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 WR 136 (HD 192163) 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.
