Hydra, the Night Sky’s Largest Constellation

Hydra, the Night Sky’s Largest Constellation
Source: www.stellarium.org

Hydra, the Water Snake, resembles the creature from which its name derives, with it huge body slithering across 100 degrees, and covering 3.158% of the night sky, making it the largest of the 88 recognized constellations. Despite its grand status amongst the constellations, and taking almost seven hours to rise fully, Hydra boasts few prominent features, and has just one star of second-magnitude, Alphard, an orange giant representing the heart of the snake.


In the southern hemisphere, Hydra is best seen in autumn, while in the northern hemisphere it can be seen from January to May. Look for the constellation just below Cancer where its head is located, and follow it onwards to a point lying between Centaurus and Libra, where its tail terminates. Being so large, Hydra borders quite a number of other constellations, including Antlia, Cancer, Canis Minor, Centaurus, Corvus, Crater, Leo, Libra, Lupus, Monoceros, Puppis, Pyxis, Sextans and Virgo.

Hercules Constellation Family

Hydra belongs to the Hercules family of constellations, the largest family group consisting of 19 constellations, namely Aquila, Ara, Centaurus, Corona Australis, Corvus, Crater, Crux, Cygnus, Hercules, Lupus, Lyra, Ophiuchus, Sagitta, Scutum, Sextans, Serpens, Triangulum Australe and Vulpecula.


The constellation of Hydra is associated with the mythological creature that Heracles was tasked with slaying to complete the second of his labors. The giant nine-headed snake lived in the swamp near Lake Lerna, and proved a formidable challenge for the Greek hero, as once one of its heads was cut off, another two would grow in its place. The goddess Hera also added to Heracles’ already difficult task by sending a giant crab, now represented by the constellation Cancer, to attack him while he battled the creature.

Fortunately, the ever resourceful Heracles enlisted the help of his nephew, Iolaus, to burn each decapitated neck stump in order to prevent any regrowth. Interestingly, King Eurystheus, who originally set Heracles ten Labors, voided this task because of the help he received from Iolaus. He also later voided the fifth labor of Heracles involving the Augean stables, and so set the legendary strongman two additional labors making twelve in total before his punishment was considered complete.

Meteor Showers

Two (rather less than spectacular) meteor showers originate from Hydra, one of which is the Alpha Hydrids that runs from the January 15th to 30th each year, and peaks around the 20th when fewer than 10 meteors per hour can be seen. The other shower associated with Hydra is the Sigma Hydrids, which runs from December 5th to 15th, and peaks on the 11th when it yields only 5 meteors per hour.

Hydra, the Night Sky’s Largest Constellation

Notable Stars

Despite its huge size, the actual shape of Hydra is made up of an asterism of only 17 stars, although some depictions include 20 stars. The constellation contains no first magnitude stars, but it does contain about 100 stars of 6th magnitude and brighter, including the six of magnitude 3 and 4 which make up its head, and the second magnitude star which represents its heart. Hydra also contain 18 stars with confirmed planets, two of which are mentioned below.

– Alphard (Alpha Hydrae), the most luminous star in Hydra, is an orange star located about 177 light years away that shines with a magnitude of 2. It is only about 420 million years old, and with 50 times the Sun’s diameter, and three times its mass, Alphard falls somewhere between being a luminous giant and an orange giant, giving it a stellar classification of K-3 II-III. In earlier times, Alpha Hydrae was also known as “Cor Hydrae”, the “heart of the serpent”, which was a name given to it by the famous Danish astronomer Tycho Brahe. The name Alphard derives from the Arabic for “the solitary one”.

– Gamma Hydrae, the constellation’s second most luminous star, is a yellow giant (G8 III) found 134 light years from our solar system with a visual magnitude of 2.993. It is about 372 million years old, and is roughly 13 times as big as the Sun, and 115 times more luminous.

– Hydrobius (Zeta Hydrae), the third most luminous star in Hydra, is a white-yellow subgiant situated 167 light years distant of visual magnitude 3.1. It is very young at 400 million years old, and has 4.2 times the mass of the Sun, 18 times its diameter, and is 132 times brighter.

