The Veil Nebula is a super nova remnant. Approx. 5000-8000 years ago a star several times bigger and heavier than our sun exploded. The expelled gas jammed with high speed into interstellar gas clouds and excited them to glow in the colors of excited hydrogen ( red) and oxygen ( blueish green). The nebula is approx. 1500 light years away.
The image is a H-alpha - OIII composit with the color mapping Ha:OIII:OIII = R:G:B.