H I region

An H I region is an interstellar cloud composed of neutral atomic hydrogen (HI). 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 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).

Mapping HI 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.

References

Anderson, Kevin J & Churchwell, Ed (1985). "The Anatomy of a Nebula". Astronomy 13: 66-71. 

See also

• H II region

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