NGC 6888 (the Crescent Nebula or Caldwell 27) is an emission nebula in the Cygnus constellation, about 5000 light years away. It is formed by the fast stellar wind from the Wolf-Rayet star HD 192163 (or WR 136) colliding with and energizing the slower moving wind ejected by the star when it became a red giant around 400,000 years ago. The result of the collision is a shell and two shock waves, one moving outward and one moving inward. The inward moving shock wave heats the stellar wind to X-ray emitting temperatures. An emission nebula is a cloud of ionized gas ( a plasma) emitting light of various colors. ... For the software company, see Cygnus Solutions. ... A light year, abbreviated ly, is the distance light travels in one year: roughly 9. ... A solar wind is a stream of particles (mostly high-energy protons ~ 500 keV) which are ejected from the upper atmosphere of a star (in the case of a star other than the Earths Sun, it may be called a stellar wind instead). ... Artists impression of a Wolf-Rayet star Wolf-Rayet stars (often referred to as WR stars) are evolved, hot, massive stars which have very strong stellar winds. ... According to the Hertzsprung-Russell diagram, a red giant is a large non-main sequence star of stellar classification K or M; so-named because of the reddish appearance of the cooler giant stars. ... In fluid dynamics, a shock wave is a nonlinear or discontinuous pressure wave. ... In the NATO phonetic alphabet, X-ray represents the letter X. An X-ray picture (radiograph) taken by Röntgen An X-ray is a form of electromagnetic radiation with a wavelength approximately in the range of 5 pm to 10 nanometers (corresponding to frequencies in the range 30 PHz...
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