The Crescent Nebula (NGC 6888) is an emission nebula lying along the "spine" of Cygnus the Swan among clouds of ionized hydrogen gas. The nebula itself is an interaction of stellar wind from the hot, massive central Wolf-Rayet star (WR 136) with its own outer envelope, which was apparently shed in an earlier episode of ejection. These extremely hot stars have largely depleted their supply of hydrogen and have moved on to fuse helium and/or heavier elements in their core. WR 136 is expected to explode as a supernova within a few hundred thousand years.