NGC 6888: 'The Crescent Nebula'
Object types:Bright (emission and/or reflection) nebula in Cygnus
Size: diameter ~ 20'
Constellation: Cygnus
Exposure data:
Date: 2007-07-07
Location: Postalm/Austria (1300m)
Telescope: TMB 80/480 (3.1" Apo f=480mm) + 0.8 TV reducer/flattener
Camera: Starlight XPress SXV-H9
Astronomik H-alpha filter (13nm)
Image type: Ha
Binning: 1x1
Exposure time: Ha
7x20m
Exposure time total: 2h 20m
Notes: windy, seeing was not optimal, good transperancy
The Crescent Nebula (NGC 6888 or Caldwell 27) is an emission nebula in the Cygnus constellation, about 5000 light years away. It is formed by the fast stellar wind from the Wolf-Rayet star HD 192163 (or WR 136) colliding with and energizing the slower moving wind ejected by the star when it became a red giant around 400,000 years ago. The result of the collision is a shell and two shock waves, one moving outward and one moving inward. The inward moving shock wave heats the stellar wind to X-ray emitting temperatures.
http://en.wikipedia.org/wiki/Crescent_Nebula