250,000 - 400,000 years ago, the star in the middle of the Crescent Nebula (number HD192163) turned into a red giant. As a result, the star expanded rapidly, burned hotter and shed lots of material. Later, it became a so-called Wolf Rayet star burning even hotter and shedding more material faster. The Crescent nebula is from the area where the second, faster wave hits the first, slightly slower wave. The interaction of the two waves makes the material glow.
What's even more exciting, this star will become a supernova - we just don't know when. Although, because of its distance of 5000 light years, the supernova might have already happened. We just haven't seen it yet.
I used the same technique as earlier, but a different color palette (Red=SII+(0.8*Ha), Green=(0.075*Ha)+OIII, Blue=OIII) which gave this nebula great color while preserving the vast amounts of nebulosity in the background.