Hydra
“The Water Snake” · Southern · best around April evenings
Real star positions and magnitudes (HYG database via D3-Celestial, BSD-3). Lines are the conventional stick figure; north is up.
The story
The largest constellation of all 88 — the many-headed monster Hercules slew, stretched across a quarter of the sky, taking six hours to rise completely. Lonely orange Alphard ('the solitary one') marks its heart.
How to find it
Its head is a compact knot south of Cancer; the body trails to below Libra.
✦ What lives inside it
- M83 Southern Pinwheel galaxy
- Ghost of Jupiter nebula
The deep dive
Researched for the Atlas from Wikipedia — Hydra (constellation) (8,334 characters read) · updated Sep 20, 2026
01 The sky's longest constellation by stretch
Hydra does not merely claim the title of largest constellation by area — at 1303 square degrees it is also the longest, spanning more than 100 degrees of arc from one tip to the other. To picture that reach, hold both arms fully outstretched to your sides: the angle between your fingertips is roughly 180 degrees, and Hydra fills more than half of that sweep on the dome of the sky. Its northern end brushes Cancer while its southern tail curves down to touch Libra and Centaurus, meaning the serpent's body winds past an extraordinary number of neighboring constellations along the way. Despite commanding so much real estate, the constellation straddles the celestial equator, placing it accessible to observers in both hemispheres. That combination — enormous yet equatorial — is unusual among the 88 modern constellations, and it makes Hydra a useful landmark for learning how the celestial sphere is organized.
02 A Babylonian serpent older than Greece Deeper
The roots of Hydra reach back well beyond Greek astronomy. The Babylonian star catalog known as MUL.APIN already included a serpent constellation — recorded as MUL.DINGIR.MUŠ — that loosely corresponds to the region of sky we call Hydra today. What makes the Babylonian original distinctive is its nature: it was not a simple snake but a mythological hybrid combining serpent, lion, and bird into a single creature. The Greek figure is therefore an adaptation, not an invention. Hydra is just one of two Greek constellations that trace their ancestry to Babylonian serpent figures; the other is Serpens. This lineage places Hydra among the most ancient recognized star patterns, predating Ptolemy's canonical list of 48 constellations compiled in the 2nd century CE — a list that Hydra was included in. The long chain of transmission from Babylon through Greece and into the modern IAU list makes Hydra a rare case where we can actually trace a constellation's documented history across cultures and millennia.
03 Why the crow, the cup, and the snake fly together
The three neighboring constellations Corvus the crow, Crater the cup, and Hydra the water snake are connected by a single Greek myth, and understanding it makes the star grouping feel deliberate rather than arbitrary. Apollo sent a crow to fetch water in a cup. The crow, distracted or deceitful, delayed and returned with a water snake as an excuse for the wait. Apollo saw through the fraud, and in his anger hurled all three — crow, cup, and snake — into the heavens together. Today, Corvus and Crater both perch on Hydra's back in the sky, exactly as the myth implies. A separate and more famous myth connects the constellation to the multi-headed monster slain by Hercules. That story adds a practical twist: every time Hercules severed one of Hydra's heads, two more grew back in its place. The deadlock was broken only when Hercules' nephew Iolaus seared each severed neck with a torch, preventing regrowth and allowing Hercules to finally defeat the creature.
04 Alphard: the solitary heart of a vast snake
For all of Hydra's enormous extent, it contains only one moderately bright star — Alphard, designated Alpha Hydrae. Its traditional name translates directly as "the solitary one," an apt label for the lone bright point in such a large region of sky. Alphard is an orange giant shining at magnitude 2.0, located 177 light-years from Earth. Moving across Hydra's body, Gamma Hydrae, now formally named Naga, is a yellow giant of magnitude 3.0 sitting 132 light-years away — closer than Alphard but noticeably dimmer to the eye. Beta Hydrae is a blue-white star of magnitude 4.3 at 365 light-years. The head of the serpent carries its own named star, Sigma Hydrae, known also as Minchir from the Arabic for "snake's nose," though at magnitude 4.54 it is quite faint. The relative scarcity of bright stars is one reason Hydra, despite its record-breaking size, is not a dominant feature of the night sky — it sprawls quietly rather than blazing.
