The Caldwell Objects

it near the Swan? There is a still more
remarkable one south of the equator, called the
Coal Sack, as a sort of nickname that has a
farcical force from its very inadequacy. In these
our sight plunges quite beyond any twinkler
we have yet visited. Those are deep wells for
the human mind to let itself down into, leave
alone the human body! and think of the side
caverns and secondary abysses to right and left
as you pass on!
As early as 1889 Arthur Cowper Raynard, the
editor of the London scientific magazine
Knowledge, believed that these dark clouds were
not Herschelian holes but "opaque matter, dust
clouds or fog-filled space." Raynard spent years
trying to convince Edward Emerson Barnard,
who had been photographing these dark regions,
of his beliefs. But Barnard strongly promoted the
notion of Herschel's vacancies — and he
continued to do so until, as William Sheehan
explains in his Barnard biography, The Immortal
Fire Within, he changed his mind on "one
beautiful and transparent moonless night" in
1913:
I was struck with the presence of a group of
tiny cumulous clouds scattered over the rich
star-clouds of Sagittarius. . . . Against the bright
background they appeared as conspicuous and
black as drops of ink. They were in every way
like the black spots shown on photographs of
the Milky Way, some of which I was at that
moment photographing. The phenomenon was
impressive and full of suggestion. One could
not resist the impression that many of the small
spots in the Milky Way are due to a cause
similar to that of the small black clouds
mentioned above.
Barnard died seven years before he could
have learned the truth. Not until 1930, when
Robert J. Trumpler published his "Preliminary
98 & 99
Results on the Distances, Dimensions and Space
Distributions of Open Star Clusters," did
astronomers have conclusive proof that the
galaxy was full of diffuse, obscuring matter.
Today we recognize dark nebulae as cold, dusty
clouds of molecular hydrogen gas that cloak or
dim the light of whatever lurks behind them.
Trumpler also discovered that interstellar dust
reddens starlight; it scatters blue light more
efficiently than red, and the effect points to dust
particles ranging between 1/1000 and 1/100 of an
inch in diameter — making them something akin
to smoke particles encased in ice. The coldest of
these dark clouds are only a few degrees above
absolute zero.
Nearly all investigations to date find the
Coalsack to be a stable, inactive nebula without
any signs of hidden star formation. Ultraviolet
spectrometers aboard the Voyager 1 and Voyager
2 spacecraft stared toward the Coalsack on four
occasions. On each occasion they recorded
intense diffuse emission with a spectrum
characteristic of a B-type star. Present in all four
observations, this spectral signature may be the
result of starlight scattered by interstellar dust in
the foreground of the main mass of the Coalsack.
Dust-dimming (or extinction) data and distance
determinations for 280 stars in the Coalsack
region suggest that the Coalsack is not a single
sheet of dust, but a conglomerate of two
overlapping dark clouds at distances of 610 and
790 light-years, respectively (These two clouds
are among the closest to Earth.) The Coalsack's
closest parts span about 54 light-years. The
Coalsack may be part of a larger interstellar
structure that includes the Chamaeleon-Musca
dark clouds some 15° away. If it is, the larger
structure may extend like a sheet at right angles
to the galactic plane.
To the naked eye the Coalsack recalls H. P
Lovecraft's "unsealed wells of night," or Edgar
The Caldwell Objects 393