NGC 6888, also known as the Crescent Nebula, is a faint emission nebula in the constellation of Cygnus. Although shining at magnitude +7.4, its extremely low surface brightness means a very transparent night or large optics coupled with a nebula filter is required to spot it.
This nebula is located in a dense region of the northern Milky Way. Positioned 2.75 degrees to the northeast is bright star, Sadr (γ Cyg - mag. +2.23), with open cluster, M29, just over a couple of degrees directly east of the nebula. NGC 6888 was discovered by William Herschel on December 15, 1792 and is best seen from northern latitudes during the months of July, August and September. It's object 27 in the Caldwell catalogue.
The Crescent Nebula is about 5,000 light-years away. It has a spatial diameter of 26 light-years and appears as an arc or bubble of gas. The centre star is classified as a Wolf-Rayet star (WR 136) that sheds its outer envelope in a strong stellar wind, ejecting the equivalent of the Sun's mass every 10,000 years. The complex structure of this nebula is likely a result of the stellar wind interacting with previously ejected material. The central star is nearing the end of its stellar life, which probably will ultimately result in a spectacular supernova explosion.
In total, NGC 6888 spans 18 x 12 arc minutes of apparent sky and is visible in large binoculars under superb skies. However, for most a telescope of a least 200mm (8-inch) aperture with a UHC or OIII filter is required. It can be difficult to pick out against the surrounding Milky Way star fields, but those with some patience will be rewarded. Averted vision also helps. Thorough large reflectors, an arc curving among several bright stars can be made out. Some observers claim it resembles a ghostly Euro sign or C shape. Overall, this is a difficult and challenging object, but well worth the effort.
NGC 6888 Data Table
|Object Type||Emission Nebula|
|RA (J2000)||20h 12m 06s|
|DEC (J2000)||+38d 21m 18s|
|Apparent Size (arc mins)||18 x 12|
|Other Name||Sharpless 105|