but nothing to get excited about.

Crater the Cup - (click for larger image)

  • Delta Crateris,- its brightest star is an orange giant located approximately 196 light years from Earth. The star is also known as Labrum (Latin for “the lip”), due to the fact that it was sometimes associated with the story of the Holy Grail.
  • Next is Alpha Crateris, another orange giant located approximately 174 light-years from Earth which is 80 times more luminous than our Sun. It is also known as Alkes, derived from the Arabic “the cup”. It is a high velocity star
  • Beta Crateris is a white sub-giant that is located approximately 266 light years from Earth. This star is also known by the name Al Sharasif, which means “the ribs” in Arabic.
  • Delta Crateris is yet another orange giant with an apparent magnitude of 3.56. In time, Delta will eventually become a Mira-type variable star before ending its life as a white dwarf.

Finally, look for R Crateris, a variable star with a lovely red colour that can be observed with binoculars. The change of magnitude is from 8 to 9.5 in a period of about 160 days.

R crateris - in Crater - deep red variable star (click for larger image)

So although quite faint, the water snake and company actually have more on offer to the observer than at first glance. Pick a moonless night and take a slide down the coils of Hydra. 


Chart credits:- Stellarium and Starry Night Pro plus 8, unless otherwise stated