Part of the Cygnus loop, the remains of a supernova that is estimated to have exploded about 5000-10 000 years ago. At the time it would have appeared as a very bright star, easily visible to our ancestors in broad day light. What remains today are the faint remnants of this massive explosion, shockwaves of ionized gas that emit strongly in the Halpha and OIII regions of the visible spectrum due to the presence of ionized hydrogen and oxygen gas.

In this image Halpha light is mapped to red and OIII is mapped to green. The blue channel has been created as an 80:20 blend of these two signals.

read more: http://en.wikipedia.org/wiki/Veil_Nebula