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Designations:
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NGC 6888, Caldwell C27, Crescent Nebula
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Object Type:
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Constellation:
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Cygnus
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20 hr 12 min 0 sec (2000.0)
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+38° 21 min (2000.0)
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8.8
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Size:
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18 X 13 arcminutes
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Distance:
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4700 light years
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Discoverer:
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William Herschel, 1792
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NGC 6888 is commonly known as the Crescent Nebula because, when seen through a
backyard telescope, it's 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
magnitude
less "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 seems to be much richer targets sprinkled across the Cygnus
Milky Way?
After all, NGC 6888 is
but a tiny patch of light about 2 3/4 degree southwest of 2nd-
magnitude
Gamma Cygni, a bright star
that lies at the heart of an enormously large and rich nebula complex spanning some 3 degrees. The
thought of tackling tiny NGC 6888 seems almost ludicrous in the 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-Rayet stars 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 across, that is tilted at a 45 degree angle to our line of sight. The Einstein and Rosat spacecraft revealed that X-rays emanate not from the nebula's cavity but from its visually brightest parts. The X-ray emissions originate in filamentary structures close to the nebula's inner border, suggesting that they are generated as material cools after being shocked by the fast wind for HD 192163. The nebula's outer edges, by contrast show the first point of contact between the advancing Wolf-Rayet wind and the atmosphere of the former red supergiant.
The revolutionary Infrared Astronomical Satellite has shown that a vastly larger (1.8° X 1.5°)
shell surrounds the Crescent and is also centered on the Wolf-Rayet star. This extended structure
may be the infrared signature of a 100,000-year-old
supernova remnant.
Since HD 192163 is believed
to be a binary star, it's possible that the outer infrared shell was generated by the explosion of the
primary star, while a very recent outflow from the secondary star formed the Crescent Nebula. Indeed,
Wolf-Rayet stars are believed to be the progenitors of some
supernova remnants,
Cassiopeia A among them.
But Argentinean astronomer Cristina E. Cappa (Argentine Institute of Radio Astronomy) and her colleagues
have questioned the notion that the HD 192163 system has hosted a supernova explosion. In a 1996
Astronomical Journal paper they argue that HD 192163 could have ejected the extended outer shell
during the star's life on the main sequence; material ejected by the star later in life then encountered
the innermost wall of the outer shell and created NGC 6888. However David A. Green (Mullard Radio
Astronomy Observatory, England) still lists a possible
supernova remnant
in A Catalogue of Galactic
supernova remnants,
dated 1998. Although the jury is still out on the origin of the outer infrared
nebula, it's safe to say that to see the Crescent Nebula is to see the results of several stages of
energetic stellar activity. |
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Telescope:
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Focal Length:
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1200 mm
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Mount:
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Camera
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Guider:
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SBIG AO-7 adaptive Optics unit
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Exposures:
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8 5-minute Luminance exposures
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Location:
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Heart of America Star Party south of Kansas City, MO
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Software:
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CCDSoft for image acquisition, processed with CCDStack and Photoshop CS2
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