Answer: To produce H II regions, a star must emit a significant number of photons with energies above 13.6 eV (ultraviolet photons capable of ionizing hydrogen). Only the hottest stars, specifically O-type and early B-type stars have the required temperature and energy output to ionize hydrogen and are thus responsible for the creation and maintenance of H II regions. These regions not only highlight areas of new star formation but also significantly affect the chemical and physical structure of the interstellar environment around them.
