NGC 6888 is located approximately 4,700 light years from Earth and is known as the Crescent Nebula because of its distinctive shape. Technically it is classified as a Wolf-Rayet nebula. Such a nebula is formed when a very hot, massive star ejects its outer layers in a strong stellar wind. In the case of the Crescent Nebula the star responsible (WR 136) is the bright star in the center of the image above. As the star ages it begins to shrink and grow much hotter. Its stellar wind becomes more rarefied but much faster. This fast stellar wind then collides with the older ejected gas and pushes it into a relative dense bubble. WR 136 is in the final stages of its stellar life and is estimated to explode in a supernova event within the next million years.
The image above was captured using SII, Ha, and OIII narrowband filters. The image was assembled using the standard Hubble Palette with SII mapped to red, Ha to green, and OIII to blue. The stars were overlaid with data obtained using red, green, and blue filters.
A Near True Color (Ha OIII OIII) version of the image may be viewed here.