The Crescent Nebula (NGC 6888) is a rapidly expanding, luminous shell of gas that surrounds the Wolf-Rayet star WR 136 in the constellation Cygnus. The Crescent Nebula started to form about 250,000 years ago as the central star began shedding its outer envelope in a strong stellar wind. As this wind rammed into the surrounding interstellar medium, it compacted it and sent shock waves into it, resulting in a series of shells, which ultraviolet radiation from WR 136 subsequently ionized and made visible.