05 Double and triple stars worth splitting Deeper
Hydra rewards patient observers with a range of multiple-star systems spread across its enormous length. Epsilon Hydrae is the most challenging of these: a binary with a very slow orbital period of 1000 years, making it appear almost stationary, and it is described as difficult to split in amateur telescopes. The primary is a yellow star of magnitude 3.4 and the secondary a blue star of magnitude 6.7, separated in the sky but bound by gravity at a distance of 135 light-years from Earth. Since 1 June 2018 the IAU has formally given this star the proper name Ashlesha, drawn from the Indian lunar zodiacal constellation Āshleshā Nakshatra, which corresponds to the head of the snake. By contrast, 54 Hydrae — a binary 99 light-years away — is described as easily divisible in small amateur telescopes, pairing a yellow primary of magnitude 5.3 with an unusually colored purple secondary at magnitude 7.4. The triple star 27 Hydrae offers a layered experience: binoculars reveal two components, while a small telescope shows a third, the secondary itself being a binary composed of a magnitude 7 and a magnitude 11 star.
06 Variable stars from barely-there to blood-red Deeper
Hydra hosts a diverse set of variable stars that span very different types of stellar behavior. R Hydrae is a Mira-type pulsating giant 2000 light-years away, and it ranks among the brightest Mira variables at its peak: magnitude 3.5 at maximum, fading all the way to magnitude 10 at minimum — a brightness change of more than 400-fold — with a period of 390 days. U Hydrae is a semi-regular variable with a deep red color, 528 light-years out, ranging from magnitude 4.2 at its brightest to 6.6 at its faintest over a period of 115 days. Most dramatic in color is V Hydrae, described as unusually vivid red, lying some 20,000 light-years from Earth. It varies between magnitudes 6.6 and 9.0. That enormous distance — roughly five times farther than the center of the Milky Way is from us — makes it remarkable that V Hydrae is detectable in amateur instruments at all. Together these three stars give observers a ready laboratory for watching stars breathe, pulse, and glow across different timescales within the same constellation.
07 A nearby star system with a habitable-zone world
Tucked inside Hydra's borders is one of the more scientifically compelling nearby star systems: GJ 357, an M-type main-sequence red dwarf only 31 light-years from the Solar System. At that distance it is practically a next-door neighbor in cosmic terms — roughly 293 trillion kilometers away, compared to the Milky Way's diameter of about 100,000 light-years. The star hosts three confirmed exoplanets. The most scientifically exciting of these is GJ 357 d, classified as a Super-Earth and noted as sitting within the star's circumstellar habitable zone, meaning it occupies the range of distances where liquid water could theoretically exist on a planetary surface under the right conditions. While habitability is far from confirmed, the combination of proximity to Earth and a potentially temperate world makes GJ 357 a target of serious ongoing interest. It represents one of the rare cases where a specific constellation contains a named exoplanet candidate of direct relevance to astrobiology research.
08 NGC 5694: a globular cluster at the galaxy's edge Deeper
Among Hydra's globular clusters, NGC 5694 stands out for its extraordinary distance. While M68 lies 31,000 light-years from Earth and is comfortably within the Milky Way's halo, NGC 5694 is located 105,000 light-years away — placing it at or near the outermost fringes of the galaxy. At magnitude 10.2 it requires a telescope to see at all. The cluster's history is layered: William Herschel detected it as a non-stellar object in 1784, but its true nature as a globular cluster was not established until 1932, when Clyde Tombaugh — better remembered for discovering Pluto in 1930 — examined photographic plates taken of the region near Pi Hydrae on 12 May 1931. This connection to Tombaugh earned it the informal designation "Tombaugh's Globular Cluster." It is classified as a Shapley class VII cluster, indicating intermediate concentration at its nucleus — neither loosely scattered nor densely packed at its core. Its remote location makes it an object of interest for studies of the Milky Way's outer structure.
