The Crescent Nebula (also known as Caldwell 27 or Sharpless 105) is an emission nebula in the constellation Cygnus, about 5,000 light-years away from Earth.
It was discovered by Friedrich Wilhelm Herschel in 1792.
It's formed by the fast stellar wind from a nearby star, colliding with and energizing the slower moving wind ejected by the same star when it became a red giant around 250,000 to 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.
Equipment:
- Vixen AX103S 103mm f/8.0 ED Apochromatic Refractor telescope.
- Avalon Linear Fast Reverse Mount.
- QSI 683 CCD camera, Kodak KAF-8300 8.3 mega-pixel sensor cooled to -25 °C.
- Astronomik L-RGB, Astrodon 5nm Ha and Astrodon 3nm Oiii filters.
Image exposure (3+ hours in total):
- 11 x 20min Ha.
