The Hydrogen line, also known as the 21-centimeter line, refers to the specific wavelength (21 centimeters) of electromagnetic radiation emitted by neutral hydrogen atoms. This emission occurs when the electron in a hydrogen atom transitions between two hyperfine levels of its ground state. The 21-cm line is significant in astronomy because it is a key spectral line used for studying the distribution and structure of hydrogen in the universe.

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Hydrogen line by Ciro Santilli 37 Updated 2025-07-16
21 cm is very long and very low energy, because he energy split is very small!
Compare it e.g. with the hydrogen 1-2 spectral line which is 121.6 nm!