Sky and Telescope - January 2018 - 57

Going Deep by Alan Whitman

Dark Clouds in Taurus
Set yourself a challenge and spend a night or more navigating through
nebulae in an under-observed part of the sky.

PHILIPP SA L ZGEBER

he ﬁnest dark nebulae for telescopes
are those that are silhouetted against
a bright background, such as the striking, chevron-shaped dark lane within
the high surface brightness Carina
Nebula, or those silhouetted against the
Milky Way's brightest star clouds. One
example of the latter is Barnard 92, a
foreground object in front of distant
M24 (the Small Sagittarius Star Cloud).
Dark nebulae are also detectable
in much fainter portions of the Milky
Way, but while they may look obvious
on images, they're fairly challenging
at the eyepiece. Since the Milky Way
background is too faint visually to
silhouette the dust clouds, they can
only be detected by the paucity of faint
stars in comparison with the rest of the
ﬁeld or surrounding ﬁelds. But note that
there may not be a marked shortage of
brighter stars since many of them will
be closer to us than the dust cloud is.
The Millennium Star Atlas (MSA),
unfortunately out-of-print, has long
been my favorite guide for hunting these
absorption nebulae. I ﬁnd that the dark
nebulae boundaries on MSA charts are
usually quite accurate. If I star-hop to
the correct location for a Barnard dark
nebula, and note a marked decrease in
the density of faint stars precisely where
the atlas says that I should, then I consider that I have detected something that
obscures the more distant stars - that is,
a dust cloud absorbing the light of stars
located behind it. I see no difference
between detecting a dark nebula due
to a marked absence of faint stars, and
detecting a loose open cluster because of
a statistically signiﬁcant increase in star
density at the cluster's plotted position.
The Milky Way is certainly faint in
Taurus. Nevertheless, my 8-inch f/6

Dobsonian, working at 30× with a 1.25°
ﬁeld of view, revealed three large winding dust clouds. Spending two nights
observing at such low power was very
relaxing. On both nights, M33 was
visible with the unaided eye, indicating
that the transparency was excellent.
The easiest one was B19-B22, the
6°-long Taurus Dark Cloud. I could
follow all of it, although the northwestern ﬁnger was just detectable. There
were almost no faint stars visible in the
8-inch between RA 4h 28m and 4h 34m,

and there was a good southern boundary
in that section. East of there the broad
dark nebula was still easy to follow since,
while there was a smattering of faint
stars, there were far more outside of the
boundaries plotted in MSA. The eastern
end, beyond RA 4h 38m, was nearly empty
of faint stars, and this prominent area
continued farther than plotted in MSA,
all the way to RA 4h 48m. This was one
of the very few times that a dark nebula
has appeared to be signiﬁcantly larger
than plotted in the MSA, but Antonin

p The dark sooty shapes of the nebulae are clearly seen against the starry backdrop in this widefield photo. The field of view also encompasses well-known objects such as the Pleiades on the
right and Aldebaran at the bottom, as well as the open clusters NGC 1746 and NGC 1647, which
are both visible with the unaided eye at true dark sites.
sk yandtelescope.com * JA N UA RY 2 018

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Sky and Telescope - January 2018

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