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 William 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^{[citation needed]} 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.

Crescent Nebula

Emission nebula
The Crescent Nebula, as taken by an amateur astronomer
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
See also: Lists of nebulae

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".

• 
  NGC 6888 in Hydrogen and Oxygen Light. HOO Pallet. Imaged by Don Christopher Deaver
• 
  Crescent Nebula in H-alpha and OIII. Imaged with a 715mm focal length telescope. The hydrogen gas is red. The oxygen is blue.
• 
  Image of NGC 6888 using H-alpha filter.
• 
  Picture of NGC 6888 captured in narrowband by amateur astronomer Luca Moretti
• 
  NGC 6888 imaged in 3 nm hydrogen-alpha and oxygen-III narrowband at 2800mm focal length by amateur astronomer Patrick Hsieh.

See also

• Cygnus Bubble

References

External links

Wikimedia Commons has media related to Crescent Nebula.

• "Crescent". National Optical Astronomy Observatory.
• "Crescent Nebula: Live Fast, Blow Hard and Die Young". Chandra X-Ray Observatory. 14 October 2003.
• Crescent Nebula on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images
• Nemiroff, R.; Bonnell, J., eds. (11 November 2007). "NGC 6888: The Crescent Nebula". Astronomy Picture of the Day. NASA. Retrieved 25 March 2018.

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