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Designations:
NGC6888, Caldwell (C27),
Crescent Nebula
Object Type: Emission
Nebula
Constellation: Cygnus
RA:
20h 12m 0s (2000.0)
Dec: +38deg
21m
(2000.0)
Visual Magnitude: 8.8
Size:
18' X 13'
Distance: 4700
light years
Discoverer: William
Herschel, 1792
Visual Description: NGC6888 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 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 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 NGC6888 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 deg X 1.5 deg) 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.
Seeing Conditions:
clear
and stable
Telescope:
TMB 152mm APO Refractor
Focal Length: 1201 mm
Mount: Takahashi NJP 160
Camera: SBIG
ST10XME
Exposure: 8 5-minute
Luminance exposures
Other: SBIG AO-7
adaptive Optics unit
Copyright(c) 2007 Doug Sanqunetti. All rights reserved
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