H I region

An interstellar region containing predominantly neutral atomic hydrogen. Its hydrogen atoms emit a characteristic 21-centimeter radio line that lets astronomers map the gas.

Hydrogen line: A spectral line produced when a hydrogen atom changes between two energy states. The 21-centimeter line is the signature used to detect neutral hydrogen.
Atomic hydrogen: Hydrogen consisting of individual atoms rather than molecules or ions. H I denotes hydrogen in this neutral atomic form.
Milky Way rotation curve: A plot of orbital speed versus distance from the center of the Milky Way. Doppler measurements of H I trace rotation beyond the bright stellar disk.
Leiden–Argentina–Bonn survey: An all-sky survey of Galactic neutral hydrogen using 21-centimeter observations. Its data provide a detailed view of the Milky Way's H I distribution.
Hyperfine structure: Small shifts in atomic energy levels caused by interactions between nuclear and electronic magnetic moments. Its spin interaction creates the transition responsible for the 21-centimeter emission.
Interstellar medium: The gas, dust, and energetic particles occupying the space between stars in a galaxy. H I regions are one component of this material between stars.
Galactic spiral structure: The arrangement of spiral arms and related features in a disk galaxy. H I maps reveal large-scale gas concentrations along the Milky Way's arms.
Canadian Galactic Plane Survey: A multiwavelength survey of the northern Milky Way's disk and plane. Its high-resolution radio data resolve fine structure in Galactic H I.
21-centimeter radiation: Radio emission at a wavelength of about 21 centimeters from neutral hydrogen's hyperfine transition. This is the characteristic signal used to observe H I gas.
Molecular hydrogen: A molecule made of two hydrogen atoms, the most abundant molecule in the interstellar medium. It is the molecular form of hydrogen, unlike the atomic gas defining H I.

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