HULST, HENDRIK CHRISTOFFEL VAN DE

born Nov. 19, 1918, Utrecht, Neth. Dutch astronomer who predicted theoretically the 21-centimetre radio waves produced by interstellar hydrogen atoms. His calculations later proved valuable in mapping the Milky Way Galaxy and were the basis for radio astronomy during its early development. In 1944, while still a student, van de Hulst made theoretical studies of hydrogen atoms in space. The magnetic fields of the proton and electron in the hydrogen atom can align in either the same or opposite directions. Once every 10,000,000 years or so a hydrogen atom will realign itself and, van de Hulst calculated, emit a radio wave with a 21-centimetre wavelength. Van de Hulst was appointed a lecturer in astronomy at the University of Leiden, Neth., in 1948 and four years later was made a professor there. In addition to his work in radio astronomy, he made valuable contributions to the understanding of light scattering by small particles, the solar corona, and interstellar clouds.