The Crescent nebula, aka NGC6888 is one of the stranger emission nebulae formed by the interaction of a red giant and a high energy Wolf-Rayet star. The Crescent is located in the constellation Cygnus at a distance of 5,000 light-years. It was first discovered by William Herschel in 1792.
Normally higher density materials such as oxygen (blue envelope which envelops the reddish hydrogen) move through space at a slower speed than the lighter hydrogen and remain closer to the core of the nebula. However in the case of the Crescent the nebula is enveloped in a shroud of oxygen. It’s also believed that the Wolf-Rayet star may explode into a supernova. If it did go supernova about 5,000 years ago we should have a good show here on Earth since we are relatively close at only 5,000 light-years!