The Northern Coalsack

We recently toured the Coalsack in the Southern Cross, the most famous of the so-called “dark nebulae”.  But northern observers have a “coalsack” of their own.  Nestled in the wings of Cygnus, the Swan, the “Northern Coalsack” is easily visible with binoculars or the unaided eye in dark skies.  Here’s how to find it…

The Northern Coalsack extends over a 6ºx5º span of sky between the stars Deneb, Sadir, and Gienah in northeastern Cygnus. There are many dark nebula in the area. But the Northern Coalsack marks the beginning of the Great Rift of the Milky Way, a huge dark cloud of gas and dust stretching from Cygnus into Sagittarius.  This cloud, which gives the northern Milky Way a mottled appearance, contains some 1 million solar masses of material.

Here’s a map to help you find it.

The Northern Coalsack in the constellation Cygnus

And here’s a spectacular image of the Great Rift in the northern Milky Way.  Make sure to move your mouse over the image to get find out which feature is which.

While not nearly as famous as the southern Coalsack, this northern incarnation has been known since antiquity.  The ancient Greeks, who loved a good story, tell of the legendary Phaeton who took the reins of the sun-god’s chariot and lost control during a joyride across the sky.  Zeus struck Phaeton down with a lightning bolt, and he fell into the river Eridanus.  But his brother, Cygnus, gathered Phaeton’s remains and buried them properly.  Zeus rewarded Cygnus’ devotion by placing him in the sky as a Swan.  Phaeton’s reckless ride across the sky is marked by the darkness of the Great Rift which ends (or begins) at the Northern Coalsack.

The Northern Coalsack is easy to spot if you have dark sky.  You’ll need no optics, though binoculars give you a closeup view of a few foreground stars and the attenuated light from background stars.  Don’t bother with a telescope… it will show you only a tiny fraction of the cloud.  Many believe the Northern Coalsack is not nearly as dark at the southern version in Crux.  But accurate measurement shows each is similar in darkness.  The southern Coalsack simply appears darker by contrast because it appears in a brighter part of the Milky Way.

So go out and see the Northern Coalsack.  While it may seem odd for a stargazer to look at absences of stars, you may acquire a fondness for observing dark nebula.  As astronomy writer Garrett Serviss said nearly a century ago, “Infinity seems to acquire a new meaning in the presence of these black openings in the sky”.