How did radio observations reveal the Milky Way’s spiral arms?

Detecting the Milky Way’s spiral structure is difficult using optical telescopes because interstellar dust obscures starlight and the solar system is located within the galactic disk. Astronomers were unable to confirm the Galaxy's spiral nature until 1953, when they began utilizing radio telescopes to map the distribution of neutral hydrogen atoms.12

Radio waves at the 21-centimetre line are long enough to penetrate interstellar dust, allowing astronomers to observe gas concentrations throughout the Galaxy. By measuring the velocities of this neutral hydrogen, researchers could estimate the distances to these gas clouds when used in conjunction with a rotation curve for the Galaxy. These radio maps revealed the large-scale structure of the Milky Way, showing that the gas is organized into spiral arms. Additionally, molecular clouds are also used in these radio studies to discern the Galaxy's structure through the dust.132