The towering Eagle: winging its way along the plane of our galaxy, Aquila, the Eagle, is home to many lesser-known deep-sky treats.
--Marcus Manilius, Astronomica
THE EAGLE is the servant of Jupiter in Roman mythology, and some writers portrayed him carrying the thunderbolts that the king of the gods hurled at hapless victims of his wrath. On late-summer evenings, we see an eagle eternally soaring among the stars as Aquila, no doubt on an errand for his master.
The realm of the Eagle boasts no renowned deep-sky wonders that are easily brought to mind by most stargazers. Yet the Milky Way plunges through Aquila, and thus it would be most surprising if nothing noteworthy lay within. Indeed, Aquila harbors quite a variety of interesting sights, so let's sample a few of these lesser-known denizens of the deep.
An object's obscurity doesn't limit it to the grasp of large telescopes. In fact, our first target is visible in 50-millimeter binoculars under a dark sky. Popularly known as Barnard's E, this inky letter written on the sky is composed of the dark nebulae Barnard 142 and Barnard 143. They lie in the same binocular or finder field with Gamma ([gamma]) Aquilae, or Tarazed, which lies 1.4[degrees] to the east. The nebulae are intricately patchy in my 4.1-inch (10.5-centimeter) refractor at 17x. The most obvious features include a fat, irregular C shape (B143) and a broad oblong patch (B142) just south of it that combine to form Barnard's E. Shadowy lacework reaches eastward from the C to a pair of 7th-magnitude stars near Tarazed. The oblong contains two 9th-magnitude stars and has a discontinuous extension that dips southward before turning toward Tarazed. Barnard's E is about 1[degrees] tall and half as broad.
These dark bars also captured the attention of German astronomer Max Wolf, who discovered them in a photograph he took in 1891. Wolf called this formation the Triple Cave and wrote, "The broadest arm of the dark structure appears as if it were the nearest, and the smallest arm as if it were the furthest from the observer, so that it would seem to give a perspective view into space of the heavens in the Milky Way. But this is probably mere illusion." See if you can imagine Barnard's E tipped away from you in the depths of space.
The open cluster NGC 6709 is another binocular object in Aquila. It lies 4.9[degrees] southwest of Zeta ([zeta]) Aquilae and makes a slender right triangle with Zeta and Epsilon ([epsilon]) Aquilae. In 50-mm binoculars, the cluster is just a bright misty glow, but 14 x 70s show me 10 faint stars over haze. It appears about 10' across and is set amid a rich Milky Way star field.
NGC 6709 is very pretty through my little refractor at 87x. Within 15' there are about 60 stars arranged in chains and bunches, the brightest gathered into a roughly triangular group. Starless voids are prominent, especially near the center, including a C-shaped void about one-third the cluster's diameter. The eastern point of the triangle is marked by three of the brightest stars, the westernmost pair forming the double Burnham 1464 (or [beta]1464). The primary is orange with a white companion 22" north-northeast. The third star is also a double, h870. (The lowercase h denotes doubles cataloged by John Herschel, rather than those by his father, William.) Look for its faint companion 12" southwest. Another orange star lying west-southwest of center decorates the cluster.
Moving southeast, we come to a quartet of objects lying along the arc of a circle whose center is near Delta ([delta]]) Aquilae. The first is NGC 6781, which makes a short isosceles triangle with Delta and Mu ([mu]) Aquilae. My 4.1-inch scope at 47x shows an easily spotted, round, fairly large planetary nebula. Its brightness is very slightly uneven at 87x. In my 10-inch (25.4-cm) reflector at 166x, NGC 6781 spans 11/2' and has unevenly brighter segments along the rim, but the annulus is interrupted by a much dimmer area in the north. Adding an oxygen III filter accentuates the planetary's variations in brightness.
The next objects in the arc are NGC 6755 and NGC 6756, a pair of open clusters that share a low-power field of view. The brighter one, NGC 6755, lies 2.9[degrees] due east of the nicely matched, white double Theta ([03b8]) Serpentis. Through my 4.1-inch refractor at 68x, the cluster displays 18 faint to very faint stars sprinkled over a 15' mottled background. Just 1/2[degrees] to the north-north-east, NGC 6756 embraces a few very faint stars in a 3 1/2' haze. In a 10-inch scope at moderate power, NGC 6755 is a rich cluster of several dozen faint stars, its dim northern reaches separated from the main mass by a comparatively starless lane. Clumps and chains of stars embellish the group. NGC 6756 exhibits a score of stars and an unresolved knot just northeast of its center.
NGC 6755 is about 50 million years old and 4,600 light-years away. NGC 6756's similar age and distance--60 million years and 4,900 light-years--may imply a physical relationship.
