Dark Markings in the Sky
                                                       Bok globules in IC 2948

             While  William Herschel  first discovered "holes  in the heavens" where there seemed to be  fewer  stars than might be expected,it was  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:

       Many of the regions he first noticed have now been studied extensively and it would interesting to compare his analysis (done by visually analyzing photographic plates) with
what is known today (with 20th century technology) about these regions .And perhaps as we learn more about the processes in the ISM(in this course !!) One could continue to add  new understanding to what is known about these clouds in terms of what we've learnt .

These are  some of his favorite dark clouds  :

               Barnard noticed the obscuration of a large part of the sky near Rho Ophiuchus by a great and beautiful nebula .He wondered about the nature of the matter in the nebulae and noted that it was probably not gaseous (Slipher et al: spectroscopic analysis).
           It is now known that Rho Oph region is one of the nearest (160pc) extended molecular cloud complex . The main cloud (L1688) is totally opaque in the optical plates but harbors some of the brightest spots of the solar neighborhood in the 12CO ,13CO, and 18CO lines. Young embedded B stars have been detected via their associated compact HII regions . A large no of Young Stellar Objects have been identified.It is thought to be  a region of active star formation.There is an unusually high IR  emission associated with this cloud.
            A lot of work is being done on the  diffuse emission in the mid IR range ( Abergel et al) And this emission is used to study the spatial structure of the clouds, the interaction of young stars with nearby dense gas and the evolution of dust within the ISM. Deep absorption features imply high column densities in compact structures.
             Also the magnetic field along the line of sight in the Ophiuchus dark cloud complex has been mapped (Goodman et al)and influence of magnetic fields have been proposed to explain the large scale orientation of dark cloud filaments (Vrba etal), condensations associated with young stars (Loren, Wooten etal)etc .
Some great pictures of these clouds : Barnard saw extinction of starlight in this cloud near the star  Nu Scorpii   . He also observed what he called "some strong and irregular condensations " (  Barnard E).
   He wondered on the intimate connection between  luminous nebulae and the dark absorbing matter and stellar condition.
           Scorpius is also considered to be an active star formation region .and a lot of research is being done to map the regions of extinction to better understand the filamentary structure of the clouds. Here are some links to some recent works:
 Mapping of the extinction in giant molecular clouds using optical star counts
 Protostellar Candidates
 Dense Cores of Dark Clouds ...by Vilas-Bodas,Myers etal  The clouds in Taurus seemed to suggest to Barnard that "a nebula may  lose some or all of its light" and speculates in his paper  Some of the dark Markings of the Sky and what they suggest(ApJ 1916)  that " perhaps  some of the nebulae over time become wholly dark or wholly bright"
        Some recent studies of the Taurus Cloud .
 Extinction Study of the Taurus Dark Cloud Complex
 Optical Polarization maps of Star Forming Regions in Perseus ,Taurus and Ophiuchus  by Goodman etal

              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

Barnard  noticed that some of the  well defined spots are uniformly gray  while others are  are entirely black or have blacker spaces in them .
A recent paper entitled
 Observational Evidence of Supershell Blowout in The Scutum Supershell
Did Barnard see evidence for any of all this?

Some Interesting Web sites

                                 Dark and Dusty Nebulae

                        Anglo-Australian Observatory Astronomical Images