Sky and Telescope - August 2018 - 62

Spectroscopic Stunner

p EMISSION IN ACTION In this sketch of NGC 6543's emission line
spectrum as seen through the author's 28-inch f/4 reflecting telescope, the
darkest vertical line corresponds to the O III lines at 495.9 nm and 500.7
nm. The fainter line to the left is Hβ 486.1; it marks the visual end of the
green-blue section of the spectrum. The broad gray zone at the right is
likely centered on the red Hα 656.3 line. Even the brightest emission line
was not bright enough to show color, although if it were visible it would be
the same turquoise as seen in the author's full-color sketch of NGC 6543
on the facing page.

Then I was ready for NGC 6543. I felt a little of the suspense and awe that Huggins described, and even hesitated a
moment before putting my eye to the eyepiece just to savor
the moment. My notes from the evening of July 30, 2017:
"Not just one line but two! It took a minute to ﬁgure out
which was the primary spectrum, and which was the nebula
because the image of the primary is so
much brighter! Seeing a second line is
unexpected, but the appearance of the
spectrum is so different than a stellar
spectrum that I can't help but think of
Huggins's exclamation that he knew at
ﬁrst glance that this spectrum proves that
at least some nebulae are gaseous. This
is especially remarkable because there's a
continuous stellar spectrum right next to
6543's lines.
"This is a memorable observation not
only for duplicating Huggins's observation, but that I can (do it) so easily and
with such an inexpensive spectrograph
. . . 15-mm Plössl / 5 spacers, 21.34 SQM.

A last look showed a bit more spectrum, a faint patch on the
opposite (red) side of the bright line from 6543!"
Recreating Huggins's observation was thrilling not only
for commemorating his landmark discovery but also because
I saw the vast difference between NGC 6543's emission-line
spectrum and a continuous stellar spectrum. It was obvious
why he understood its signiﬁcance so quickly.
Of course, NGC 6543's bright turquoise color and internal
structure are a joy to observe with a regular eyepiece, particularly when the seeing is steady. I've had several excellent views
of its inner helix-like shape and outer halos at various times,
and of all the planetary nebulae I've observed its color stands
up best to high magniﬁcation. In retrospect, it makes perfect
sense that it shows a line spectrum.
I re-observed the Cat's Eye Nebula and its spectrum at
the 2017 Oregon Star Party. One evening I devoted an hour
explaining the signiﬁcance of spectroscopy to astronomy
and science in general to my family, and how Huggins used
it to solve the riddle of the nebulae. Just
as I had for myself a few weeks before,
I showed them Vega with its bright and
colorful spectrum and easily seen dark
absorption bands. Its bright colors are also
a nice change of pace compared to the
shades of grey most objects show without
the spectroscope.
Everyone in the group quickly understood that the absorption bands are signa-

t MODERN EQUIPMENT The diffraction grating
(lower left) screws onto a 1.25-inch eyepiece like a
filter. The lens (top right) fits over the eyepiece and
widens the resulting spectrum, making absorption
and emission lines easier to see.

In a Nutshell

". . . they believed that if and
when the great breakthrough
were to come, it would be thanks
to expert use of a fine telescope.

AUGUST 2 018 * SK Y & TELESCOPE

It was an unquestioned conviction
that both guided and constrained
all their thinking about the
celestial challenges they faced.
Having lived so long inside
this deceptively comfortable
box, those actively working on
the nebular riddle expected
the answer would arrive out of
imaginable improvements to
methods and instruments with

which they were familiar. Thus,
until August of that year, they
continued to row through the fog
in a boat firmly tied to the pier."
Miller and Huggins were the
first to figure out how to cast off the
lines and trim the spectroscopic
sails. It took only a modestly sized
telescope and the use of a visual
spectrograph to decipher the
cryptic Cat's Eye Nebula.

HOWA RD BA NICH (3)

Barbara J. Becker, author of Unravelling Starlight: William and
Margaret Huggins and the Rise of
the New Astronomy sized up the
thinking of many visual astronomers about solving the riddle of
the nebulae in August 1864:

Sky and Telescope - August 2018

Table of Contents for the Digital Edition of Sky and Telescope - August 2018