Using only binoculars, we explore a host of inky dust clouds, the dark nebulae that smudge the Milky Way on late summer nights.
Lots of seasoned amateurs recommend binoculars over a telescope as an easy and inexpensive way for a beginning skywatcher to learn more about the denizens of the night sky. Once you're familiar with the brighter constellations, binoculars can take you to the next level and reveal the brighter deep-sky objects, numerous asteroids, all the planets, a potpourri of double stars ... and dark nebulae!
I've looked at lots of dark nebulae in telescopes, but many are easily visible in binoculars, too. So I thought I'd take my own advice and make for the countryside with only a pair of binoculars and a star atlas instead of a 75 pounds of telescope.
Dark nebulae might be called anti-deep-sky objects as most don't look any different from a proverbial "hole in the sky," more absence than presence. So it seemed to generations of skywatchers until about a century ago, when American astronomer E. E. Barnard and others came to realize the holes were really massive clouds of dark dust blocking the light of more distant suns. The same way passing clouds appear in black silhouette against the glow of the Milky Way, cosmic dust clouds, fashioned by galactic tides into every imaginable shape, shadow the stars that lie beyond.
Lots of dark nebulae are big enough and opaque enough to see in ordinary binoculars. The wide-angle variety are best, since they provide an ample field of view against which to see the dark forms. I use 10×50 Minoltas that combine enough magnification, a large 6.5° field of view, and comfortable eye relief. The vast majority of dark nebulae snake through the plane of the galaxy, so the best place to find them is in the band of the Milky Way from Sagittarius–Scorpius north through Cepheus. Here the dense, starry backdrop makes an ideal backdrop for these inky clouds to strut their stuff.
Dark nebulae were cataloged by Barnard in his Catalog of Dark Markings in the Sky and later by American astronomer Beverly Lynds in the Lynds' Catalog of Dark Nebulae based on the National Geographic - Palomar Observatory Sky Survey. Barnard objects are labeled with a capital "B" and the Lynds' nebulae by LDN or simply "L." The nebulae are further described by their size and opacity, a measure of how dense (dark) they appear against the stellar background. The scale runs from 1 to 6 in order of increasing opacity.
Generally, the higher the opacity, the greater the contrast and the easier a nebula is to see. In the descriptions below, I've abbreviated opacity to "op" and listed the size of each nebula in arcminutes.
I had to restrict this observing guide to nebulae north of the Sagittarius–Ophiuchus region because these regions are now low in the sky from my latitude (47°N) and have been compromised in recent weeks by a nightly veil of smoke from forest fires in California and British Columbia. We'll go there another time. For this round, let's leave the horizon haze behind and head north, starting with Aquila.
No question that B143 (60′ × 60′, op. 6) and B142 (40′ × 40′, op. 5) are two of the finest dark nebulae in any instrument. I could even glimpse B143 as a small, dark hole with the naked eye under dark skies. The larger and more obvious of the two, its dark prongs pair up with neighboring B142 to scrawl out the letter the "E" in homage to Edward Emerson Barnard.
If you now slide your glass 6° southwest of this pair you'll find a second, more widely-spaced pair of dark patches, B330 (30′ × 30′, op. 4) and B140 (60′ × 60′, op. 3). While not nearly as inky as the previous duo, they're still obvious, particularly B140, as it's framed on all sides by several 7th-magnitude stars.
Midway between Delta (δ) and Lambda (λ) Aquilae we arrive at one my favorites, B138 (180′ × 10′, op. 2) and its extensions, B137 (12′, op. 3) and B139 (10′ × 2′, op. 5). This 3°-long, mostly vacant arc has a curious, smoky-gray mottled texture. I like the shape which resembled a feathery quill dipped in an inkwell.
Six degrees to the west of the quill, look for a thick, horizontal gap in the stars about 2° long and elongated east–west. This is LDN 617 (~180′ × 15′, op. 5), which resides along the eastern edge of the prominent dark lane that splits the Milky Way into two in this region of the sky. It's vague, but after a few tries with averted vision, you'll have it. The tail of Aquila, where it borders with Scutum, is pocked with many dark nebulae that are difficult to distinguish from one another, but together they create a star-poor area about 4.5° across.
Cygnus is so rich with chunks of the Milky Way that where the stars thin out, you'll swear you're seeing one dark nebula after another. It's easy to fall prey to these galactic hijinks, so be sure to use the map above, an atlas like Uranometria 2000.0 or an online planetarium-style program like Stellarium.
Let's start big with B144 (6° × 3°, op. 1), which in spite of its weak opacity is a delightful, field-filling sight in the 10×50s. I see lots of mottling with areas of higher and lower opacity centered on the 4th-magnitude star Eta (η) Cygni. B144 sits squarely in the spectacular Cygnus Star Cloud, making it one of the most seductive regions to sweep in a low-power glass. From here, head north to Gamma (γ) Cygni (Sadr) for an impressive view of the narrow, north-south-elongated nebula L889 (1.5° × ~15′, op. 5). I think you'll like how it snakes up along the east side of the star.
L935 (90′ × 20′, op. 4) is a striking zigzag of black lightning just east of Deneb separating the North American Nebula from the Pelican Nebula, both of which I can faintly see in my binoculars. If you next look 8° north of Deneb, you'll stumble across the Funnel Nebula, also called Le Gentil 3, a huge gash at least 10° long that bisects the Milky Way. Put down your binoculars and soak in the naked-eye view. Yes, dark nebulae can be this amazing.
We finish up Cygnus with B168 (100′ × 10′, op. 4) a narrow, dark streak almost 2° long that cuts like a dagger through the rich stellar backdrop. Easy to see, the nebula lies just 1.5° west of the attractive binocular cluster, M39.
After the studded star fields of Cygnus, the Milky Way thins in Cepheus. I found several dark nebulae within its confines, but they were generally of low opacity and required averted vision to confirm. The easiest was a trio of three overlapping nebulae that includes B171 (op.5), B169, and B170. Combined, they make a 1°-wide dark patch ~1° west of Zeta (ζ) Cephei. Smaller B173 (4′, op. 6) and B174 (19′, op. 6) extend just north of the complex.
Likewise, I can make out the lower opacity neighboring blots, B365 (22′ × 3′, op. 4) and B160 (30′ × 15′, op. 4) 2.5° south of the pretty red supergiant star Mu (μ) Cephei. If you look 6° southwest of Alpha (α) Cephei using averted vision, you'll also discern B354 (60′ × ~25′, op. 2), a vague, relatively starless vacuity that extends north-south.
As you explore the dusty recesses of the galaxy, consider that every dark nebula had a bright past as dust blown from the atmospheres of red giant stars and set loose in supernovae explosions. In time, gravity will gather and re-forge that material into new star clusters. Dark clouds will give way to sunshine again.