A cloud of glowing gas in interstellar space. Interstellar clouds consist primarily of hydrogen, which can be excited or ionized by the action of the ultraviolet radiation from hot stars embedded within it. Energy is released by the recombination of ions and the collision of electrons with ionized atoms of heavier elements, such as oxygen and nitrogen, which are also present. The most intense radiation from hydrogen gives rise to the typical pinkish colour observed in ionized hydrogen clouds H II regions such as the Orion Nebula.