By Jim Kaler

To the north of Scorpius, Libra, and Sagittarius lie two of the odder constellations in the sky, the intimately linked Ophiuchus (the Serpent Bearer) and Serpens, the giant Serpent that surrounds him. Related to the ancient healer Asclepius and to the Lacoön myth, the dual constellation of snake-wrapped Ophiuchus is with us today in the physician's symbol. Ophiuchus made the news in 2011, when the press discovered that it is considered by some to be the 13th constellation of the Zodiac. Indeed, the Sun takes longer to move through Ophiuchus's modern boundaries than it does through neighboring Scorpius. Nevertheless, it is not part of the classical astronomical Zodiac. (The press also discovered precession of the equinoxes, and that the signs of the astrological Zodiac, which are pinned to the Vernal Equinox, now overlay themselves roughly one constellation to the west. Never mind that knowledge of precession is more than 2000 years old.)

It's hard to talk about one constellation without bringing in the other. Giant Ophiuchus, one of the larger sky-figures, divides the Serpent into two parts. To the west of the Bearer we find the Head (Serpens Caput), to the east, the tail (Serpens Cauda). Though completely separated, they are treated as one. Serpens Cauda, as well as eastern Ophiuchus, fall within the rich Milky Way that streams around the center of the Galaxy.

Bayer separated Serpens as well, his Greek letters flipping from one side then to the other. The third magnitude (nearly second) luminary, Unukalhai (Alpha Serpentis, the Arabic proper name from a phrase meaning "the Serpent's neck"), a standard class K giant 74 light years away, falls into Serpens Caput. As do Beta through Epsilon (a fourth magnitude class A "metallic-line" star in which the chemical composition of the outer layers is messed up by a combination of radiative lofting of some elements, gravitational settling of others). Then we switch to the other side, where we oddly begin with faint fifth magnitude Zeta, next bumping up to third magnitude Eta Ser, the brightest in the snake's Tail and yet another K giant. The prize of this side, though, is fourth magnitude Alya (the only other star with a proper name, which has nothing to do with the Serpent), or Theta Ser. Through the telescope, it breaks into a pair of lovely white class A stars some 22 seconds of arc apart reminiscent of Mizar. They have maintained that separation for more than 250 years, and obviously belong to each other even though at least 1100 Astronomical Units apart. In the middle of it all is Ophiuchus himself, who is topped by second magnitude Rasalhague (from an Arabic phrase meaning "the Head of the Serpent Collector"), Alpha Oph, a white class A giant 49 light years off.

But two constellations do not seem to be enough. Ophiuchus is home to one of the sky's more prominent asterisms (informal figures), Poniatowski's Bull, a vee-shaped configuration a bit over 10 degrees southeast of Rasalhague that looks somewhat like the head of the Zodiac's Taurus. It might be ignored but for two stars, one of which, Barnard's Star, lies just to the northwest of the "vee." Though only 10th magnitude, Barnard's, a classic red dwarf, is the second closest star (or star system, the closest being triple Alpha Centauri), just 5.95 light years away. It also holds the record for "proper motion," zipping across the line of sight at a rate of 10.4 seconds of arc per year, the movement easily followed through a small telescope. And at 138 kilometers per second relative to the Sun, it's really moving, the star a lower-metal "subdwarf" just passing through from a more ancient part of the Galaxy. Half a century ago, Barnard's was made famous by the "discovery" through its wobbling of an orbiting planet (the first so claimed), which turned out to be caused by adjustments in the telescope lens. Lesser known is fourth magnitude 70 Ophiuchi. Part of the "vee" itself, 70 Oph is a classic orbiting binary. Just 17 light years away, the star breaks into a pair of orange class K dwarfs that range between 2 and 7 seconds of arc apart over a full period of 88 years, allowing a patient telescopic observer to watch the gravitational interaction between the two work its magic.

The current stellar king though is probably the recurrent nova RS Ophiuchi, which explosively brightens every 20 or so years, from near 12th magnitude to near naked-eye brightness, the last time in 2006. Novae are all close double stars in which a more ordinary but tidally-distorted companion feeds mass onto a dense white dwarf. When the fresh hydrogen later gets compressed and hot enough, it goes off in a nuclear blast. In RS Oph, the feeder is a red giant, while the accretor is a white dwarf probably near the white dwarf limit of 1.4 solar masses, the high mass of the white dwarf causing it to erupt with some frequency. One theory holds that recurrent novae are the progenitors of Type Ia supernovae, caused when the donating companion actually sends the white dwarf over the line, causing it to collapse and explode. As possibly happened (also in Ophiuchus) to produce Kepler's Star, the supernova of 1604 (the last supernova seen in the Galaxy), though that may well have been of the massive core-collapse variety.

