Explanation:
The emission nebulae emit their own light because the hydrogen atoms present in the gas that compose it are excited by the powerful ultraviolet light of nearby hot stars. This causes the hydrogen to ionize, generating the characteristic red light of the nebula.
It should be noted that this red color is due to the emission of H-alpha ([tex]H-\alpha[/tex]) lines from hydrogen after ionization, where the [tex]H-\alpha[/tex] line is centered on a wavelength of approximately 656 nm, which corresponds to red.
