Planetary Nebulae
32 SEPTEMBER 2019 • SKY & TELESCOPE
Planetary. A disklike halo surrounds a bright central star.
Concentrating intensely on the star with direct vison causes
the nebula to seemingly disappear. Averted vision makes the
nebulous disk easily visible, so shifting one’s gaze back and
forth makes the object appear to blink. After having fun
with that phenomenon, be sure to notice the subtle fl iers
(fast low-ionization emission regions) that manifest as two
opposing brighter knots to the east and west in the otherwise
uniformly bright nebula.
Two curious planetaries fl ank the expansive North
America Nebula (NGC 7000). NGC 7026’s disk is linearly
bifurcated roughly along the east-west axis. Two brighter lay-
ers sandwich a dark inner layer, giving rise to its nickname:
the Cheeseburger Nebula. The greenish NGC 7027 is known
as the Green Rectangle. It has a very bright knot toward its
central region that must surround the central star, which is
itself too faint to detect in amateur scopes.
Campbell’s Hydrogen Star is a lesser-known object in
southern Cygnus about 3 degrees northeast of Albireo. It’s
bright enough to be easily visible as a star in an 8-inch scope,
but rather diffi cult to detect as an extended nebulous object.
The relatively bright 11th-magnitude central star overwhelmsqNGC 7048 This ghostly ring fl oating in Cygnus offers a good example
of how applying an O III fi lter will greatly enhance the view.the tiny disk. A hydrogen-beta fi lter may aid in its detection,
but only in larger-aperture scopes. These fi lters have a very
narrow bandpass (just 9–12 nm wide) around the hydrogen
emission line at 486 nm. When conditions allow, Campbell’s
may show a reddish hue when the fi lter is removed.
For many observers, the Dumbbell Nebula is the only
deep-sky object they visit in the small constellation of
Vulpecula. But several other fi ne objects exist there, includ-
ing NGC 6842. This planetary is a rather ghostly disk that’s
nearly invisible without the aid of a narrowband fi lter. I fi nd
it necessary to fi x its position and extent in the fi ltered view
before it’s detectable without a fi lter as a round ethereal disk.
It’s just a short hop to Sagitta where we fi nd NGC 6879.
Like a great many planetary nebulae, NGC 6879 appears stel-
lar. In most backyard telescopes, it can be distinguished from
ordinary stars only by employing a narrowband fi lter. The fi l-
ter dims NGC 6879 less than neighboring stars, exposing the
object’s true nature as a planetary. Rapidly switching between
the fi ltered and unfi ltered view makes detection easier. You do
this either by employing a fi lter wheel or slide bar, or by devel-
oping your handheld blinking technique. Hold a fi lter between
thumb and forefi nger and quickly move the fi lter into and out
of the light path by passing it between eye and eyepiece. It’s
harder than it sounds and requires a bit of practice, but it’s a
valuable skill to develop. Tiny objects like NGC 6879 possessSTEF
ANHEUTZ^ /STEF
ANBINNEWIES/^ JOSEF
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