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. 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.
|Observation data: J2000.0 epoch|
|Right ascension||20h 12m 7s|
|Apparent magnitude (V)||+7.4|
|Apparent dimensions (V)||18′ × 12′|
|Designations||NGC 6888, 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".
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.