What radio line first mapped the Milky Way's spiral structure after 1952?

The radio line that first mapped the Milky Way’s spiral structure after 1952 was the 21-centimeter line. It is emitted by neutral atomic hydrogen, or H I, when the proton and electron reverse their relative spin alignment.

The line has a wavelength of about 21 centimeters and a frequency of 1420.4 MHz. Hendrik van de Hulst predicted it in 1944, and Harold Ewen and Edward Purcell detected it at Harvard in 1951. Dutch and Australian teams soon confirmed the result.

Radio waves at this wavelength can pass through much of the interstellar dust that blocks visible-light views across the Galactic plane. By measuring the line’s strength and Doppler shift in different directions, astronomers traced hydrogen clouds, estimated their motions, and reconstructed the Galaxy’s large-scale structure.

A common mix-up is confusing the 21-centimeter line with H-alpha, which is an optical emission line from ionized hydrogen. H-alpha is valuable for star-forming regions, but the dust-penetrating neutral-hydrogen line was the key tool for the first broad radio maps of the Milky Way’s spiral arms.