NGC 6888
| Emission nebula | |
|---|---|
Image of NGC 6888 using H-alpha filter.
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| Observation data: J2000.0 epoch | |
| Right ascension | 20h 12m 7s |
| Declination | +38° 21.3′ |
| Distance | 5,000 ly |
| Apparent magnitude (V) | +7.4 |
| Apparent dimensions (V) | 18′ × 12′ |
| Constellation | Cygnus |
| Designations | NGC 6888,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. 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 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".
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Wikipedia