This nebula began to form 250,000 years ago, when the
massive, energetic bright star near its center (called a
"Wolf-Rayet" star) began blowing off material from its outer
layers at tremendous speed. That material collided with the
gas and dust that had been ejected by the star millennia earlier.
The faster moving material slammed into the slower expanding gas
and dust, creating the complex shockwaves apparent in this image
and causing the hydrogen to glow. Eventually (estimates range from
100,000 to 1,000,000 years), this star will end in a supernova.
The Crescent Nebula is 5000 light years away, in the constellation
Cygnus, and is 25 light years in diameter.

Image details:
Camera: SBIG ST-4000XCM
Exposure: 10 minutes x 24
Telescope: 10" Schmidt-Newtonian, Baader MPCC