uTHE BLINKING PLANETARY Top: This
Hubble Space Telescope image of NGC 6826
reveals several phenomena associated with
dying stars: The outer green bubble is thought
to be gas that used to make up about half
the star’s mass, while the hot remnant star
produces a fast wind that produces shocks —
seen as the brighter rim — as it rams into the
previously expelled material.uNGC 7026 Middle: This planetary is
nicknamed the Cheeseburger Nebula. In the
Hubble image starlight is colored in green,
glowing nitrogen gas in red, and oxygen in
blue. In fact, the oxygen appears greenish in
reality, but the color in the image has been
shifted so as to increase the contrast.uNGC 7027 Bottom: Hubble Space Tele-
scope images of this planetary nebula in Cyg-
nus clearly reveal the central white dwarf, in
addition to the complexities of the mass-loss
processes. In this false-color image, infrared
(purple and yellow) and visible (blue) data are
combined.the advantage of having relatively high surface brightness,
making them detectable in less than pristine skies.
IC 4997, also in Sagitta, is another example of a tiny plan-
etary nebula that’s detectable even in urban skies. It forms a
pair with a brighter star (10th-magnitude HD 355464) and
can be confi rmed as a planetary by employing a fi lter. In a
dark sky and with large apertures, the nebula appears as a
tiny blue-green disk.
Delphinus, the Dolphin, is home to the Blue Flash Nebula,
NGC 6905. It makes for a particularly pleasing view in that it
sits in a pretty star fi eld and within a triangle of 10th-, 11th-,
and 12th-magnitude stars. Nicknames always add interest
and popularity to celestial objects.
The evocative moniker Blue Flash is
attributed to amateur astronomer John
H. Mallas, who described the little
planetary as “glimmering and fl ash-
ing between the triangle [of stars]”. It
seems to have multiple personalities —
it sometimes appears blue, but as often
it appears nebular grey. It’s a round
disk but can appear vaguely annular
when a UHC fi lter is used.
Aquila is fi lled with interesting
planetary nebulae that we toured in
“Aquila’s Gems” in the August 2018
issue (page 34). We’ll revisit just one
here that defi nitely deserves another
look. NGC 6781 is visible with as
little as 6 inches of aperture and is
a remarkably intricate object whenviewed with larger scopes. It’s vaguely annular with a notice-
ably darker center. The 17th-magnitude central star is too
faint for detection in backyard telescopes. An O III fi lter
will bring out some additional structure, but the unfi ltered
view is my favorite. Its northern edge seems to fade into a
delicate extension. In contrast, the rest of the boundary is
more sharply defi ned. Sometimes the object gives an excel-
lent impersonation of the Owl Nebula (M97) when the inner
portion resolves into two dark voids.
The term planetary nebula itself comes down to us from Sir
William Herschel’s description of NGC 7009 in Aquarius. It
was probably the fi rst nebula he discovered and one of several
objects that he described as planetary
due to their round shape and ability to
respond to magnifi cation in much the
same way as the planets. Eventually the
term came to label the whole class.
William Parsons, the third Earl
of Rosse, bestowed on NGC 7009 its
nickname, the Saturn Nebula. Two
extensions, properly termed ansae, give
the object an appearance reminiscent
of the planet. The central star is some-
times hidden in the bright blue-green
disk, but in the right conditions it’s
more easily seen. The ansae require
moderate aperture or good seeing for
reliable detection. Large instruments
and high power reveal quite an intri-
cate inner structure of looping rings
that stand out from the disk. In theNG
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TYIMAGESPLUSskyandtelescope.com • SEPTEMBER 2019 33Ƌ e term planetary
nebula itself comes
down to us from Sir
William Herschel’s
description of
NGC 7009 in
Aquarius. Eventually
the term came to
label the whole class.
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