The Crescent Nebula, NGC 6888 or Caldwell 27, is an emission nebula in the constellation Cygnus.
NGC 6888 is a visually interesting object. Its 20' x 10' NE-SW halo is unevenly illuminated, the brightest portion an arc along the north side. The arc is most conspicuous on its northeast, just where it begins a sharp curve to the south. A multitude of Milky Way field stars shine through the nebula, including a bright keystone with a 9th and 10th magnitude star forming the east side and two 7th magnitude stars forming the west.
About 4,700 light years away, NGC 6888 is formed by a fast stellar wind from the Wolf-Rayet star HD 192163, also known as WR 136. The star is shedding its outer envelope, ejecting the equivalent of the Sun's mass every 10,000 years. This stellar wind is colliding with, and energizing, a slower-moving wind ejected by the star when it became a red giant around 400,000 years ago. The result of the collision is a shell about 25 light-years across. Two shock waves, one moving outward and one moving inward, heats the stellar wind to X-ray-emitting temperatures. The central star WR 136 will probably undergo a supernova explosion sometime in the next million years.