OF THE ROGER N. CLARK

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NGC 6888, THE CRESCENT NEBULA IN
CYGNUS
R.A. 20h 12.5m, Dec. 38° 25' (2000.0)
Technical. NGC 6888 is a diffuse shell of
gas being blown from an unusual object: a
Wolf-Rayet star, a rare class of star (spectral
type W) that is extremely hot, massive, and
hydrogen-poor. The nebula is rather large,
with a true size of 16 by 24 light-years, and
contains about 2 solar masses of gas. Its origin
is somewhat unclear. It may all have been
ejected from the central star, or most of it
may already have existed in the neighborhood
and is just being pushed away from the
star by a powerful "stellar wind."
The 7th-magnitude central star is known
as HD 192163. It is one of only about 150
VISUAL ASTRONOMY OF THE DEEP SKY
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Wolf-Rayet stars known in our Galaxy. One
rẹaso the are unusual is that the absorption
hnes In their spectra are very broad instead of
narrow. The strongest spectral features of
this star are due to nitrogen, and no features
to indicate the presence of carbon. A second
type of Wolf-Ray et star has the spectral sign_
ature of carbon but no nitrogen.
HD 192163 is about seven times larger
than the Sun and seven times hotter: 40 000
kelvin. Wolf-Rayet stars constantly shed
mass as a thick stellar wind flowing outward
at great speed. The wind's velocity is similar
to that of an expanding nova: about 3000
kilo meters per second. If this were to keep up
the typical Wolf-Ray et star would lose all of
its mass in just a few million years.
Visual. NGC 6888, often called the Crescent
Nebula, is harder to see than the Veil Nebula
10° to the southeast. No magnitude or surface
brightness estimates are available for NGC
6888. The star HD 192163 appears off-cen ter
in the egg-shaped loop. Much of the loop is
very fa int and probably cannot be detected
visually. The brightest portion is a crescent
on the north side. Its shape is rather reminiscent
of the much larger Veil.
The Crescent is easily seen through moderate-size
telescopes under good skies. I t should
be visible through small telescopes in excellent
conditions. A magnification of 117 X on
the 8-inch in good skies gave the best view.
The crescent was dimmest near the middle of
the arc. A number of stars appeared superimposed
upon it.
A VISUAL ATLAS OF DEEP-SKY OBJECTS
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Photograph of NGC 6888. South is up. (Courtesy
Martin Gerrnano.)
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Scale: 3. 5 arc-min/ cm
8-inch f/ 1 1.5 Cassegrain
20mm Erfle (11 7 x
best view)
12.4mm Erfle (188X)
Drawing of NGC 6888.
Viewing Distance (cm)
25X: 39
50X: 20
100X: 10
200X: 5
300X: 3
400X: 2
air mass 1. 24, faintest star 14.0 at zenith, 188X; no
tracking
9/ 1 1/ 83 9: 35- 10:20 UT at Barbers Point, Hawaii;
R. Clark
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