The Crescent Nebula (NGC 6888)
Object Type: Emission Nebula
Scope: AP 5"
Exposure 114 Minutes
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 4.6 million year old star is about 20 times the mass of the sun, with surface temperature of 70,000 Kelvin and is 660,000 times more luminous than the sun. It is expected to go supernova in the next 100,000 to 200,000 years.
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.