This object is the Crescent Nebula, NGC 6888. I had imaged it before, in LRGB in August 2018, and the number of stars in that image was rather overwhelming. So this time I decided to go for HOO. I had one night's worth of data from March 2022, with the sensor at -5C, but it was 6 months before I collected the rest of the data, in Nov 2022, with the sensor at -10C (this is all a question of ambient temperature). I decided to dump the March data and only use the November sub-frames. 

The Crescent Nebula is about 5000 light-years away, and it is about 25 light-years across. At its centre is a bright blue Wolf-Rayet star, which is very massive, hot and intrinsically luminous. Wolf-Rayet stars are a late stage in the life of very massive stars, after passing through a red supergiant phase. Being massive, they have short lifetimes. This one, called WR136, is estimated to be only 4.7 million years old, about a thousand time less than the age of our Sun. To put it even more in perspective, the asteroid impact which caused the dinosaurs to die out was about 70 million years ago. WR136 has only a short lifetime ahead, as it is expected to explode as a supernova within a few hundred thousand years.

When WR136 was a red supergiant, about 120,000–240,000 years ago, it blew off a shell of gas travelling at a speed of about 80 km/s and this is still expanding. When it became a Wolf-Rayet star it started emitting a very fast stellar wind due to its extremely high temperature, in which gas is ejected at about 1,700 km/s: a characteristic of these stars. The collision of these streams of gas is energising the shell and causing it to emit light, which is what we see as this glowing shell-like structure.