H II region
Terminology

The term H II is pronounced "H two" by astronomers. "H" is the chemical symbol for hydrogen, and "II" is the Roman numeral for 2. It is customary in astronomy to use the Roman numeral I for neutral atoms, II for singly-ionised—H II is H in other sciences—III for doubly-ionised, e.g. O III is O, etc. H II, or H, consists of free protons. An H I region consists of neutral atomic hydrogen, and a molecular cloud of molecular hydrogen, H. In spoken discussion with non-astronomers there is sometimes confusion between the identical spoken forms of "H II" and "H ".
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