Dark clouds of dust and gas in the Milky Way Galaxy become visible due to their position against bright background fields of stars. An American astronomer named Edward Emerson Barnard cataloged 370 of these dark nebulae (or nebulas), including Barnard 33, or the famous Horsehead Nebula.
Dark Nebula Defined
In The Facts on File Dictionary of Astronomy, dark nebulae are also referred to as dark clouds and absorption nebulae. A dark nebula is "a cloud of interstellar gas and dust that is sufficiently dense to obscure partially or completely the light from stars and other objects lying behind it and sufficiently large and suitably located to produce a noticeable effect."
Dark nebulae are visible in the Milky Way Galaxy and also in distant galaxies where a dark dust lane obscures part of a spiral arm or the bulge, such as in the Black Eye Galaxy. These dark areas are often dense locations where dust is coalescing to form new stars. Dark nebulae are composed predominantly of molecular hydrogen.
Some small dark nebulae are named Bok Globules. Bok Globules are cool clouds of gas and dust that look like drops of water with their mostly spherical shapes. Named after American astronomer Bart J. Bok, Bok Globules are believed to be the source of lower mass stars.
Famous Dark Nebulae
The most famous of the dark nebulae is the Horsehead Nebula, also known as Barnard 33. The Horsehead Nebula is found in the constellation Orion. Orion is well known for the three stars that make up its belt, and the Horsehead Nebula lies beside the leftmost belt star, Alnitak. Amateurs are able to spot the Horsehead Nebula with large telescopes, excellent seeing conditions, and the help of a filter. So while the Horsehead Nebula may be the most famous of the dark nebulae, it is not the easiest to spot.
The Coalsack Nebula is generally considered the easiest dark nebula to see. It is visible to the unaided eye as a dark, cloudy patch in the Milky Way, located in the constellation Crux. Unfortunately for the majority of the population, the Coalsack Nebula is only found in Southern Hemisphere skies.
The Snake Nebula, Barnard 72, is an S-shaped dark nebula found in the constellation Ophiuchus. Other dark nebulae are found nearby the Snake Nebula, such as B68, which is also considered a Bok Globule. Viewing any of these dark nebulae requires skies free of light pollution.
Just observing with the eyes alone in the countryside can allow an observer to spot the cloudy lanes in the Milky Way Galaxy and reveal the darker side of the night sky.
Sources:
Illingworth, Valerie and Clark, John E. O., The Facts on File Dictionary of Astronomy, Checkmark Books, 2000.
Kelly Whitt - I have been a professional writer for more than a decade, covering many topics from science-based articles to do-it-yourself home ...
