This is NGC 6888, the Crescent Nebula, in Cygnus. Looking at Cygnus is seeing into the heart of our galaxy, straight through the plane of the Milky Way - there's a LOT of stars there. Because it is in this deepest, darkest neighbourhood of the galaxy, there's a lot of interesting things there to look at but it's all obscured by stars and dust clouds. With the magical deceit of computer wizardry I have generated a series of images with gradually reducing star fields, culminating in this nebula being seen in just reflected starlight and it's own emitted light. This makes the images dark but very dramatic.

I'm paraphrasing Wikipedia here. This nebula is an emission nebula about 5000 light-years away from Earth. It was discovered by William Herschel in 1792. It is formed by the fast stellar wind from the Wolf-Rayet star WR 136 (HD 192163) colliding with and energizing the slower moving wind ejected by the star when it became a red giant around 250,000 to 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.

I didn't intend to image this nebula at all - I was aiming at an open cluster called Messier 29 and this appeared when I was trying to improve the framing of the cluster stars.