The sprawling constellation of Hydra was one of the 48 constellations listed by Ptolemy and endures today to be the largest of the 88 modern constellations adopted by the International Astronomical Union. Spanning an incredible 1303 square degrees of night sky and containing 17 primary stars in the asterism, Hydra contain 75 stars with Bayer/Flamsteed designations. It is bordered by the constellations of Antlia, Cancer, Canis Minor, Centaurus, Corvus, Crater, Leo, Libra, Lupus, Monoceros, Puppis, Pyxis, Sextans and Virgo. Position south of the ecliptic plane, Hydra is visible to all observers at latitudes between +54° and ?83° and is best seen at culmination during the month of April.
In mythology, Hydra represents the Snake – not much of a stretch of the imagination given all the twists, turns and distance this constellation takes across the sky. According to legend, Apollo sent the Raven, Corvus, off with the Cup (Crater) to fetch a drink. When the Raven spent his time waiting for a fig to ripen instead of returning with Apollo’s refreshment, he realized he’d made a mistake and grabbed a water snake to offer to the sky god as atonement for his tardiness. Infuriated, Apollo tossed the whole lot of them into the sky where they remain until this day… Some legends also refer to Hydra as one of the many labors of Hercules, too!
Shall we begin with a binocular tour of Hydra? Then let’s start first with the small asterism of stars which marks the “head” of Hydra located between bright stars Regulus and Procyon. When you’ve picked out this distorted circlet, focus your attention on the northernmost of these stars – Epsilon – the backward “3” on our map. While to binoculars it might seem rather ordinary, Epsilon is actually a fantastic multiple star system with at least five members! The primary is a yellow-white giant star with a white subgiant star orbiting so close that it is considered a spectroscopic binary star. A bit further away is another binary pair, the G and F star… and further away yet is a class M dwarf star. Be sure to take out your telescope and have a look….
Now move southeast for the brightest star in Hydra – Alpha – the “a” symbol on our map. Its name is Alphard and it is located about 175 light years away from Earth. Shining in a very soft orange color, this giant star reaches temperatures of about 4000 degrees Kelvin and if at home in our solar system would be about 400 times brighter than our Sun. What makes Alphard unique? Its barium content. At one time Alphard, too, was a binary star, but its massive companion is long gone. Alphard happily collected its by-products of nuclear fusion and left us with evidence of what once was!
Keep your binoculars handy and use the two points of reference you’ve just learned to find our next target – Messier 48. By connecting Epsilon and Alpha as the base of an imaginary triangle with the top pointed southwest, aim your binoculars at the apex and behold one very nice – and very bright – open star cluster. Discovered by Charles Messier in 1771 and also cataloged as NGC 2548, you might even be able to distinguish this stellar field as a hazy spot unaided from a dark sky location. With an estimated age of about 300 million years, you’ll see a very large group of about 50 stars which can resolve into as many as 80 members in larger telescopes. When you see M48, you can thank Caroline Herschel for fixing Messier’s position mistake on this one!
Hop along to Lambda Hydrae – the upside down “Y” on our map. Lambda is a visual double star in binoculars, but it is also a true spectroscopic binary star as well. As you continue south, then east and pass by Xi (the squiggle), keep in mind Xi is unique, too. Xi is an evolved giant star with solar-like oscillations… the very first time science has proved the existence of vibrations in a giant star 10 times the size of our Sun! If you place Xi to the western edge of the field in binoculars, you’ll also see 5th magnitude Beta Hydrae, too. Seeing two there instead of one? You should. Beta is a visual double star and the pair are only separated by about half a magnitude.
Now head northeast towards Gamma, but stop by Messier 68 along the way. This class X globular cluster was discovered by Charles Messier on April 9, 1780, but was resolved into stars by Sir William Herschel who said; “A beautiful cluster of stars, extremely rich, and so compressed that most of the stars are blended together; it is near 3′ broad and about 4′ long, but chiefly round, and there are very few scattered stars about.” M68 will look like a small, round fuzzy in binoculars, but larger telescopes will resolve this 33,000 light-year distant Milky Way resident out!
Are you ready for Gamma Hydrae? It’s the “Y” shape on our map. If you got lost, just use the lower two stars of Corvus to point east towards it. Gamma is located about 132 light years away from our solar system and shines approximately 105 times brighter than Sol. In the not-to-distant past, Gamma decided to shut down its hydrogen fusion factory, which means it may possess a dead helium core. What’s in Gamma’s future? Chances are it will grow larger and less luminous as the core shrinks – then it will fire up to fuse carbon and oxygen. When it does it will become six times brighter and five times bigger! If you’re looking with a telescope and see another star there, you’re right… but it’s an optical companion.
To the east of Gamma is R Hydrae. Now here is one classy variable star! Located about 2000 years away from Earth, R Hydrae’s changes take a period of 389 days to happen, but they happen in a big way. The magnitude of this crazy star jumps from a very dim and telescopic only 11.0 to an easy unaided eye 3.2! R is the third Mira-type variable discovered and may have been noted as early as 1662 by Johannes Hevelius. R Hydra is also special because it has a “declining period” – it has changed its times by 100 days in the last couple of hundred years. So what’s happening? A helium shell is building up around the exterior – just waiting for the day to reach a critical mass and ignite, creating more carbon and oxygen. This is called a “helium shell flash” and it signals the end of life for the giant star. Eventually the layers will just expand into space and the carbon-oxygen core will shine as a white dwarf star. Look around while you’re there… Because you just might spot a companion!
Now drop almost due south for Messier 83 (RA 13:37.0 Dec -29:52). In binoculars this superb spiral galaxy will appear as a soft round glow, but telescopes will reveal wonderful spiral galaxy structure (dependent on observer latitude). With a classification somewhere between a normal and barred spiral galaxy, large telescopes can expect to at least see three traces of spiral arm structure. For astrophotographers, you’ll find terrific star forming regions will appear and dark dust lanes follow the spiral structure throughout the disk.
Ready to do a telescope object in Hydra? Then look no further than NGC 3242 (RA 10:24.8 Dec -18:38). This 8th magnitude planetary nebula is best known as the “Ghost of Jupiter” for its magnificent size! Be sure to look for a double halo structure and the 11th magnitude central star. Even small telescopes will catch a faint blue color to this superb object!
For larger telescopes, let’s try some galaxies. First off, NGC 3621 (RA 11:18.3 Dec -32:49) located about about 3 degrees west/southwest of Xi. You’ll find this fairly larger and bright spiral galaxy sitting inside a box of faint stars! Need a pair of galaxies? Then try NGC 3923 and NGC 3904 (RA 11 h 51 min Dec – 28 48′). Use low magnification and a wide field eyepiece to capture this spiral and elliptical galaxy in the same view.
There’s plenty more deep sky in the constellation of Hydra to be explored, so be sure to get a good star chart and charm the “Snake”!