The Flame Nebula (also known as the Burning Bush) is lit up by Alnitak, the nearby left hand belt star of Orion. The energetic light of Alnitak ionizes or disassociates the hydrogen gases of the Flame into it's constituent electrons and protons. Much of the glow results from the light given off when the electrons and ionized hydrogen (protons) recombine.

10" F6.3 Schmidt Cassgrain reflector, Canon 300D at Prime focus
 300 sec, ISO 800
 6 photos averaged together, then brightness and contrast enhanced, cropped to fit screen