Summary by Bence Beky
It would be unwise to assume that all the dark places shown on photographs of the sky are due to intervening opaque masses between us and the stars. In a considerable number of cases no other explanation seems possible, but some of them are doubtless only vacancies. I do not think it necessary to urge the fact that there are obscuring masses of matter in space. This has been quite definitely proved in my former papers on this subject. If any doubt remains of this it will perhaps be readily dispelled by a close examination of the photographs previously printed.
During the class on 2011-01-27, we discussed Edward Emerson Barnard’s 1919 article On the dark markings of the sky, with a catalogue of 182 such objects. This paper is by far the earliest among all that we shall study during this course, and consequently features a distinct style. As many of us remarked, this is a descriptive catalogue with statements mostly based on morphological visual observations, with little reasoning behind the inferred nature of the obscurities that would be considered to be scientific up to today’s standards. Barnard also cites a spectroscopic observation, suggesting that one great nebula about Rho Ophiuchi is not gaseous, without giving any further detail on its composition. The question about the exact meaning of the word nebula at the time the paper was written emerged during the discussion, and we concluded that it must have been a general term describing any extended object on the sky. This is, in fact, in agreement with what Wikipedia has to tell us.
As Greg pointed out, the (maybe only) strong point of the article is describing how there obscure areas are brighter than the surrounding sky background, concluding that they must be clouds covering what’s behind them. However, this background was incorrectly contributed to an unresolved, dense collection of faint stars, therefore this argument is bogus, as there is no reason to assume that these stars exist uniformly at every part of the sky, including behind the presumed clouds: the dark areas might as well be lack of stars, as mentioned in the very first paragraph of the paper. Now we know that in fact the background is due to light scattered by ISM.
The objects investigated in the paper are nowadays called dark nebulae. One of them, the Pipe Nebula, was identified as five separate objects in Barnard’s catalogue. It’s distance is 600 to 700 lightyears, comparable to the scale height of the galactic thick disk at the position of the Sun. This suggests that such nebulae should be observable in other directions in the sky than the galactic plane. However, a brighter background, such as the Milky Way, makes obscuring clouds more noticable, hinting that Barnard’s objects should be concentrated around that region. This prediction is confirmed by the map showing the distribution of Barnard’s objects.