<ul><li>Astronomers use a tool called 21cm radiation to map out the Milky Way.</li><li>Hydrogen atoms in the Milky Way constantly emit 21cm radiation, which can travel through interstellar dust.</li><li>By studying the redshift and blueshift of 21cm radiation, astronomers can determine the rotation and dynamics of the Milky Way.</li><li>The same techniques can be used to estimate the masses of distant galaxies by examining the 21cm radiation emitted by neutral hydrogen gas clouds.</li></ul>

How astronomers make deep maps of the Milky Way

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