The coalsacks of Cygnus

Crisp September nights often bring high-contrast skies, which allows us to look up and see darkness well.

That may sound paradoxical, but we need bold swaths of starlight to see the “deep wells” of darkness that mar the Milky Way like celestial graffiti. Mariners of the 16th century called them “coalsacks”; today, we categorize them as dark nebulae. Let’s explore some prominent examples in Cygnus.

The Northern Coalsack is an elliptical splash of darkness between Deneb (Alpha [α] Cygni), Sadr (Gamma [γ] Cyg), and Epsilon (ε) Cyg. Despite its popularity, the Northern Coalsack is not particularly obvious to unaided eyes for two reasons. First, it is large (7° by 5°), and second, it is not fully surrounded by bright patches of Milky Way. I’ll return to this later.

To target a smaller object, slip over the North America Nebula (NGC 7000) and look for Barnard 352, which sits slightly less than 3° east-northeast of Deneb. On a globe, this 20′ by 10′ dark cloud would lie at about the position of the Northwest Passages above Hudson Bay. Try to spy it first through binoculars. If that’s too difficult, use a telescope at low power.

Arguably the best Cygnus coalsack is Le Gentil 3, sometimes called the Northern Inkspot. You’ll find it 8° north-northeast of Deneb. French astronomer Guillaume Le Gentil recorded this 5°-wide naked-eye cloud in 1749, noting that it “seems opaque and very dark.” And while Le Gentil 3 is as large as the Northern Coalsack, the Milky Way surrounding it is more uniform in intensity, boosting its contrast and enhancing the darkness.

When the 19th-century French astronomer Étienne Léopold Trouvelot drew the naked-eye Milky Way, he wrote, “It enters Cygnus, where it becomes very complicated and bright, and where several large cloudy masses are seen terminating its left branch, which passes to the right, near the bright star Deneb, the leader of this constellation.” In his drawing, this terminating branch is separated by a dark gap at the position of Le Gentil 3. Most telling is that he does not draw (or failed to notice) the dark Northern Coalsack.

Now use your scope and low power to look roughly midway between the North America Nebula and open cluster M39 for B361, which I call the Little Cygnus Inkspot. It lies about ¼˚ west of the 9th-magnitude open cluster IC 1369. Through a 4-inch scope under a dark sky, the 20′ well of darkness stands out prominently against the surrounding Milky Way. The view is enhanced by the star cluster’s presence, which is only half the dark nebula’s size.

We’ll end this survey with B168, one of the northern sky’s most visually stunning dark nebulae through binoculars and telescopes. It’s also a great naked-eye challenge. The Cocoon Nebula (IC 5146) lies in a pool of darkness at the southeast end of this nearly 2°-long stream of darkness. Look just south of Pi2 (π2) Cygni, where the Milky Way appears tangled in a cobweb of naked-eye dark nebulae. It is out of this web that a surgeon’s cut of darkness trickles like blood into the pool surrounding the Cocoon. It’s difficult to see this dark stream without optical aid because it is so narrow. But try sweeping your gaze up and down and left to right for a few minutes and see if you catch glimpses of it. If not, binoculars will show it clearly slicing across the Milky Way.

As always, share what you see and don’t see at sjomeara31@gmail.com.