H I region

An H I region is an interstellar cloud composed of neutral atomic hydrogen (H). These regions are non-luminous, save for emission of 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 H 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 H region is very small at around 10-4 (i.e. one particle in 10,000).
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of neutral hydrogen (i.e. atoms) in interstellar space that emits photons of wavelength 21 cm.