The Crescent Nebula (NGC 6888)

I first thought this was a super nova remnant, but it’s actually what happens when the stellar wind of a star quickly changes speed when the star enters its red super giant phase. The shock wave of expanding gas from the red giant collided with the slower stellar wind, and the resulting shockwave is hot enough to emit all the way up through x rays.

The blue color is from the oxygen rich nature of the gas. I haven’t worked out why it’s so rich in oxygen.