Barnard's initial catalog of dark nebulae, first published in 1919, described 182 entries and was superceded by a 1927 follow-up publication which was expanded to include 349 entries.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: Dark nebula B159 in Cygnus is an irregular area measuring approximately 25' in diameter and which is characterized with multiple patches of varying degrees of nebulosity in a generally east-west direction. Of interest is the presence of the bright carbon star 75-Cygni (mag 5.11, B-V=+1.87), far left in the image below, which makes the location and identification of the nebula very easy. With an opacity index of 5, the dark nebula is quite rich in contrast and certainly stands out against the background stars in the field of view. Immediately due north of B159 is B158 (3' in diameter) and slightly further northeast lie B155 (13' in diameter) and B156 (8' in diameter).