An Etymological Dictionary of Astronomy and Astrophysics

A component of the → interstellar medium consisting of
an extremely tenuous (density 0.1 to 10 cm^-3) and relatively warm gas (temperature about
8,000 K) filling the space between denser neutral and ionized gas. Hydrogen is partly ionized, partly atomic and observed by the → 21-centimeter line in emission.

See also: → warm; → intercloud medium.