Hot O and B stars embedded in an emission nebula emit a great deal of ultraviolet radiation. But UV radiation is not visible to the unaided eye. So where does the visible red color come from?

Question

Quizplus · Accepted Answer

The visible red color in emission nebulae comes from the process of ionization and recombination of hydrogen atoms. Ultraviolet radiation from hot O and B stars ionizes hydrogen gas in the nebula. When the electrons recombine with the protons, they emit light in various colors, including the characteristic red light associated with the H-alpha emission line. This process is responsible for the visible coloration of emission nebulae, not the direct visibility of ultraviolet radiation, redshifting, or reddening through interstellar dust.

Hot O and B Stars Embedded in an Emission Nebula