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How does the Doppler effect help radio astronomers measure the motion of hydrogen clouds?

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

Radio astronomers use the Doppler effect to measure the motion of interstellar hydrogen clouds by observing shifts in the frequency of 21-centimetre radiation.12

Neutral hydrogen atoms naturally emit radio waves at a specific frequency of 1,420 megahertz. When these hydrogen clouds move relative to an observer on Earth, the frequency of the detected radiation shifts due to their relative velocity.134

Blueshift: If a hydrogen cloud is moving toward the observer, the frequency of the detected radio waves increases.34
Redshift: If the cloud is receding from the observer, the frequency decreases.34

By measuring these frequency shifts, astronomers can calculate the velocity of the gas. Because this radiation penetrates interstellar dust, it serves as a vital tool for mapping the large-scale structure and kinematics of the Milky Way Galaxy. Comparing these measured velocities with the expected dynamics of the galaxy further helps researchers determine the location and distribution of these hydrogen sources.321