ionized hydrogen

Hydrogen in which electrons have become separated from their parent protons. (A neutral hydrogenatom consists of a single proton, forming the nucleus, and one electron.) Hydrogen clouds in interstellar space become ionized largely through the absorption of ultraviolet photons, which have enough energy to detach the single electrons from the atoms. Ionized hydrogen is the main constituent of H II (or H+) regions, discrete hot clouds that are roughly spherical and up to 600 light years across. Ionization is produced by intense ultraviolet radiation from young O and B stars embedded in the clouds. H II regions are strong sources of radio waves, emitted by the free electrons, and of recombination lines. The Orion Nebula is a giant H II region and one of the nearest. Ionized hydrogen is also present in supernova remnants and the shells of planetary nebulae.