Barnard's initial catalog of dark nebulae, first published in 1919, described 182 entries and was superceded by a 1927 follow-up publication which was expanded to include 349 entries.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: The Barnard dark nebula B33 in Orion, commonly referred to as the Horsehead Nebula, is perhaps the most-recognized and photogenic of all the dark nebulae within the Barnard catalog. Lying approximately 30 arc-minutes to the south of Alnitak (ζ-Ori, mag 1.89), the immediate area around B33 is bathed within the emission nebula IC 434 and the intervening thick dust between the earth-based observer and IC 434 leads to this characteristic formation. The Horsehead Nebula is elongated running from east to west and measuring approximately 8'x6' and lies at a distance of 1,500 light-years away.