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The interstellar matter is made mainly of Hydrogen and, in smaller measure, Helium and minimal
part of heavy elements and dust.
At the equilibrium, several phases of interstellar matter with different density and temperature can coexist.
Therefore, using a classification based on the Hydrogen atomic states, it is possible to distinguish
the molecular regions formed from molecular Hydrogen, the HI regions in which Hydrogen
is neutral, and the ionized Hydrogen regions HII.
Densities and temperatures of the dominant phases of galactic and intergalactic diffuse media.
For each class of objects, the characteristic size in log(cm) is given.
Figure from [Dopita & Sutherland, 2003]