– Beta Hydrae, one of several multi-star systems in Hydra, is located around 370 light years away. The principal component of the system is a chemically peculiar class B star with a strong magnetic field, classified as an “alpha-2 Canum Venaticorum type variable”. The combined apparent visual magnitude of the system is 4.276, but this value varies by as much as 0.4 magnitudes within a period of 2.344 days.

– HD 122430 is the only star with a confirmed planet in Hydra that is visible without optical aid, and then only when seeing conditions are exceptionally good. It is an orange giant (K2III) located about 440 light years from Earth that shines with an apparent visual magnitude of 5.47. The planet orbiting the star is about four times larger than Jupiter, and has an orbital period of 345 days, which places it at about the same distance from its parent star as Earth is from the Sun.

– Gliese 433, only 30 light years away, falls way below naked eye visibility with a luminosity of 9.81, and although it is on the main sequence, it is a cool red star with a M1V classification. It has a confirmed planet 5.3 times the size of Earth, orbiting it at a distance of only 5.4 million miles, which equates to 0.05809212096282 AU.

Notable Deep Sky Objects

The constellation of Hydra contains three Messier objects, as well as a number of NGC objects, including star clusters and galaxies.

– Messier 48 (M48, NGC 2548), a large open cluster discovered by Charles Messier in 1771, is the only Messier object in Hydra that you will be able to see with binoculars from the northern hemisphere, and even then only during good seeing conditions. The 300 million year old cluster contains around 80 stars, is about 1,500 light years away, and has an apparent visual magnitude of 5.5.

– Messier 68 (M68, NGC 4590) is a globular cluster of over 2,000 stars that was discovered by Messier in 1780. It is around 33,600 light years from Earth, and has an apparent visual magnitude of 9.67, making it a challenging target for small telescopes. In fact, telescopes with apertures of at least 6″ to 8″ are recommended to get the best view of this beautiful globular cluster.

Southern Pinwheel Galaxy– Southern Pinwheel Galaxy (M83, NGC 5236), at a distance of 14.7 million light years, and with a visual magnitude of 7.54, is amongst the most luminous and closest barred spiral galaxies discovered to date. This makes it an easy binocular target, while a large telescope will yield some fairly good details. The galaxy bears a strong resemblance to the Pinwheel galaxy (M101) in Ursa Major, and to date, six supernova explosions have been identified in M83 over the past 100 years. M83 was discovered by the French astronomer Nicolas Louis de Lacaille in 1752 during a visit to the Cape of Good Hope, and by 1781 Charles Messier had added it to his famous list.

– Hydra Galaxy Cluster (Abell 1060), as galaxy clusters go, is slightly bigger than average, and contains 157 bright galaxies which span more than ten million light years. The three largest members include the elliptical galaxies NGC 3309 and NGC 3311, and the spiral galaxy NGC 3312, which at around 150,000 light years across are significantly bigger than the Milky Way with a diameter of 100,000 light years. Nonetheless, the Hydra Cluster of galaxies is better known for the abnormally large proportion of dark matter that makes up its combined mass. The Hydra Cluster is about 190 million light years away, and forms a part of the much larger Hydra-Centaurus Supercluster of galaxies that include galaxies that properly fall into the constellations of Centaurus and Norma.

Ghost of Jupiter– Ghost of Jupiter (NGC 3242, Caldwell 59) is a planetary nebula around 1,400 light years away that shines with an apparent visual magnitude of 8.60. Also known as the Eye Nebula, but more often as the Ghost of Jupiter, this planetary nebula is an easy target even for amateur telescopes with modest apertures. In 1785, William Herschel discovered this beautiful nebula, and during the 1830’s his son, John Hershel, also observed it from the Cape of Good Hope, but only included it in his 1864 edition of the General Catalogue.

– Tombaugh’s Globular Cluster (NGC 5694, Caldwell 66), although discovered by William Herschel as long ago as 1784, was only determined to be globular cluster by Clyde Tombaugh in 1932, who had discovered Pluto two years earlier. At 12 billion years old, NGC 5694 is one of the oldest known globular clusters of the Milky Way, and shines with an apparent visual magnitude of 10.2.

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