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:
These are some of his favorite dark clouds :
Sagittarus
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