Trifid Nebula. Optical image of the Trifid Nebula (M20, or NGC 6514) in the constellation Sagittarius. North is at top. This nebula is a combination of a reflection nebula (blue) and an emission nebula (red). Reflection nebula reflect the light of nearby stars, appearing blue as blue light is scattered more than red. Emission nebulae glow as the hydrogen gas they contain is excited by radiation from stars embedded within them.