NGC 6888 is a bubble blown by a Wolf-Rayet star in Cygnus. It was discovered by William Herschel on September 15, 1792. It is in the second H400 observing program. It may be about 5000 light-years distant. Seligman has a good write-up on this one so I'll save my fingers and paste in his words.

"The Crescent Nebula is an emission nebula caused by the collision of two waves of gaseous emissions by the "bright" (7th-magnitude) star near its center, WR 136. The star is an approximately 4.5 million year old Wolf-Rayet star of perhaps 40 to 80 solar masses. Wolf-Rayet stars are very massive, extremely hot stars (originally O-type Main Sequence stars) which are near the end of their lives. A few hundred thousand years ago the star swelled up to become a red super-giant and ejected a few tenths of a solar mass of gas at about 20 thousand miles per hour. About 200 thousand years later, it heated up to several hundred thousand degrees, and began ejecting about a solar mass of super-heated gas per ten thousand years, at nearly 1% the speed of light (3 to 4 million miles per hour). As the faster moving, the hotter gas reached the slower-moving, cooler gas previously released, it created a supersonic shock wave, causing the nebula to emit visible (primarily red H‘) light, as well as ultraviolet and X-radiation. The complex filamentary structure of the nebula is real, but its spherical structure is tissue-thin in comparison to its size. Only the surface of the structure is glowing; the hot gas streaming away from the star is essentially invisible. Within a few thousand years the current nebula will fade away, as its gas disperses into the surrounding space; but within a hundred thousand years, a new and even more spectacular nebula will form when the star supernovas. WR 136 and its nebula are about 4700 light years away. Given that and the approximately 18 by 12 arcmin apparent size of the nebula, NGC 6888 is about 25 light-years across."

This was my first attempt at merging H alpha with color data back in 2007. I didn't do it right as I was working in ignorance. Nor did I take the data correctly for such a combine. This is another image I need to retake but over 10 years later that hasn't happened. Due to my errors, I'm reproducing this one at 2" per pixel rather than my normal 1" per pixel as it hides some of my errors.

14" LX200R @ f/10, Ha=3x30', RGB=3x5', STL-11000XM, Paramount ME