An emission nebula is a glowing gas cloud with a strong source of radiation—usually a bright star—within or behind it. If the source gives off enough high-energy ultraviolet radiation, some of the gas is ionized, which means the electrons and nuclei of the gas molecules become separated and fly freely through the cloud. When the free electrons recombine with the free nuclei to become atoms again, the gas gives off light of specific colors. Which colors they emit depends on the temperature, density, and composition of the gas. The Orion Nebula, for example, glows mostly green and red.