An emission nebula in the constellation of Cygnus, lying somewhat to the southwest of Sadr, the central star of Cygnus' Northern Cross formation. The distance to this nebula is uncertain, with estimates ranging from 4,700 light years to 5,400 light years from the Solar System.
The nebula represents that last stages of development of the star V1770 Cygni or HD 192163, a star that evolved into a red giant hundreds of thousands of years ago, and is expected to shortly undergo a supernova. When it entered the red giant phase of its existence, the star cast of its outer shell, and that shell has now expanded across an area of space some twenty-five light years in diameter.
Meanwhile, fusion has continued in the surviving core of the star, generating a strong stellar wind rushing outwards fast enough to overtake the expanding shell. Where the wind and shell meet, their interactions generate radiation, making the nebula visible as vast 'bubble' in space.
The effect creating the nebula is not symmetrical, and one side of the bubble shape is more strongly visible than the other. The outcome is a bright arc of material enveloping the central star in a crescent shape as seen from Earth, a shape that gives the nebula its common name.
The Crescent Nebula in Cygnus (C27 or NGC 6888) is not to be confused with NGC 6445, a rather smaller and fainter planetary nebula in the constellation of Sagittarius, which is also sometimes known by the name 'Crescent Nebula'.