A dark nebula is one of a group of celestial phenomena, generally known simply as nebula, (or nebulae in the plural,) consisting of clouds of gas and dust grains, slowly swirling tightly together in interstellar space. The term nebula was once applied to any celestial object that had a hazy and cloud-like appearance. Norman Sperling, in his 2004 article for astrosmart.com, ‘Nebulous Categories’, notes that, following the realisation that many of these objects were, in fact, other distant galaxies, the term dark nebula was limited, by the late 1800s, to describing actual clouds of dust and gas that occur within our own Milky Way galaxy.
Sperling tells us that it is believed a nebula can be created following the death of a star, in a hugely destructive and spectacular explosion known as a supernova, or by the gravitationally directed collection of tiny pieces of space debris over time. Reflecting their composition of either dense, cold dust particles, or light dust and highly reactive gasses, nebulae are generally classified either as bright or dark.
Bright nebulae, as their name suggests, are observed by astronomers as light signals in the visible spectrum, either reflected from nearby stars or produced from their own glowing, gaseous interior. Dark nebulae, on the other hand, are visible only by indirect means, as they neither emit nor reflect any light at all, and are frequently so dense that they absorb even the background light passing from nearby stars. According to ‘The Facts on File Dictionary of Astronomy’, it is for this reason that dark nebulae are also known as absorption nebulae.
Dark nebulae are detected, then, as dark, starless regions in the night sky, observed only by inference, from the blocking of light and other forms of radiation from known stellar sources in specific parts of the sky. Since they neither reflect nor emit any light, dark nebulae are believed to be cold regions of space, and together account for about 2% of the visible sky.
When viewed at the right time and against a background of bright stars or nebulae, it is possible for some dark nebula to be detected by the naked eye. Kelly Whitt’s April 2010 article, at Suite101.com, ‘Dark Nebula Facts and Information’, gives an excellent example of this: the Coalsack Nebula, which is located in the vicinity of the Southern Cross and Crux constellations.
The mysterious, loosely defined, irregular dark clouds known as dark nebulae continue to puzzle scientists and laypeople alike, and have provided some memorable images. The Horsehead Nebula, in particular, has long been a favourite for astronomy photographers, but there are many other exceptionally beautiful images of dark nebula to be found, such as the Triffid Nebula, with its dark tendrils and the E Nebula, named for its shape, oddly evocative of the letter ‘E’ in the alphabet.