| Emission nebula | |
|---|---|
 The Crescent Nebula, captured with a color camera, narrowband filter, and high-speed amateur telescope | |
| Observation data: J2000.0epoch | |
| 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 | |
TheCrescent Nebula (also known asNGC 6888,Caldwell 27,Sharpless 105) is anemission nebula in the constellationCygnus, about 5000light-years away fromEarth. It was discovered byWilliam Herschel in 1792.[2] It is formed by the faststellar wind from theWolf-Rayet starWR 136 (HD 192163) colliding with and energizing the slower moving wind ejected by the star when it became ared giant around 250,000 years ago[3]. The result of the collision is a shell and twoshock waves, one moving outward and one moving inward. The inward moving shock wave heats the stellar wind toX-ray-emitting temperatures.
The Crescent Nebula is a rather small object located about 2 degrees Southwest ofSadr. While considered bright by astronomical imaging standards, visually it is relatively faint. For most telescopes it requires aUHC or OIII filter to see. Under favorable circumstances a telescope as small as 8 cm (with filter) can see its nebulosity. With higher magnification, it has been reported to have a resemblance to aEuro sign shape leading to a nickname the "Euro Sign Nebula."[4]
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