Sky and Telescope - April 2017 - 31

fully between the two, though the boundary between them is
somewhat, well, nebulous. ISM, the dust and gas between the
stars, is distributed throughout our galaxy, but can be found
relatively close by - say 200 light-years from us at its nearest
- in the galactic plane. Galactic cirrus, on the other hand,
technically lies at high galactic latitudes (above and below
the zodiac) and thousands of light-years distant.
Whatever you choose to call it, you may be surprised to
learn that you can see it with amateur observing equipment.
For instance, I can detect faint glows tangling with wellknown objects like the Pleiades and the Andromeda Galaxy
with my improved richest-ﬁeld (the widest possible view at
lowest possible magniﬁcation) telescopes. My sketches at the
eyepiece match deep digital images of the dark clouds, letting
me know it's not just my eye playing tricks on me.
My ﬁrst view of galactic cirrus was accidental and astonishing, and it made me wonder: Who else had spotted this
ghostly dust? Turning to my library, my astonishment only
grew as I uncovered a history of visual observations stretching back to William Herschel.

Orion's Nebula

Barnard's
Loop

M42

BA R N A RD'S LOOP: SCOT T ROSEN; PLEIA DES: VA LERIE ROSEN; A LL SK E TCHES: MEL BA RTELS

A Dusty Discovery
The typical discovery story you'll read for galactic cirrus goes
something like this: It was ﬁrst noticed on glass plates taken
with the 48-inch Schmidt telescope at Palomar Observatory,
one of the most productive telescopes of the modern age.
Cataloged as dark clouds or nebulae in 1965 by Beverly T.
Lynds, the dust's infrared emissions were discovered in the
early 1970s and studied in detail by IRAS in the 1980s. In the
1970s, David Malin also found cirrus on images taken with
the 1.2-meter UK Schmidt Telescope at Siding Spring Observatory, Australia, and Allan Sandage wrote about it and the
scattering of optical light in 1975-76.
The discovery story you won't read goes back another 150
years, however, to one of the greatest visual astronomers of
all time, William Herschel, who detected very faint nebulosity across large areas of the sky. What we call the sky
background, Herschel called the sky "bottom" or "ground."

M45

p TRUE NEBULA Even casual observers may be familiar with Orion's
Nebula (M42), a naked-eye object in suburban skies. Less well-known is
the nearby arc comprising Barnard's Loop, an emission nebula included
by Herschel in his list of 52 nebulous regions.

From 1783 to 1811, Herschel cataloged 52 nebulous regions
of the sky bottom, including the emission nebula behind
the Horsehead Nebula in Orion (Williamina Fleming, a
computer at Harvard College Observatory, discovered the
Horsehead Nebula itself in 1888; E. E. Barnard's 1919 catalog
of dark nebulae popularized it). Herschel's observations also
included the North American Nebula and Barnard's Loop, as
well as the nebulosity surrounding M81 and M82.
Herschel initially believed that even these faint, diffuse structures were resolvable; that is, the nebulosity
could be explained as aggregations of stars too faint to be

t CAPTURING THE
BUBBLE A five-hour total
exposure reveals not just
the Merope reflection
nebula, but also the dust
through which the Pleiades cluster happens to
be passing. The so-called
Pleiades Bubble defines
the margins of the galactic
cirrus.
t t FINDING THE EDGE
The author sketched the
Pleiades Bubble as seen
through a 6-inch f/2.8
richest-field telescope with
no filter.
s k y a n d t e l e s c o p e .c o m

* APRIL 2017

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Sky and Telescope - April 2017

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