Crescent Nebula

Crescent Nebula
Emission nebula

Image of NGC 6888 using H-alpha filter.
Observation data: J2000.0 epoch
Right ascension 20h 12m 7s[1]
Declination +38° 21.3′[1]
Distance 5,000 ly
Apparent magnitude (V) +7.4
Apparent dimensions (V) 18′ × 12′
Constellation Cygnus
Designations NGC 6888,[1] Sharpless 105, Caldwell 27

The Crescent Nebula (also known as NGC 6888, Caldwell 27, Sharpless 105) is an emission nebula in the constellation Cygnus, about 5000 light-years away from Earth. It was discovered by Friedrich Wilhelm Herschel in 1792.[2] 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 250,000[3] 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.

It is a rather faint object located about 2 degrees SW of Sadr. For most telescopes it requires a UHC or OIII filter to see. Under favorable circumstances a telescope as small as 8 cm (with filter) can see its nebulosity. Larger telescopes (20 cm or more) reveal the crescent or a Euro sign shape which makes some to call it the "Euro sign nebula".

See also

References

  1. 1 2 3 "SIMBAD Astronomical Database". Results for NGC 6888. Retrieved 2007-04-23.
  2. ↑ Erdmann, Robert. "NGC 6888". The NGC/IC Project.
  3. ↑ "WR 136".
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