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How do radio telescopes detect hydrogen gas in the Milky Way Galaxy?

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Radio telescope Lovell Telescope, a fully steerable radio telescope at Jodrell Bank, Macclesfield, Cheshire, England.

Radio telescopes detect hydrogen gas in the Milky Way by observing 21-centimetre radiation, a specific radio wavelength emitted by cold, neutral interstellar hydrogen atoms.12

This emission occurs due to a hyperfine transition within the hydrogen atom. Normally, the atom is in its lowest energy state when the spins of its proton and electron are antiparallel. When collisions excite the atom, the spins become parallel, creating a tiny amount of extra energy. When the atom eventually returns to its lower energy state, it releases this energy as a photon with a wavelength of 21 centimetres (a frequency of approximately 1,420 megahertz).1

Because this radio wavelength is long, it easily penetrates the interstellar dust that obscures optical observations, allowing astronomers to map the galaxy's spiral structure. Furthermore, by using sensitive spectrometers to measure changes in the wavelength of these lines caused by the Doppler effect, astronomers can determine the rotational velocities and motions of hydrogen clouds throughout the Milky Way.123