The last object in our arc is the small but fairly bright globular cluster NGC 6760. With 21 and 23 Aquilae it forms an isosceles triangle that fits within the field of a finderscope. In my little refractor at 87x, a faint 4' halo surrounds a large, bright core. Several elusive points of light occasionally blink into view. My 10-inch at 170x reveals a sparse scattering of stars across the halo and outer edge of the core. An 11th-magnitude star sits just beyond the northeast edge.
Our final target is Palomar 11, located halfway between Kappa ([kappa]) and 56 Aquilae. Look for it 4' south-southeast of an 8.6-magnitude star. This globular cluster is much more challenging than NGC 6760, but that's not to say that it's impossible to see in a small telescope. Finnish amateur Jaakko Saloranta has accomplished this feat with his 3.1-inch (80-mm) refractor at a high elevation in the Canary Islands. This is not an accomplishment I expect to duplicate, but I did tackle Palomar 11 with a somewhat more substantial telescope--my 14 1/2-inch (36.8-cm) reflector. At 245x this low-surface-brightness globular appears about 3' x 2 1/4' aligned northeast to southwest. It looks mottled, and there are a half dozen superposed stars, at least some of which may be foreground objects. (The brightest stars of Palomar 11 itself are red giants with apparent magnitudes of around 15 1/2.) A skinny triangle of 11th- and 12th-magnitude stars sits off the northeast edge. A 13th-magnitude star hugs the eastern side of the cluster, and a slightly more distant one lies a bit west of south.
Palomar 11 is one of several globulars discovered in the 1950s by American astronomer Albert G. Wilson while he was examining photographs from the National Geographic Society-Palomar Observatory Sky Survey. Palomar 11 and NGC 6760 both lie near the plane of our galaxy, where they suffer from obscuration by dust clouds. They are 42,000 and 24,000 light-years away, respectively. Compare NGC 6760 to the relatively unobscured cluster M5 in Serpens, which lies at the same distance. NGC 6760 is intrinsically less than one magnitude fainter than M5, but because of obscuration it appears over three magnitudes fainter in our sky.
Sue French can be contacted by e-mail at email@example.com.
Where the Eagle Flies Object Type Magnitude Size/Sep. Barnard 142 Dark nebula - 40' x 15' Barnard 143 Dark nebula - 30' NGC 6709 Open cluster 6.7 15' Burnham 1464 Double star 9.2, 9.7 22' h870 Double star 9.8, 11.3 12' NGC 6781 Planetary nebula 11.4 1.8' NGC 6755 Open cluster 7.5 15' NGC 6756 Open cluster 10.6 4.0' Theta (q)Ser Double star 4.6, 4.9 22' NGC 6760 Globular cluster 9 9.6' Palomar 11 Globular cluster 9.8 10' Object RA Dec. Barnard 142 [19.sup.h][39.7.sup.m] +10 [degrees] 31' Barnard 143 [19.sup.h][41.4.sup.m] +11 [degrees] 00' NGC 6709 [18.sup.h][51.5.sup.m] +10 [degrees] 20' Burnham 1464 [18.sup.h][51.5.sup.m] +10 [degrees] 19' h870 [18.sup.h][51.6.sup.m] +10 [degrees] 19' NGC 6781 [19.sup.h][18.5.sup.m] +6 [degrees] 32' NGC 6755 [19.sup.h][07.8.sup.m] +4 [degrees] 14' NGC 6756 [19.sup.h][08.7.sup.m] +4 [degrees] 42' Theta (q)Ser [18.sup.h][56.2.sup.m] +4 [degrees] 12' NGC 6760 [19.sup.h][11.2.sup.m] +1 [degrees] 02' Paloar 11 [19.sup.h][45.2.sup.m] -08 [degrees] 00' Object MSA U2 Barnard 142 1243 85L Barnard 143 1243 85L NGC 6709 1246 85R Burnham 1464 1246 85R h870 1246 85R NGC 6781 1269 85L NGC 6755 1269 105R NGC 6756 1269 105R Theta (q)Ser 1270 105R NGC 6760 1293 105R Palomar 11 1315 125L Angular sizes or separations are from recent catalogs. The visual impression of an object's sized is often smaller than the cataloged value and varies according to the aperture and magnification of the viewing instrument. Right ascension and declination are for equinox 2000.0. The columns headed MSA and U2 give the chart numbers of objects in the millennium Star Atlas and Uranometria 2000.0, 2nd edition, respectively.
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|Title Annotation:||deep-sky wonders|
|Publication:||Sky & Telescope|
|Date:||Sep 1, 2005|
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