The Bubble Nebula, NGC 7635 in Cassiopeia, with the star cluster M52 on the left and the small nebula Sh2-159 at top right. The Bubble is blown by a single star, BD+60 2522, about 45 times the mass of the Sun, whose wind streams out at more than 4 million miles per hour and inflates a shell into the cloud of hydrogen around it. The cluster M52 is not part of the scene: it sits roughly 5,000 light-years away, in front of the nebula, which is about twice as far.
This is a close-up from the same data as my Lobster Claw image, shown in the Foraxx palette: a blend of the same three gases (sulphur, hydrogen, oxygen) that lets hydrogen show warm red where there is little oxygen, and oxygen show cyan where it is strong, around the Bubble itself. The stars are their real colours, from separate red, green and blue exposures.