By Jim Kaler

Long lines of stars seemed to intrigue the ancients. Draco the Dragon wraps itself around the northern pole, Eridanus (the River) drains the sky from Orion's foot to its end in the southern hemisphere at brilliant blue Achernar, and Hydra, the Water Serpent, slithers to the east- southeast from its scary head below Cancer nearly to Libra, making it not only the longest constellation in the sky but also the largest, just barely beating out Virgo for the dubious honor. As if a Water Serpent had not been enough, early explorers of the southern hemisphere invented Hydrus, the Water SNAKE, which Bayer included in his Uranometria of 1603. While the pairing of the two can be confusing, Ian Ridpath points out that they fall into the "major-minor" tradition that pairs big and small bears, dogs, triangles, horses, as well as northern and southern couplets that include crowns, triangles, and flies (the Minor Triangle and Northern Fly no longer recognized). Hydra is peculiar in that it has two ancient constellations riding on its back, Corvus the Crow (which perhaps makes some sense) and dim Crater, the Cup (which doesn't), dividing it into crude thirds. Hydrus is peculiar in that it winds so as to avoid the two Magellanic Clouds, our brightest satellite galaxies.

While celestial treasures within the two constellations are not numerous, the ones that are there grab attention. Hydra's luminary, Alphard, leads the naked- eye list. At the heart of the Serpent, the name from Arabic means "the solitary one," the second magnitude (1.98) star shining in lonely splendor to the southeast of Regulus in Leo. While a relatively common orange helium-fusing K giant 180 light years away, Alphard has an unusually high mass 4.5 times that of the Sun and radiates at a rate of nearly 1000 solar luminosities. It has the distinction of being a mild sort of "barium star" that was contaminated by heavy elements from the wind of a more massive companion that is now a white dwarf. No degenerate neighbor, however, has ever been found, and in fact the star might not be contaminated at all. Stellar oddities include a fourth magnitude class A0 dwarf near Hydra's extreme western edge that goes by both "c" Hydrae (from Bayer) and 30 Monocerotis (from Flamsteed), the star once included in Monoceros (the Unicorn) but orphaned by modern boundaries. While no planet has been discovered, 30 Mon is surrounded by a dusty disk similar to that around Vega in which one or more planets may well orbit. Hydra also fools us, as the second brightest star is not Beta Hya (an interesting fourth magnitude binary 310 light years away with a magnetic main component), but third magnitude Gamma, another common G8 helium- fusing giant 134 light years away. Follow Corvus's two western stars downward to find Beta and the bottom two eastward to locate Gamma, which lies below Spica.

The most interesting Greek- lettered star in the huge constellation may well be the northernmost in Hydra's evil-looking head, Epsilon, a multiple system 129 light years away with at least four components and a rarity in having not one, but TWO determined orbits. Separated by only a few tenths of a second of arc, Epsilon Hydrae A (a class G giant) and B (an A8 dwarf) revolve around each other every 15 years at an average distance between them of 17 Astronomical Units. Then Epsilon C (an F7 dwarf) orbits the inner AB pair with a period of 590 years 130 AU apart, the visual double of AB and C separated by a couple seconds of arc and accessible to the amateur's telescope. The spectrum then reveals that Epsilon Hya A (the G giant) has a much closer low mass companion that orbits in just 9.9 days. Given that "A" has a mass of around 2.5 Suns, the little one goes around just 0.13 AU from its giant master and close to its surface. The tight double radiates X-rays, so one or both are magnetically active rather like the Sun but aided by the duplicity. Way outside these are D, E, and F. All three, however, are just line of sight coincidences, so we can quit at four.

Hydra's most famed star may be the classic Mira variable R Hydrae, a red M6-M8 giant located just a couple degrees east of Gamma. A dying star 400 light years away, radiating in the tens of thousands of Suns, R Hya has become highly unstable, varying between a readily-visible fifth magnitude (or brighter) and ninth or tenth over a decreasing period of around 388 days. Though visually much fainter, V Hydrae takes a notable position as one of the weirder stars, not just of Hydra but of anywhere. It's a very red carbon-rich giant that varies by 1.5 magnitudes over 130 days upon which is superimposed another variation twice that of 17 years, probably as a result of ejection of carbon-loaded dust shells. A by-product of helium fusion, the star's carbon has been brought up by convection from below. Most of the carbon in the Universe, including that here on Earth, was created by such stars.

As Mira and related variables lose their outer envelopes, they expose their hot cores, which will eventually become carbon white dwarfs. A hot wind blowing from the core shovels the fleeing envelope into a complex shell, which in turn is ionized and illuminated by the hot core's ultraviolet radiation, birthing an ephemeral planetary nebula. Just west of Crater, Hydra holds one of the real classics, bright NGC 3242, the "Ghost of Jupiter." It's a prototypical double-shell nebula with bipolar jets of the kind best seen in Herschel's discovery nebula, NGC 7009 in Aquarius. Around 1600 light years away and half a light year wide, NGC 3242's twelfth magnitude central star shines at around 90,000 degrees Kelvin with a luminosity of 1000 Suns, almost all in the invisible ultraviolet. Far less known is NGC 2610 in western Hydra. It's a particularly high excitation planetary nebula with a central star so hot that standard temperature determinations do not work, but at least show that it tops 100,000 Kelvin.

Hydra is home to three very different sorts of Messier objects. In far western Hydra find the near-naked-eye sprawling open cluster M 48. Nearly a degree across, 2500 light years away, almost half a billion years old, M 48 is a nice sight in binoculars. Next, pointed downward to by Corvus's eastern edge, M 68 is a tiny globular cluster, one of the few in the celestial hemisphere that lies opposite the Galaxy's center. Even more special, it has one of the lowest metal contents known for any of its kind, its iron-to-hydrogen ratio just 0.38 percent of that found in the Sun. The cluster is a true halo object formed as the Galaxy was being born. Third is M 83, a prominent barred spiral galaxy called the "southern pinwheel." Eight degrees south-southeast of Gamma Hydrae, it lies 15 million light years away and recedes from us at a speed of 515 kilometers per second.

Far to the south, Hydrus serves up three rather diverse third magnitude stars. At magnitude 2.80, Beta Hyi just barely tops the short list. As a G2 subgiant just 24 light years away rather similar to the Sun, it's clearly the most interesting as well, though no planets have been found. The closest bright star to the South Celestial Pole, Beta Hyi served as the southern pole star around 150 BC, but has now moved away thanks to the 26,000-year wobble of the Earth's rotational axis. Magnitude 2.86 Alpha Hyi, an F0 dwarf, so served around 2900 BC. At magnitude 3.24, Gamma Hyi provides contrast as a class M2 red giant around 215 light years away.

While Hydrus manages to miss the two Magellanic Clouds (the Small Cloud 60,000 light years away in neighboring Tucana, the Large Cloud 50,000 light years away in Dorado), it still manages to get a tiny piece in the form of a relatively young globular cluster, NGC 643, in the SMC's outer halo. Beta Hyi however provides a beautiful guide to one of the greatest of all globulars, 47 Tucanae five degrees to the north, which helps maintain the old (paraphrased) adage that the Good Lord put most of the astronomers in the north but most of the good stuff in the south.

Copyright © James B. Kaler, all rights reserved. These contents are the property of the author and may not be reproduced in whole or in part without the author's consent except in fair use for educational purposes. First published in the August 2015 Newsletter of the Lowestoft and Great Yarmouth Regional Astronomers, who are gratefully acknowledged.