(via APOD: 2012 August 16 - NGC 6888: The Crescent Nebula)
Image Credit & Copyright: J-P Metsävainio (Astro Anarchy)
The Crescent Nebula is born of the dying actions of a Wolf-Rayet star (WR 136) in Cygnus, the swan. As a star begins to die, it starts shedding matter as the nuclear reactions shut down temporarily, then start-up again as gravity provides enough pressure for the next set of fusion reactions. This creates a series of dust and gas clouds spreading out from the star, and then a bunch of stellar winds and radiation coming off the star again and interacting with the ejected material. This “catching up” of the radiation with the matter is what creates the wispy complex structure of this nebula.
Its next step will be somewhat more definite and final. As the star builds up to more complicated atomic structures in the fusion reactions, it will, at some point jump from fusing carbon atoms to fusing iron, at which point a big problem occurs. Fusing iron actually saps energy from the process, leading to a cataclysmic shutdown of the fusion reactions…with a bunch of really hot carbon around an iron core, which suddenly isn’t providing any outward pressure.
Carbon-detonation supernova, Type Ia, the brightest nova events in the sky. As all that carbon collapses in the gravity, it suddenly ignites in one last burst of fusion, throwing off everything but the iron core.
In case you were wondering what it looks like when a WR star’s stellar wind catches up with the ejected red-giant shell in x-rays, Chandra’s got you covered:
Credit: X-ray: NASA/UIUC/Y. Chu & R. Gruendl et al. Optical: SDSU/MLO/Y. Chu et al.