As befits a large constellation at the edge of the Milky Way, Ophiuchus is packed with deep-sky sights for observers with small and large telescopes. Open and globular star clusters abound here, along with many fine double stars. Let’s have a short tour of a handful of the highlights of the constellation, moving from easy objects to more difficult sights.
One of my favorite regions of the sky is centered near the little asterism called Taurus Poniatowski. The little bull-shaped asterism on the northeastern edge of Ophiuchus is easy to spot in dark sky. It was named in 1777 by Abbe Poszobut after King Stanislaus Poniatowski of Poland. It’s a pretty little group. In binoculars, the background is flecked with fainter 9th and 10th-magnitude stars that straggle off the western edge of the Milky Way. The V-shaped head of the bull consists of three stars: 67, 68, and 70 Ophiuchi. The two stars at the back end of this little beast are γ and β Ophiuchi (Cebalrai).
Now grab your telescope and direct your optics and attention to the star 70 Ophiuchi in the head of Taurus Poniatowski. 70 Oph is one of the best-known and widely studied binary star systems. It’s relatively nearby, just 16 light years away. You’ll need about 75x to resolve the pair cleanly with a telescope. The two components of 70 Oph have magnitude 4.2 and 5.9; the brighter star is yellow-gold while the fainter looks orange-red, with some observers reporting a tinge of violet. The pair is separated by about 5″. Move your telescope out of focus just a touch to see the colors best. Each star has an intrinsic brightness only a fraction that of our Sun.
The components of 70 Oph complete a revolution about their common center of mass in just 88 years. So this is one of the few double stars you can see move appreciably during a human lifetime.
Now look for the lovely open star cluster IC 4665 just 1.3o northeast of Cebalrai (beta Ophiuchi). In dark sky, IC 4665 is just visible to the unaided eye. If you’ve got a little light pollution, you’ll need binoculars to spot it. The cluster is spread out over a full degree, more than twice the diameter of the full Moon, so it looks fainter than its integrated magnitude of 4.7. We’ve covered this lovely little cluster in these pages before, and you can learn more about it here.
Up for a couple more open clusters? Then look east and slightly north of the head of Taurus Poniatowski to see a pair of splendid star groups, NGC 6633 and IC 4756. Some call this pair the “Ophiuchus Double Cluster”, which isn’t exactly accurate because the latter lies just over the border in the constellation Serpens
Caput Cauda. IC 4756 is much larger, covering a full degree of sky, so it’s an excellent cluster for observing with binoculars or with low magnification in a telescope. NGC 6633 is smaller and tighter and can handle a little more magnification. Both clusters are the same distance away, about 1,300 light years, so their comparative sizes reflect their true dimensions.
Now to a couple of globular clusters. The pair M10 and M12 inside the pentagon of Ophiuchus are fainter than the more famous globular cluster M5 in Serpens. But they still look good in a small scope. M10 lies just 1o west of the orange star 30 Ophiuchi. M10 appears brighter than its neighbor M12, and certainly more concentrated towards the center. The cluster is visible in binoculars or a finderscope as a dim smudge. In a 4” scope at 70x, you will see some granularity on the outer edges of the cluster. A 6” or larger scope resolves a couple of dozen stars towards the center.
M12 is just 3o northeast of M10. The two are visible in the same finder field of view, and even in a wide-field telescope at low magnification. The cluster is certainly less dense than M10, and a good 6” scope may resolve some stars near the core. Try a range of magnification to see what works best for you.
M10 lies some 14,000 light years from Earth, while M12 is about 16,000 light years distant. Since they are physically separated by just 2,000 light years, a planet around a star in either cluster would see the other cluster shine at a brilliant 2nd magnitude.
Back to Taurus Poniatowski for the most challenging object on this tour, the pretty little planetary nebula NGC 6572, sometimes called the “Emerald Eye Planetary”. The name comes from the somewhat greenish hue of this small bright gem-like nebula. The color comes from the light emitted by OIII oxygen ions excited by the nebula’s hot central star. Look for this little celestial jewel above the horns of Taurus Poniatowski, and about 1.3o southeast of the star 71 Ophiuchi (see the first map at the top of the page).
This 8th-magnitude nebula appears quite small, so it looks star-like at low magnification (<50x). The nebula reveals a small disk only at 70x or higher. Try as much magnification as you can to bring out detail in this tiny object. At 100x, you may begin to see a diffuse elliptical halo. Try an OIII or UHC filter if you have one to bring out more contrast and detail.
Most planetary nebulae last for just 50,000 years or so before they expire, which is not long in astronomical terms. But NGC 6572 is young even by these standards: it’s just 2,600 years old. So it will likely expand, dim slightly, and change its shape slowly over the coming centuries.