Looking for Dark Nebulae

Posted by Brian Ventrudo

Follow the arc of the Milky Way on a dark night away from city lights, and you’ll see knots and ribbons of darkness weaving among the bright star clouds.  Many new stargazers think such dark regions are simply absences of stars.  But their true nature is far more interesting.  These dark regions are immense clouds of gas and cold interstellar dust, much of it made from the dregs of dead stars that exploded long ago.  In time, some of these so-called dark nebulae will contract, heat up, and recycle themselves by collapsing into clusters of hot, new stars.

Dark nebula Barnard 68 (from European Southern Observatory)

Dark nebulae appear dark because of tiny ice-covered dust grains that scatter background star light.  The grains are cold, just 10K (or -263 C), and less than 1/1000 of a millimeter across.  Within the cloud there may be just 100 dust grains per cubic centimeter, still nearly a vacuum by earthly standards.  But these clouds are tens of light years thick, so light from background stars is slowly but surely blocked and scattered in all directions.

Dark nebulae are a little tricky to see because you’re looking for, well… nothing… an absence of stars.  But in time, you learn to see these clouds as they protrude irregularly into the starry background.  The regions from Scorpius through Crux, and the band of Milky Way that cuts through the constellation Cygnus hold dozens of dark nebulae.  To get the best view, slowly sweep these parts of the sky with binoculars and linger on each patch for a time until your eyes and brain learn to look at voids in the starry background..  Dark sky is essential.

A warning… as you see dark nebulae, you may at first find the lack of stars quite unsettling.  In his 1882 novel Two on a Tower, Thomas Hardy called dark nebulae “deep wells for the human mind to let itself down into”.