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Neutral hydrogen atoms emit electromagnetic radiation at a wavelength of 21.1cm as a result of the atom's single electron changing from having spin parallel to that of the nucleus, to having spin antiparallel to that of the nucleus. (In other words, the electron turns around and starts spinning the other way.) While this is an incredibly rare transition, neutral hydrogen accounts for a disproportionately large percentage of the universe's mass and creates a strong galactic emission line.

This wavelength is in the range of wavelengths that are most useful to radio astronomy, and neutral hydrogen observations make up a bulk of the work done at radio telescopes. 21cm radiation is particularly useful since there are no other strong emission lines near it, which means (assuming your RFI countermeasures are good) that a signal anywhere in the vicinity of 21cm is almost assuredly from neutral hydrogen. Armed with this, one can calculate the redshift of the object and estimate its distance using Hubble's law.

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