Crescent Nebula (NGC 6888)

Some larger stars go through a red giant or supergiant phase as they age. The core of the star by then has exhausted its supply of hydrogen and fusion takes place in a shell around the core. The star inflates to many times its original size and it becomes redder and more luminous. Sometimes red giants eject their outer layers and form a so-called planetary nebula. This is what happened to the central star WR 136 in the Crescent nebula around 200,000 years ago; it blew off around five solar masses of material. Subsequent to that, WR 136 exhausted its hydrogen fuel altogether and began fusing helium into heavier elements. At that point, it became a Wolf-Rayet star, becoming even brighter--about 600,000 times as bright as the Sun. The surface temperature is around 70,000 K (compared with our Sun’s 6,000 K). WR 136 produces a fast stellar wind of charged particles blowing out (at speeds of 3.8 million mph) into the planetary nebula that it formed earlier. This collision of the stellar wind with the older material produces the unusual texture of the nebula. The Crescent nebula is about 5,000 light-years away in the constellation Cygnus. The red and blue areas are produced by glowing hydrogen (H-alpha) and oxygen (OIII).