Crescent Nebula

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 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 ascension20h 12m 7s[1]
Declination+38° 21.3′[1]
Distance5,000 ly
Apparent magnitude (V)+7.4
Apparent dimensions (V)18′ × 12′
ConstellationCygnus
DesignationsNGC 6888,[1] Sharpless 105, Caldwell 27

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".

  • 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 3nm hydrogen-alpha and oxygen-III narrowband at 2800mm focal length by amateur astronomer Patrick Hsieh.

See also

References
  1. "NGC 6888". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2007-04-23.
  2. Erdmann, Robert. "NGC 6888". The NGC/IC Project. Archived from the original on 2012-05-20.
  3. "WR 136". jumk.de. Retrieved 25 March 2018.
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