The constellation Ophiuchus (pronounced off-ee-OO-cuss) the Serpent-bearer isn’t all that easy to pick out, primarily because of its large size and the relative dimness of its brightest star, Rasalhague (Alpha [α] Ophiuchi). This giant white star emits about 25 times the light of the Sun, but sits some 50 light-years away, so it glows at magnitude 2.1 — just slightly fainter than Polaris (Alpha Ursae Minoris). That makes it the 56th-brightest nighttime star. Only four other stars in Ophiuchus make the list of the top 200 brightest stars in the sky: magnitude 2.4 Sabik (Eta [η] Ophiuchi) is the 83rd brightest; magnitude 2.6 Zeta (ζ) Ophiuchi is 95th; magnitude 2.7 Yed Prior (Delta [δ] Ophiuchi) is 115th; and magnitude 2.8 Cebalrai (Beta [β] Ophiuchi) is 122nd.
The Serpent-bearer ranks 11th in size out of 88 constellations, covering 948.34 square degrees (2.3 percent) of the sky. Ophiuchus is visible in the evening from late spring through early autumn in the Northern Hemisphere, and its center lies at right ascension 17h20m and declination –8º. The best date each year to see Ophiuchus is June 11, when it stands opposite the Sun in the sky, reaching its highest point at local midnight. With respect to visibility, anyone living north of latitude –76° and south of latitude 60° can view the entire figure at some point during the year. And because it lies near the celestial equator, portions of Ophiuchus are visible worldwide.
The Serpent-bearer contains seven Messier objects — all globular clusters — as well as many other non-Messier globulars. Apart from those, it features three nice planetary nebulae, several worthy open clusters, and even a spiral galaxy. The last is a surprise because this star figure lies along the Milky Way, which normally blocks the light from objects such as distant galaxies.
Get comfortable, take your time, and enjoy your trek through this remarkable region of sky. Good luck!
Globular cluster M12 lies some 8° east-northeast of Yed Posterior (Epsilon [ε] Ophiuchi). It glows at magnitude 6.1 and measures 14.5' across, roughly half the width of the Full Moon.
Because globular cluster M10 lies some 14,000 light-years away, intervening dust dims its stars by nearly a magnitude. Still, it glows at magnitude 6.6 and is an easy catch through binoculars.
Ray and Geoff Weavill/Adam Block/NOAO/AURA/NSF
The Box Nebula (NGC 6309) glows softly at magnitude 11.5. However, because it spans only 18", you’ll have to spot it through a 6-inch or larger telescope. Crank the power past 250x to see the Box’s shape; an Oxygen-III filter will help.
The Snake Nebula (Barnard 72) is a cloud of dust and cold gas. Your best views of it will come through a telescope and eyepiece combo that yields a field of view around ½°. To find it, look 1½° north-northeast of Theta (θ) Ophiuchi.
Jim and Janet Castano/Adam Block/NOAO/AURA/NSF
NGC 6384, the single spiral galaxy in Ophiuchus worth observing, is magnitude 10.4. It lies 3.7° northwest of Cebalrai (Beta [β] Ophiuchi) and measures 6.2' by 4.1'. You’ll see its roughly rectangular shape through an 8-inch scope at 150x.
The Captain Hook Cluster (NGC 6633), located toward the upper right of this image, is an open cluster that shines at magnitude 4.6 and measures 27' across. You’ll find it 8° west-northwest of Alya (Theta [θ] Serpentis). Keep the magnification low through large scopes so you don’t look “through” this cluster. Open cluster IC 4756 in Serpens is also visible at left here.
The Emerald Nebula (NGC 6572) lies 2.2° south-southeast of 71 Ophiuchi. Although this planetary is small, it has a high surface brightness and is quite colorful. This shot was taken using a 5.1-inch refractor, but an 8-inch or larger scope will reveal the Emerald’s oval shape.
The Black Swallowtail Butterfly Cluster (IC 4665) — seen here above the cyan-tinted star Cebalrai — is a relatively bright open cluster at magnitude 4.2. It won’t look bright, however, because it measures 70' across. That means it covers 5½ times the area of the Full Moon. Observe it through a low-power eyepiece.
