Sky & Telescope - August 2022 - 57

OLEG BOUEVITCH
Going Deep by Ken Hewitt-White
GORGEOUS WRECKAGE Adrift roughly
5,000 light-years from Earth, the emission nebula
NGC 6888 is a vast shell of gas sculpted
by high-velocity winds from 7.5-magnitude
WR 136. The exceedingly hot, massive WolfRayet
star shines some 600,000 times brighter
than our Sun and is doomed to die in a supernova
explosion. NGC 6888 is a subtle target for
observers, who know it as the Crescent Nebula
for its visual appearance in telescopes.
known as Sadr. To get there, I enjoy
a scenic Gamma-to-Crescent starhop
down the Cygnus Milky Way. My
most recent ramble occurred on an
exceptional night in August 2019 atop
6,100-foot (1,860-meter) Mount Kobau
in southern British Columbia. Kobau's
zenith sky can be described in a single
word: black.
Gamma is enwreathed in an espeMore
Than Just
a Pretty Crescent
NGC 6888 is a faint but seductive deep-sky wonder.
F
irst, the bare-bones truth:
NGC 6888 is no showstopper.
The pale object in central Cygnus
known as the Crescent Nebula is barely
visible without narrowband filters. But
amateur astronomers embrace challenges,
and NGC 6888 rewards those
who scrutinize it in dark skies with
large telescopes - in my case, Mount
Kobau (sounds like " oboe " ) with an
18-inch f/4.5 Dobsonian. The ghostly
Crescent doesn't repel us, it attracts us.
NGC 6888 certainly intrigues
professional astronomers. They've
designated this expanding cloud an
emission nebula that's being excited
by an enormous, decaying sun with
the cryptic name WR 136. The WR
stands for Wolf-Rayet. First noted in
1867 by French astronomers Charles
Wolf and Georges Rayet, the spectra
of Wolf-Rayet stars are characterized
by broad emission lines predominantly
of helium. Massive, luminous, and hot
(likely descended from stars of spectral
class O), they develop intense radiation
pressure and winds. A Wolf-Rayet's
violent behavior results in tremendous
mass loss. In short, the star becomes
unstable.
WR 136 is a classic case. Originally
a star of perhaps 20 solar masses, its
hydrogen-rich atmosphere has been
stripped away. Intense radiation from
the exposed helium core has since
expelled gas in a high-velocity wind that
has collided into a previously ejected
envelope to create an elliptical shell
about 25 by 16 light-years in extent,
tilted roughly 45° to our line of sight.
Getting There
NGC 6888 lies 2¾° southwest of 2ndmagnitude
Gamma (g) Cygni, also
cially rich swath of stars and nebulosity
several degrees across. Amid the Gamma
glitter are four tiny clusters. The best
of the bunch, 35′ north of Gamma, is
NGC 6910. Starting the 18-inch at 64×
that night, I saw two 7th-magnitude
stars, 3¼′ apart, connected by a wavy
string of 10th-magnitude pinpoints.
Two more jutting from the middle of the
string turned NGC 6910 into a squished
letter Y. Cute!
The other three clusters are less
appealing, but they promised doublestar
bonuses, so I hit 'em. A bit less than
55′ west-northwest of Gamma is Collinder
419, a compact clump obliterated
by a 6.0-magnitude star. This beacon
is Struve (Σ) 2666, which harbors
an 8.2-magnitude secondary sun 2.8″
away. Around 40′ on the other side of
Gamma, the cluster Dolidze 10 is basically
a minute rectangle. On its northernmost
corner is Dembowski 22, a
double comprising 8.2- and 9.5-magnitude
stars 2.9″ apart. And 58′ southsouthwest
of Gamma, Dolidze 5 yields
five or six dim dots enveloping a teeny
triangle. North of the triangle, Σ2668
presents 6.3- and 8.5-magnitude elements
separated by 3.5″. The three clusters
and their embedded binaries were a
snap at 128×.
After Σ2668, I returned to low power
and swept southwestward 1½° to a
lovely, north-south star pair, 2.2′ apart.
The 7.1-magnitude northern component
sk yand tele scope .o r g * AUGUST 2022 57
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