While William Herschel first discovered "holes in the
heavens" where there seemed to be fewer stars than might be
Edward E Barnard whose photographs
first seemed to suggest that the "dark nebulae"
or vast star deficient regions are actually obscuring
bodies closer to us was one of the first astronomers to introduce
the concept of "matter among stars" - clouds of Interstellar
matter blocking our view of the stars behind .
Barnard became interested in the structures and forms of the Milky Way while searching for comets. Barnard's curiosity about these "vacant "regions among the stars resulted in a systematic study of photographs many of which he took himself at the Lick and Yerkes Observatory and catalogue of such dark markings and nebulous regions . It is interesting to go through his publications since 1907 on the dark patches and see how he slowly came around to accepting that many of the "dark objects" was possibly "dull, feebly luminous obscuring matter" and in some cases ( Taurus ) for example ," reveal to us a nebulous substratum in certain parts of the sky". He experiments with the idea of "dead nebulae" writing:
" What would the be the condition of a nebulae that no longer emitted light ...it is likely that we should simply have a dark nebulae which would not be visible in the blackness of space unless its presence were made known by its absorption of the light of the stars beyond it".
He also toyed with the idea that nebulae need not be "transparent " and says " I believe nebulae in general are transparent yet there are some cases where the appearance is quite otherwise. The beautiful veil of nebulosity extending from the star N Scorpii gives the impression of dulling the light of stars from that direction . There is nothing to show whether the light of a star has not been greatly reduced by the interposition of nebulous matter" . So he instinctively expected extinction but did not
know it to be the case.
What is truly amazing is the instinctive realization that although he cannot explain the connection of "nebulosities with vacancies as if the darkness was something tangible" that he had stumbled upon something very important. He writes
"I believe there will one day develop facts of the greatest importance in explaining the real structure of the heavens . It would therefore be valuable work to locate all these regions and secure long exposure photographs of them"
He also noted that these objects were not confined to the Milky Way but were also present outside the galaxy.
He published his findings and spent a lot of time analyzing these photographs and speculating on the nature of these structures. Some of his publications relevant to these dark markings and partly luminous objects are:
These are some of his favorite dark clouds :
Another dark cloud that is now known to have hot young stars associated with
dusty clouds. The bright stars illuminate the tiny solid dust particles, producing blue reflection nebulae bordering some of the emission regions. The dust is also evident in silhouette, both as sinuous dark lanes winding through the luminous gas and as the
dark patches obscuring the ancient, yellow stars that populate the central parts of the Milky Way.
The dust lane dividing the Milky Way in Sagittarius
The Star Clouds of Sagittarius
Dust and reflection Nebulae in Sagittarus
Some Interesting Web sites
Dark and Dusty Nebulae
Anglo-Australian Observatory Astronomical Images