in astronomy, interstellar matter consisting of ionized hydrogen atoms. Such regions occur as hot, ionized gaseous clouds, sometimes called emission nebulae. The energy that is responsible for ionizing and heating the hydrogen in an emission nebula comes from a central star that has a surface temperature in excess of 20,000 K. The density of these clouds normally ranges from 10 to 1,000 particles per cubic cm; their temperature is about 10,000 K. Some electrons and protons in the ionized gas recombine and form neutral hydrogen atoms. Such a process results in the emission of energy at specific wavelengths, the most prominent of which are the hydrogen Balmer lines in the visible part of the electromagnetic spectrum. H II regions can emit visible light. The Orion Nebula (q.v.) is one of the most conspicuous H II regions in the sky.