An H I region (pronounced "aitch one region") is an interstellar cloud composed of neutral atomic hydrogen (H₁). These regions are non-luminous but are detectable because molecules present in small quantities emit radiation in the 21-cm (1,420 MHz) region of the radio-spectrum, even at low temperatures. 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 a neutral hydrogen H I region is very small at around 10^-4 (ie one particle in 10,000), but plasma physicists consider it to be a weakly ionized plasma.

Mapping H I emissions with a radio telescope is a technique used for determining the structure of spiral galaxies. It is also used to map gravitational disruptions between galaxies. When two galaxies collide, the material is pulled out in strands, allowing astronomers to determine which way the galaxies are moving.

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Nebulae

H-I-Gebiet | Región H I | H I 영역 | H I područje