Barnard's initial catalog of dark nebulae, first published in 1919, described 182 entries and was superceded by a 1927 follow-up publication which was expanded to include 349 entries. These "dark nebulae" are believed to be the result of molecular clouds of dust and gas which are present in our line of sight and which absorb the starlight originating behind them, thus providing the earth-based observer working in the visible spectrum the illusion of a black "void" (these dark nebulae are NOT associated with dark matter!). Some of the best-known examples of these "absorption nebulae" include B33 (Horsehead Nebula), B68 (Molecular Cloud 68), B72 ("S" or Snake Nebula) and B142-143 ("E" Nebula).

Note: Perhaps the most recognized dark nebula of the summer sky is B72 in Ophiuchus. Commonly referred to as the "Snake Nebula" and/or "S nebula" owing to its shape similar to a snake as well as the letter "S", B72 is a very dense dark nebula located northeast of the double star θ-Oph (mag 3.23) and in an area with many other dark nebulae from Barnard's catalog including B68, B69, B70 and B71 to the immediate west.

Note: The image below was characterized with a very dominant light gradient which adversely affected the final result. It will be revisted under darker skies at the first available opportunity.