NGC 6888, located in the constellation Cygnus, is an emission nebula. This nebula is lit by a central star within the nebula.
Sometime between 250,000 to 400,000 years ago, this star ejected a stellar shell when it ballooned to a red giant star. Since that time, the central star is nearing the end of it’s life and the stellar winds have greatly increased (now categorized as a Wolf-Rayet star) resulting in another stellar shell.
The two different stellar shells are colliding, creating one outward moving shockwave and one inward moving shockwave. This is why the image appears to have two distinct shells.