H I Region
Region of neutral (atomic) hydrogen in interstellar space. The temperature is about 125 K (the spin temperature of neutral hydrogen - far too low for electrons to emit radiation in the optical part of the spectrum (see 21-cm radiation).
H I Region: A region in space, seen as cloud-like features, containing mostly neutral atomic hydrogen gas. If of sufficient density, these areas may become the birthplace of stars and planets.
H I region
Neutral hydrogen regions located in interstellar space. H I regions are vast, cold clouds of hydrogen that line the spiral arms of our own galaxy.
H I region - (n.)
An interstellar region of neutral hydrogen.
H II region - (n.) ...
H I region
An H I region is an interstellar cloud composed of neutral atomic hydrogen . These regions are non-luminous, save for emission of the hydrogen line spectral line....
H II region ...
The element occurs both in stars and as interstellar clouds, in regions where it may be neutral (H I regions) or ionized (H II regions). [A84]
(b) The lightest and most common element in the universe.
The interstellar gas is electrically neutral at points far removed from any star (H I regions) but is highly ionized (the electrons are detached from their atoms) in the immediate vicinity of the most massive and hottest stars (H II regions).
H II regions are named for the large amount of ionised atomic hydrogen they contain, referred to as H II by astronomers (H I region being neutral atomic hydrogen, and H2 being molecular hydrogen).
Depending on the density, size and temperature of a given cloud, the hydrogen in it can be neutral (H I regions), ionized (H II regions) (ie. a plasma), or molecular (molecular clouds).
 See also: Element, Wavelength, Electron, Ionized, H II Region
  
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