Map 14: Equatorial Region 5
Serpens Caput to Aquila
August 12, 2007
If Map 13 contains the realm of the galaxies, then this one encompasses the realm of globular clusters. Our Milky Way contains roughly 200 globular clusters, and a third of them can be found in just 3 of the 88 constellations: Scorpius, Sagittarius, and Ophiuchus. (Ophiuchus occupies a large part of Map 15. Find Scorpius and Sagittarius on Map 20.) But this region contains more than just globulars. |
Because the Milky Way passes through Sagitta, Aquila, Scutum, Serpens, and part of Ophiuchus, open clusters and nebulae abound here. You won't find many galaxies, however. The reason is the composition of the Milky Way itself. In addition to stars, immense, diffuse gas clouds and dust permeate our galaxy's spiral arms (which we see at night as the Milky Way). This material blocks out light from more distant stars and galaxies. The drop-off in numbers of observable galaxies is so extreme that early 20th-century astronomers referred to this area as the Zone of Avoidance.
In Sagitta, find globular cluster M71 midway between Gamma and Delta Sagittae. This 8th-magnitude cluster looks irregular at low power because of foreground stars. Through a 6-inch scope, you can resolve a couple dozen stars, and the view doesn't improve much through a 12-inch — only 50 stars are visible within a 4' area.
Below Sagitta, Aquila provides an opportunity to view an easily recognized dark nebula. About 1° west of Gamma Aquilae lies a complex of dark nebulae known as Barnard's E. Dark nebulae are not voids, as was once believed, but rather clouds of dust and cold gas that block out light from more distant stars. American astronomer Edward Emerson Barnard cataloged hundreds of these objects strewn across the bright background of the Milky Way.
Use a low-power eyepiece to see Barnard 143, which forms a prominent C shape roughly 20' long oriented east-west. Just to the south lies Barnard 142, a ½°-long stretch of dark nebulosity. Together, these two form the E, which stands out well because the background is crowded with faint stars.
Not quite 4° north-northwest of Delta Aquilae lies planetary nebula NGC 6781. Through an 8-inch telescope, this magnitude 11.4 object spans more than 1.5' and stands out well against a star-filled background. For best results, use a nebula filter, and look for NGC 6781's irregularly illuminated outer edge.
Now look a bit more than 4° southwest of Delta Aquilae for globular cluster NGC 6760. Glowing at magnitude 9.1, NGC 6760 is a concentrated globular cluster easily spotted through a 3-inch scope. Even a 12-inch telescope doesn't resolve more than a handful of the globular's stars, but this size scope does show the cluster as mottled. Southwest of Aquila lies the small constellation Scutum the Shield.
Aquila contains 6 times more area than Scutum, but Aquila contains no Messier objects, and Scutum has two.
Nearly 2° southeast of Beta Scuti lies the Wild Duck Cluster (M11). This magnificent open cluster measures about 12' across and shines at magnitude 5.3, making it visible to your naked eyes from a dark site. M11 acquired its odd name when 19th-century observer William Henry Smyth wrote that it resembled a flock of wild ducks in flight. Smyth was referring to the cluster's triangular shape. Through a 6-inch telescope, you'll see more than 100 stars, many of them packed into the dense core. The brightest star shines at 8th magnitude and lies near M11's center.
M26 is Scutum's other Messier object. M26 is also an open cluster, but fainter (magnitude 8.0) and looser than M11. An 8-inch scope reveals 25 stars within a 10' area.
The lone galaxy of note on Map 14 — Barnard's Galaxy (NGC 6822) — is also a challenge to see. The galaxy's magnitude of 8.8 is misleading because the light is spread out over an area 19' by 15'. Not many bright stars reside near NGC 6822. This object lies more than 6° northeast of Rho1 Sagittarii. Use low power under a dark sky, and look for a roughly rectangular haze slightly brighter than the background.
Ophiuchus contains no less than 10 globular clusters brighter than magnitude 9. Seven belong to Messier's catalog: M9, M10, M12, M14, M19, and M62 (Map 20), and M107. Three do not — magnitude 8.2 NGC 6293 (Map 20), magnitude 8.2 NGC 6356, and magnitude 8.9 NGC 6366. Each of these globular clusters offers a unique observing experience.
For example, compare M10 and M12. Move from one to another and back again. View them at low power against a wide background. Then, view them at as high a magnification as conditions allow. By noting the similarities and differences, you'll become a better observer.
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