His main field of study was interstellar matter. In 1944, while a student of J. H. Oort, he calculated that neutral hydrogen should emit radio waves with a wavelength of 21 cm. His prediction was borne out in 1951 when the American physicists Edward Mills Purcell (1912–97) and Harold Irving Ewen (1922–2015) detected 21-cm radio emission from interstellar hydrogen clouds. The same year, van de Hulst and Oort began to use Doppler shifts in hydrogen emission to map the Galaxy’s spiral structure.