Sky & Telescope - September 2019 - 31

mber Sights
Join the author on a saunter through September skies and admire
some of his favorite planetary nebulae.

CAT'S E Y E NEBUL A: M A K IS PA L AIOLOGOU / STEFA N BINNE WIES / JOSEF
PÖPSEL; LE A F TE X T URE: ZOZODESIG N / ISTOCK / G E T T Y IM AG ES PLUS

ges ago as a novice observer, I accidentally stumbled
upon a bright nebulous object while scanning
through the Summer Triangle with my ﬁrst telescope.
The serendipitous "discovery" was so thrilling that I credit
it for my current obsession with visual astronomy. That ﬁrst
observation of the Dumbbell Nebula (M27) also sparked my
interest in planetary nebulae.
Planetary nebulae are the remnants of highly evolved
low- to intermediate-mass stars. An ionized cloud of glowing
gas - the ejected outer layers of the progenitor star - surrounds a hot compact object, the exposed core of that parent
star. Planetary nebulae exhibit a wide range of structures
that include disks, rings, bipolar and multipolar shapes, and
even bright starlike points. They're some of the most colorful
and intricate objects in the sky and remain my favorite telescopic targets. The September sky contains enough fascinating examples that this tour of planetary nebulae just might
make you a fan as well.
Fascinating certainly applies to the Cat's Eye Nebula, or
NGC 6543, in Draco, and it's the perfect place to start our
tour. A remarkable Hubble image of this object reveals a complex structure of overlapping bubbles and shells that indicate
a history of repeated mass ejections.
NGC 6543 is also of historical signiﬁcance. In 1864, William Huggins, then a little-known amateur astronomer from
London, established the gaseous nature of planetary nebulae
based on his spectroscopic examination of the Cat's Eye (see
S&T: Aug. 2018, p. 58). It continues to be a source of discovery and has been studied extensively at multiple wavelengths.
The origin of the Cat's Eye moniker is uncertain, but its
bright yellow central star embedded in a greenish nebula
does indeed evoke that image. The nebula itself is irregularly
bright, which is evidence of its multiple shells. A detached
knot to the west gets its own designation, IC 4677. This wisp
of nebulosity is quite challenging to detect, but it gives a
sense of the true extent of the outer halo of the Cat's Eye,
which is many times larger than the rather small central
region that's easily visible.
We could ﬁll our tour with interesting planetary nebulae
without ever leaving the constellation of Cygnus. The Swan

is home to some of the season's best planetaries. NGC 7048
is an exceptional example and a great object with which
to demonstrate the beneﬁt of narrowband nebula ﬁlters. It
appears as a disk without a ﬁlter, but the application of the
O III ﬁlter will show it as a nice annular ring.
I would be hard pressed to choose a favorite Cygnus planetary, but NGC 7008 is deﬁnitely a contender. The view of
this intricate dual-lobed object is enhanced by a pair of stars
to the south-southeast. Two knotlike orbs make a kidney-

Filters for Planetary Nebulae
Planetary nebulae produce their peak emission in a
narrow range of wavelengths that are visually enhanced
by the application of ﬁlters that preferentially pass
those wavelengths. Most effective are the ﬁlters that
are optimized to pass light at the wavelengths emitted
by doubly ionized oxygen (O III) at 500.7 and 495.9
nanometers. Wider bandpass ﬁlters that span the range
between 484 and 506 nm (Ultra-High Contrast, or UHC,
ﬁlters) are also very effective. The beneﬁt gained from
using a ﬁlter is somewhat aperture-dependent: They
block a good deal of light and might darken the view too
much in smaller apertures.

bean shape with the northernmost lobe being considerably
brighter. In large aperture at high magniﬁcations, this object
is nothing short of magniﬁcent! Filters aren't necessary to
enjoy this planetary, but they'll change the apparent brightness distribution and reveal some additional structure.
I really enjoy sharing NGC 6826 at outreach events. Several planetary nebulae elicit a visual effect that's quite fun:
They seem to blink when the observer alternately employs
direct and averted vision to see the object. NGC 6826 does
this so well that it has earned its nickname as the Blinking
sk yandtele scope.com * S E P TE M B E R 2 019

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Sky & Telescope - September 2019

Table of Contents for the Digital Edition of Sky & Telescope - September 2019