An
HI region (read H one) is a
cloud in the
interstellar medium composed of neutral atomic
hydrogen (HI), in addition to the local abundance of
helium and other elements. These regions do not emit detectable
visible light (except in
spectral lines from
elements other than hydrogen) but are observed by the
21-cm (1,420 MHz) region spectral line. This line has a very low transition probability, so requires large amounts of hydrogen gas for it to be seen. At ionization fronts, where HI regions collide with expanding ionized gas (such as an
H II region), the latter glows brighter than it otherwise would. The degree of ionization in an HI region is very small at around 10
-4 (i.e. one particle in 10,000). At typical interstellar pressures in galaxies like the
Milky Way, HI regions are most
stable at
temperatures of either below 100 K or above several thousand K; gas between these temperatures heats or cools very quickly to reach one of the stable temperature regimes. Within one of these phases, the gas is usually considered
isothermal, except near an expanding
H II region. Near an expanding H II region is a dense HI region, separated from the undisturbed HI region by a shock front and from the H II region by an ionization front.