The Constellation Hydra
Over the next few weeks we’ll veer off the spring ecliptic, below the constellations Cancer, Leo, Virgo, and Libra, to examine the deep-sky delights of the constellation Hydra, which stretches across a quarter of the night sky. Today, a quick tour of the constellation, and look at a lovely double star in the head of this celestial beast.
Hydra, the Water Serpent, was one of the original 48 constellations outlined by Ptolemy in the 2nd century A.D. It survives today as the largest of the 88 modern constellations, spanning over 1300 square degrees. Though Eridanus is longer, it wraps back upon itself and doesn’t stretch as far across the sky as Hydra.
The Egyptians saw the long, winding constellation as the celestial representation of the life-giving river Nile.
But the ancient Greek legend of Hydra had a little more action.
According to the Greeks, Hydra was a ghastly swamp creature with a long, dragon-like body and nine heads. The beast terrorized the village of Lerna in the northeast Peloponnesian Peninsula, killing livestock and threatening to cut off access to the village’s water supply. But the brave Hercules (or Heracles, in Greek) came to the rescue along with his nephew Iolaus. Hercules clubbed and hacked at the monster’s heads, but when he knocked one off, two grew back in its place. So Hercules schemed to have Iolaus cauterize the monster’s wounds with a torch each time the hero knocked off a head. It worked. The beast was defeated, and Hercules buried the monster in the swamp.
While its stars are not bright, Hydra is easy to find because of its long zig-zag line of some 17 stars. It immediately jumps out in darker sky; city dwellers may need to binoculars to trace its shape. Find the circular head of Hydra about halfway between the bright stars Regulus at the tip of the Sickle in Leo, and Procyon in Canis Minor (see map below). The head is a delight in binoculars and fits nicely in a 5-degree field of view. (The constellation has but one head, not nine). Then follow the rest of the constellation to zeta (ζ), alpha(α), lamba(λ), nu(ν), xi(ξ), and finally gamma (γ)Hydrae, about 15 degrees below Spica. It takes the entire constellation about three months to work its way across the sky.
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The winding constellation Hydra in the southern April sky, below Cancer, Leo, Virgo, and Libra
At 2nd magnitude, the brightest star in Hydra is the rich-orange giant Alphard, which is Arabic for “the Solitary One”, an appropriate name because of its lonely place in this part of the sky.
Physically, Alphard is fascinating. The star has an unusually high abundance of the element barium, a situation caused by a long-dead companion star that irradiated Alphard with neutrons, creating barium and other heavy trace elements in its atmosphere. Alphard itself is on the way to doom; it has long exhausted the hydrogen fuel in its core and has swelled to burn cooler and some 400 times brighter than our sun. Alphard lies 175 light year away.
Look back to the head of the constellation, just below Cancer. The northernmost and brightest star of this asterism is epsilon Hydrae, an amazing quintuple-star system about 135 light years away. In a small scope at 100x or so, you’ll easily see two yellow stars separated by 4.5″. Each of these is itself a double star. And about 19″ away from this pair lies a dim 10th magnitude red dwarf star.
The head of Hydra, below Regulus and Procyon, showing the multiple star ε Hydrae (lower left)
Enough for today. When we meet Hydra again, we’ll examine Charles Messier’s “lost” star cluster.
