Hydra: The Largest Constellation

Hydra holds the record as the largest of all 88 modern constellations, stretching across an enormous 1,303 square degrees of sky. This sinuous water serpent winds its way from the border of Cancer in the west all the way to Libra in the east, spanning more than 100 degrees of celestial longitude. Despite its impressive size, Hydra is often overlooked because most of its stars are faint. Finding its head and tracing its body is a rewarding exercise in patience and sky knowledge. Track Hydra's full length using StarGlobe.

Finding Hydra

The most distinctive part of Hydra is its head, a compact group of five or six stars forming a small irregular polygon south of Cancer. From mid-northern latitudes, the head is visible in the southern sky during late winter and spring evenings. It sits roughly between the bright stars Procyon (in Canis Minor) and Regulus (in Leo).

Once you have identified the head, follow the body southward and then eastward. The snake's heart is marked by Alphard, the only reasonably bright star in the constellation. From Alphard, the body continues to wander eastward and slightly southward, passing beneath Corvus and Crater (two small constellations that appear to ride on Hydra's back) before finally ending near the border of Libra and Centaurus.

Tracing the full extent of Hydra requires a clear southern horizon and reasonably dark skies. Its stars are mostly third and fourth magnitude, so light pollution makes the tail section especially challenging.

Alphard: The Solitary One

Alphard (Alpha Hydrae) is the constellation's sole bright star at magnitude 2.0. Its Arabic name means "the solitary one," a fitting title given the empty expanse of sky surrounding it. Alphard is an orange giant star about 177 light-years from Earth, roughly 50 times the Sun's diameter and over 900 times its luminosity.

Alphard is easy to find by drawing a line from the two stars marking the bowl of the Little Dipper through Regulus and continuing roughly the same distance southward. Its warm orange hue is noticeable even to the naked eye and makes a nice contrast with the blue-white stars surrounding it in the spring sky. Learn why stars appear different colors in our article on star colors and temperature.

Deep Sky Objects

Hydra's large area encompasses several interesting deep sky objects. M48 is a bright open cluster in the western portion of the constellation, visible in binoculars as a hazy patch and resolving into dozens of stars through a small telescope. It lies about 1,500 light-years away and contains roughly 80 stars.

M68 is a globular cluster in the southern part of Hydra, near the border with Corvus. At magnitude 7.8 and about 33,000 light-years distant, it requires at least binoculars to detect and a moderate telescope to resolve individual stars around its edges.

M83, the Southern Pinwheel Galaxy, is one of the brightest and closest barred spiral galaxies, lying about 15 million light-years from Earth near Hydra's border with Centaurus. Under dark skies, it is visible in binoculars as a faint glow, and telescopes reveal spiral arms and a bright nucleus. M83 is a prolific producer of supernovae, with six observed in the past century.

The Ghost of Jupiter (NGC 3242) is a bright planetary nebula located south of Alphard. Through a telescope, it appears as a small, vivid blue-green disk reminiscent of the planet Uranus. Its nickname comes from its apparent size, which is similar to Jupiter at low magnification. At magnitude 7.7, it is accessible to small telescopes and is one of the finest planetary nebulae visible from either hemisphere.

Mythology: The Lernaean Hydra

In Greek mythology, Hydra represents the Lernaean Hydra, a monstrous multi-headed water serpent. Slaying the Hydra was the second of the Twelve Labors of Hercules. The beast lived in the swamps near the city of Lerna and terrorized the surrounding countryside. Each time Hercules severed one of its heads, two more grew back in its place.

Hercules eventually defeated the Hydra with the help of his nephew Iolaus, who cauterized each neck stump with a burning torch after Hercules cut off the head, preventing regrowth. The Hydra's central head was immortal, so Hercules buried it under a heavy rock. He dipped his arrows in the Hydra's poisonous blood, making them lethal, though this eventually led to tragedy when he accidentally wounded the centaur Chiron (commemorated in Centaurus). More constellation stories are collected in our mythology guide.

Corvus and Crater on the Serpent's Back

Two small constellations, Corvus the Crow and Crater the Cup, sit on Hydra's back in the eastern portion of the serpent's body. According to one myth, Apollo sent the crow to fetch water in a cup, but the bird was distracted by a fig tree and waited for the fruit to ripen. When the crow finally returned, it brought the water serpent Hydra as an excuse. Apollo saw through the deception and placed all three in the sky as a warning against dishonesty.

Corvus is a small but distinctive trapezoid of four stars that is easy to spot, while Crater is a fainter and less obvious cup shape. Both are useful landmarks for tracing Hydra's body across the sky.

Observing Tips

Hydra's head is best observed in February and March, while the tail does not reach its best evening position until May. The constellation's enormous east-west extent means it can take months for all parts to pass through prime evening viewing. Alphard is visible from both hemispheres and makes a reliable anchor for identifying the serpent.

Binoculars are ideal for sweeping along Hydra's length and finding its clusters and nebulae. The Ghost of Jupiter and M83 are telescope highlights that reward patient searching. Use StarGlobe to trace the entire serpentine path of the sky's largest constellation from head to tail.