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.


Look for Hydra in the Northern Hemisphere from January to May from just below Cancer where its head is located, 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.

Part of the Hercules Family

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


The Hydra constellation is associated with the mythological creature that Hercules was sent to battle for his second labor. The giant many-headed water snake proved a formidable task 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 Hercules’ already difficult task by sending a giant crab, now represented by the constellation Cancer, to attack him while he battled the Hydra. Fortunately, the ever resourceful Hercules enlisted the help of his nephew, Iolaus, to burn each decapitated neck stump to prevent any regrowth. Interestingly, King Eurystheus, who set Hercules his Twelve Labors, voided this labor because of the help he received from Iolaus. He also later voided the fifth labor of Hercules involving the Augean stables, and so set the legendary strongman two additional labors 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. The peak occurs around the 20th of January, when fewer than 10 meteors per hour is the norm. The other shower associated with Hydra is the Sigma Hydrids, which runs from December 5th to 15th each year, and peaking on the 11th when it yields only about 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. Hydra has no first magnitude stars, but it does contain about 100 stars of 6th magnitude and brighter. However, Hydra does contain 18 stars with confirmed planets, two of which are mentioned below.

– Alphard (Alpha Hydrae); located about 177 light years away, Alphard is the most luminous star in Hydra (2.0 mag), and derives its name from the Arabic, al-fard, which roughly translates into “the solitary one”. With three times the mass of the Sun, and 50 times the Sun’s diameter, but only about 420 million years old, Alphard falls somewhere between being a luminous giant, and an orange giant, with a 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 Tycho Brahe, the famous Danish astronomer.

– Beta Hydrae is one of several multi-star systems in Hydra, and is about 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.

– Gamma Hydrae, the second most luminous star in Hydra (2.993 mag), is a 372 million-year old G8 III yellow giant star that is about 134 light years distant, and is roughly 13 times as big as the Sun. It is also 115 times as luminous as the Sun.

– Zeta Hydrae is very young, 400 million years old star (G9 II-III), that has 4.2 times the mass of the Sun, 18 times the Sun’s diameter, and is 132 times its brightness. At a distance of around 167 light years from Earth, it has an apparent visual magnitude of 3.1.

– HD 122430, is a 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 classified as K2III, that shines at an apparent visual magnitude of 5.47, and is located about 440 light years away from Earth. 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

Messier 48 (M48, NGC 2548); discovered by Charles Messier in 1771, this large open cluster is visible in binoculars during good seeing conditions. The cluster has an estimated age of only about 300 million years, and has an apparent visual magnitude of 5.5, at a distance of about 1,500 light years.

Messier 68 (M68, NGC 4590); this globular cluster was discovered by Messier in 1780, and at a distance of around 33,600 light years has an apparent visual magnitude of 9.67,  making it a challenging target for small telescopes. Telescopes with 6″ to 8″ apertures are recommended to get the best view of this beautiful globular cluster.

Hydra Galaxy Cluster; as galaxy clusters go, the Hydra cluster is slightly bigger than average, and its 157 members, which are all bright galaxies, span more than ten million light years. The two largest members, NGC 3309 and NGC 3311, are both elliptical galaxies but for the most part, the galaxies in this large cluster are all significantly bigger than the Milky Way at about 150,000 light years across. Nonetheless, the Hydra Cluster of galaxies is better known for the large proportion of dark matter that makes up its combined mass. The Hydra Cluster is about 190 million light years away, and form a part of the much larger Hydra-Centaurus Supercluster of galaxies that include galaxies that properly fall in the constellations Centaurus and Norma as well.

Southern Pinwheel Galaxy (M83, NGC 5236); discovered by the French astronomer Nicolas Louis de Lacaille in 1752 during a visit to the Cape of Good Hope, the Southern Pinwheel galaxy is the most luminous, and closest barred spiral galaxies discovered to date, which makes it an easy binocular target. It bears a strong resemblance to the Pinwheel galaxy (M101) in Ursa Major, and to date, six supernova explosions have been identified in M83 during the last 100 years. Charles Messier added the Southern Pinwheel galaxy to his famous list in March of 1781.

Ghost of Jupiter (NGC 3242, Caldwell 59); in 1785, William Herschel discovered this beautiful planetary nebula, which has an apparent visual magnitude of 8.60 and is located at a distance of about 1,400 light years from Earth. His son, John Hershel, also observed the nebula from the Cape of Good Hope during the 1830’s, but only included it in the 1864 edition of the General Catalogue. 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. It is about 1,400 light years away.

Tombaugh’s Globular Cluster (NGC 5694, Caldwell 66); although this cluster was discovered by William Herschel as long ago as 1784, it was Clyde Tombaugh who in 1932, was first to determine its nature as a globular cluster. It is also one of the oldest known globular clusters around the Milky Way, with an estimated age of about 12 billion years. The cluster has an apparent visual magnitude of 10.2.