Neutral hydrogen

Neutral hydrogen consists of hydrogen atoms with one bound electron and no net electric charge. The electron–proton spin interaction produces a spectral line at 21 centimeters, widely used to trace hydrogen gas in space.

Hyperfine structure: Small energy-level splittings caused by interactions between a nucleus and the electrons in an atom. The line is a hyperfine transition between two relative orientations of electron and proton spins.
21-centimeter astronomy: The study of astronomical hydrogen gas through its 21-centimeter radio emission or absorption. It turns the atomic line into a tool for measuring cosmic gas.
Hydrogen atom: The simplest atom, consisting of one proton and one electron in its neutral form. Neutral hydrogen’s ground-state structure supplies the proton and electron whose spins interact.
Ionized hydrogen: Hydrogen from which the electron has been removed, leaving a positively charged proton. Unlike neutral hydrogen, it lacks the bound electron needed for the 21-centimeter transition.
21-centimeter line: The radio spectral line emitted or absorbed when a neutral hydrogen atom changes between its two ground-state hyperfine levels. This is the characteristic signal produced by neutral hydrogen’s spin transition.
Neutral hydrogen cloud: A region of interstellar or intergalactic gas in which hydrogen is predominantly un-ionized. Such clouds are the structures traced by 21-centimeter observations.
Proton: A positively charged particle in atomic nuclei, composed of quarks bound by the strong interaction. Its nuclear spin combines with the electron spin to split hydrogen’s ground state.
Molecular hydrogen: Hydrogen gas whose atoms are bound in H₂ molecules. It is another major interstellar form of hydrogen, commonly traced through molecular emission rather than the atomic line.
Spin temperature: A temperature describing the relative populations of atomic hydrogen’s two hyperfine levels. It determines whether the 21-centimeter signal appears in emission or absorption.
21-centimeter emission line: Radio emission produced when neutral hydrogen atoms undergo the 21-centimeter hyperfine transition. Its intensity and Doppler shift reveal hydrogen column density and motion.

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