HIi

H II, sometimes written Hii (or hii), denotes ionized hydrogen. In astronomy and physics, H II refers to hydrogen atoms that have lost their electron, in contrast to neutral hydrogen (H I). Ionization is produced when photons with energy above 13.6 eV emanate from hot, young stars or other energetic sources.

In astrophysical environments, H II gas forms around hot, early-type stars (O- and B-type) and can create extended regions of ionized gas within molecular clouds. These regions typically have electron temperatures around 8,000–12,000 kelvin and electron densities ranging from tens to thousands of electrons per cubic centimeter. The gas emits strong optical recombination lines from hydrogen, such as H-alpha and H-beta, and also forbidden lines from heavier elements like [O III], [N II], and [S II], which are used to diagnose physical conditions and chemical abundances.

The concept of a Strömgren sphere describes a simplified, ionization-bounded region around a source of ultraviolet radiation where ionizations balance recombinations. H II regions vary in size from compact (sub-parsec) to giant (tens of parsecs) and often exhibit structures shaped by stellar winds, turbulence, and surrounding dust. They are indicators of recent star formation and are widely used to trace star formation rates in galaxies, as well as to study the properties of the interstellar medium and galactic chemical evolution.