The Caldwell Objects

2 7
Crescent Nebula
NGC 6888
Type: Emission Nebula
Con: Cygnus
RA: 20 h 12.0 m
Dec: +38° 21'
Mag: 8.8 (nebula)
Mag: 7.7, var.? (central star)
Dim: 18' x 13'
Dist: 4,700 light-years
Disc: William Herschel,
1792
W. H ERSCHEL: [Observed 15
September 1792] A double
star of the 8th magnitude
with a faint south preceding
[southwesterly] milky ray
joining to it. 8' in length and
1½' broad. (HPV-72)
GC/ NGC: Faint, very large, very much extended,
double star attached.
NGC 6888 is COMMONLY KNOWN AS THE CREScent
Nebula because, when seen through a backyard
telescope, its brightest segments look like the dim
spirit of a young Moon set against the glowing
clouds of the Milky Way. Some publications
simply list NGC 6888 as a magnitudeless "bright
nebula." With its nature apparently so obscure,
most amateurs probably shrug their shoulders at
the thought of trying to hunt down the Crescent.
Why look for some inconspicuous blip when
modern star atlases show what seem to be much
richer targets sprinkled across the Cygnus Milky
Way? After all, NGC 6888 is but a tiny patch of
light about 2¾° southwest of 2nd-magnitude
Gamma (γ) Cygni, a bright star that lies at the
heart of
The Caldwell Objects
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an enormously large and rich nebula complex
spanning some 3°. The thought of tackling tiny
NGC 6888 seems almost ludicrous in this context.
But the Crescent Nebula is one of the more
intriguing objects in our galaxy. This annular
nebula circumscribes a 7.5-magnitude Wolf-Rayet
star known as HD 192163. Wolf-Rayets are
extremely luminous and hot stars that shed mass
at enormously high rates, in many cases after
passing through a supergiant stage. This material
travels at speeds as high as 3,000 km per second
until it smashes into gases that previously
constituted the star's outer atmosphere. The
Crescent has long been believed to be the result
of such a collision. Studies show it to be a prolate
ellipsoid, 25 by 16 light-years
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