In Thomas Hardy’s 1882 novel, “Two on a Tower,” Swithin St. Cleeve speaks to Lady Constantine regarding dark nebulae thus: “Look, for instance, at those pieces of darkness in the Milky Way . . . In these our sight plunges quite beyond any twinkler we have yet visited. Those are deep wells for the human mind to let itself down into, leave alone the human body . . .”
This is an apt description of dark nebulae as these dense clouds of gas and dust absorb all light from behind causing nebulae to look like holes in space. This extinction of light is caused by tiny ice-covered dust grains (little dust bunnies) in the clouds that scatter background starlight.
The term “nebula” comes from the Latin word for “cloud.” Telescopic dark nebulae had been observed for over two centuries, but not much was known about them until Edward Emerson Barnard, the leading observational astronomer of his time, cataloged 370 dark nebulae in 1919. He proved that dark nebulae were clouds of dust and gas that blocked the light of distant stars rather than just the absence of stars.
Nebulae consist primarily of molecular hydrogen and are made up, for the most part, of dead stars that exploded long ago. Some of the largest nebulae are more than a million times as massive as the sun. The largest dark nebulae are visible to the naked eye. However, visualization can be difficult because rather than looking for stars one must actually look for an area of the sky in which stars are absent.
Some nebulae obscure light from background stars. An example of this type of nebula is the Doodad Nebula. The Doodad Nebula has well-defined boundaries which make it fairly easy to see. Other nebulae block light from emission nebulae in areas where stars are forming. An example of this type of nebula is the Horsehead Nebula, the most famous dark nebula. It is found in the constellation Orion. Its swirling clouds resemble a horse’s head with a red glow. The red color is caused by hydrogen gas from behind the nebula which is ionized by the nearby star, Sigma Orionis.
The Coalsack Nebula is considered the easiest dark nebula to see. It overlaps the constellation Crux, and parts of it are in the constellations Musca and Centaurus. It is only seen in Southern Hemisphere skies and is visible as a dark, cloudy patch in the Milky Way.
One way to visualize dark nebulae is to slowly sweep across the band of the Milky Way that cuts through the constellation Cygnus with binoculars. It is necessary to dwell on each patch for a while until the eyes can adjust to looking for voids in the star-laden background. It takes practice because one is in actuality looking for “nothing.”
Dark nebulae are among the most beautiful objects in the universe, and wondrous events take place in these dense clouds. Stars are born and other stars take their last breath, all within their boundaries. They are truly the gatekeepers of the universe.