Emission nebulae are clouds of ionized gas emitting light of various colors. The source of ionization are high-energy photons emitted from a nearby hot star. There are several different types of emission nebulae. One of them are H II regions, where star formation is taking place and young, massive stars are the source of the ionizing photons.The nebula's color depends on its chemical composition and degree of ionization. Many emission nebulae appear red due to the prevalence of hydrogen in interstellar gas. If more energy is available, other elements will be ionized, which make possible green and blue colors in the nebula.