Cutting across the edge of the western part of the Milky Way, the constellations contain so many other fine sights that it's impossible to single them all out. One thing Ophiuchus is known for is its globular clusters, which flock around the Galactic Center. (Open clusters, on the other hand, are generally few and sparse.) The Serpent Bearer contains no less than seven globular clusters from the Messier catalogue: M9, 10, 12, 14, 19, 62, and 107. All are accessible with a small telescope, some easily found with binoculars. Four (M10, 12, 19, and 62) have integrated visual magnitudes brighter than 7.0, of which M62 (magnitude 6.5) is brightest. (At magnitude 7.9, M107 is the faintest of the set.) Such magnitudes are deceptive, as the cluster's light is spread over a wide area as opposed to being a stellar point of light. At a distance 14,000 light years (distances often contended), M10 is the closest, while at 27,000 light years M19 is the farthest. Such distances are typical. Among the first denizens of the Galaxy, distributed within the Galaxy's ancient spheroidal halo, fewer than 200 globulars are known, none therefore very close. All are low- metal and old, typically 12 billion years of age. To be so bright, they have to be very luminous. Absolute visual magnitudes run from -7.0 for M 107 to -9.1 for M 62 (which lies practically on the border with Scorpius), making it the most massive, weighing in at half a million Suns. Being so close to the Milky Way, all are notably dimmed by intervening interstellar dust, M14 and M62 the most highly affected (M14 dimmed by two full magnitudes). Though at first all globulars seem alike, they take on personalities of their own. Some, like M9, are rather flat looking, while others, particularly M62, are more centrally condensed.

Globulars are the principal homes of the famed RR Lyrae stars, low mass, low metal versions of Cepheid variables. Helium-fusing giants, their lower envelope masses make them hotter, which places them into the realm of classes F and A. M14, 62, and 107 are especially rich in them, while M12 and 19 have few or none. Another dozen globulars with NGC numbers hover in far southeastern Ophiuchus near the border with Sagittarius.

Bouncing way over into Serpens Caput, we find one of the great globulars of the sky, Messier 5. An easy sight for the telescope or binoculars, M 5 can be found by sweeping roughly 20 degrees southeast of Arcturus, or better yet, seven or eight degrees southwest of Alpha Serpentis. With an integrated visual magnitude of 5.7, it's potentially visible to the naked eye. Some 23,000 light years away, M5 is among the more massive of its kind. Bright and beautiful, almost undimmed by interstellar dust, the cluster is especially rich in RR Lyrae stars.

Oph-Ser is also home to two most-famous nebulae. In far western Ophiuchus, just above Antares, is a bright diffuse nebula centered on Rho Ophiuchi. To the east extends the Rho Oph dark cloud, a finger-like appendage that hosts one of the great nearby star-forming engines. Other dark clouds abound. On the other side, in far southeastern Serpens Cauda, is the Eagle Nebula, also known as Messier 16, the Messier number taken from the open cluster that lies within it. Embedded in the Eagle Nebula is a set of three long, dark "elephant trunks." In one its most reproduced and famous images, the Hubble Space Telescope displays in exquisite detail how the northern tips appear to be "boiling off" under the harsh radiation of nearby hot massive stars, eventually to reveal newly-formed birthing stars within.

Near the other end of the evolutionary road are three glorious "NGC" planetary nebulae, all in Ophiuchus. Planetaries, named by William Herschel for their disk-like appearance, are the sloughed-off, compressed expanding envelopes of once-great red giant stars like Mira that are lit by the stars' nearly- exposed, very hot but near dead nuclear-burning cores. The best known is bright, compact (just 15 seconds of arc wide) NGC 6572, the "Blue Racquetball," which lies to the northeast of Poniatowski's Bull. About 5000 light years away (few planetaries have accurate distances; this isn't one of them), its central star shines at 13th magnitude with a temperature close to 65,000 degrees Kelvin, hotter than any ordinary star. Somewhat cooler is the core of the larger, ring-shaped planetary NGC 6369 (the "Little Ghost"). In the Milky Way well to the west of the Lagoon Nebula (M8 in Sagittarius), it is dimmed by more than three magnitudes by interstellar dust. In between the two is NGC 6309 (the "Box Nebula"), whose complex inner elliptical structure is lit by a much hotter star of around 90,000 Kelvin. All three central stars are heating and shrinking, and will at some point find themselves within the family of white dwarfs that crowd around us, Ophiuchus and Serpens containing both stellar beginnings and stellar endings.

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 Sept/Dec 2012 Newsletter of the Lowestoft and Great Yarmouth Regional Astronomers, who are gratefully acknowledged.