09 NGC 4993 and a landmark cosmic collision Deeper
One galaxy within Hydra has become famous not for what it looks like but for what happened inside it. NGC 4993 is an elliptical galaxy that was the site of three simultaneous events in August 2017: the gravitational wave event GW170817, the gamma-ray burst GRB 170817A, and the optical transient SSS17a. All three were produced by the merger of two neutron stars — the first such merger ever detected across gravitational waves and electromagnetic light simultaneously. This multi-messenger observation was a landmark moment in astrophysics, confirming that neutron star mergers are a source of short gamma-ray bursts and demonstrating that gravitational waves travel at the speed of light to within extraordinary precision. The fact that this host galaxy sits within Hydra's boundaries means that one of the most significant astronomical events of the 21st century is associated with this quiet, sprawling constellation. NGC 4993 is an otherwise unremarkable elliptical galaxy, but it now carries scientific weight far beyond its visual appearance.
10 How the Ghost of Jupiter got its name
NGC 3242 is a planetary nebula located 1400 light-years from Earth and glowing at magnitude 7.5, making it accessible in small telescopes. William Herschel discovered it in 1785, and over time observers gave it a vivid nickname: the Ghost of Jupiter. The name comes from its striking visual resemblance to the giant planet — its blue-green disk appears in a telescope eyepiece with a similar apparent size and color palette to Jupiter, though it is of course an entirely different kind of object: the expelled outer layers of a dying star, illuminated by the hot stellar remnant at its center. In a small telescope the disk is clearly visible; larger instruments reveal a surrounding halo. That combination of accessibility and visual reward places NGC 3242 high on lists of binocular and small-telescope targets within Hydra. It is one of the few deep-sky objects in the constellation that delivers an immediate, recognizable impression rather than requiring extended observation time or very dark skies to appreciate.
11 Chinese and Arabian readings of the serpent's stars
Western Greek mythology is not the only cultural lens through which Hydra's stars have been organized and named. In Chinese astronomy, the stars that correspond to Hydra fall within two entirely different celestial systems: the Vermilion Bird and the Azure Dragon, two of the Four Symbols that divide the Chinese sky into quadrants associated with seasons and cardinal directions. The serpent's body was therefore distributed across a broader symbolic landscape rather than standing alone as a single creature. Arabian sky-lore left its mark more concretely in star names: the head of Hydra was collectively known in Arabic as Min al Az'al, meaning "belonging to the uninhabited spot" — an evocative name suggesting a desolate or empty region of sky. Individual star names also carry Arabic roots: Alphard derives from Arabic meaning "the solitary one," and Minchir for Sigma Hydrae comes from the Arabic for "snake's nose." Indian astronomy further contributed through the Āshleshā Nakshatra, a lunar zodiacal mansion that corresponds to the head of the serpent, and whose name was formally adopted by the IAU as the proper name of Epsilon Hydrae in 2018.
12 Meteor showers radiating from the water snake
Hydra is the radiant point for two distinct meteor showers. The more active of the two is the Sigma Hydrids, which peak on December 6. The parent body of this shower is believed to be Comet Nishimura, formally designated C/2023 P1, a comet discovered in 2023 by Japanese astronomical photographer Hideo Nishimura. The identification of a specific parent comet gives the Sigma Hydrids added scientific interest, since debris streams can be traced back to the object that shed them. The second shower, the Alpha Hydrids, is a minor event peaking between January 1 and 7. Together the two showers mean that Hydra is not merely a backdrop for deep-sky objects but an active source region for meteors in both late autumn and early winter. For observers who take the time to learn the serpent's head position in the sky — itself a useful navigational skill — these showers offer a concrete reason to look toward Hydra on specific calendar dates each year.