Living reference work entry
An H II region is an astrophysical object belonging to the interstellar medium and consisting in a cloud of hot plasma of protons and electrons at about 10,000 K. The name stems from the old appellation H II for the ionized state of hydrogen (now H +), as opposed to the neutral state H I. The plasma is produced by photoionization of the hydrogen in an interstellar cloud caused by the ultraviolet radiation from nearby hot massive stars of spectral type O and B. The stars are generally born in a giant molecular cloudand ionize the gas around them that was not condensed into stars. The occasional recombination of a proton and an electron produces a cascade in the energy levels with emission of photons that explains why H II regions appear as luminous nebulae. Their spectrum contains mainly strong H I recombination lines, especially the Hα line responsible for the reddish color, as well as forbidden lines (transitions from metastable states, denoted in...
KeywordsBioorganic Chemistry Reddish Color Metastable State Ultraviolet Radiation Neutral State
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