Van de Hulst, Hendrik ("Henk") Christoffel (1918–2000)
Dutch astronomer, born and educated in Utrecht, who made his most important
contribution while still a student: his 1944 prediction that clouds of cold
hydrogen gas in space should emit and absorb a spectral line at a wavelength
of 21 cm (see 21-cm line). Following the
detection of this radiation in 1951 by American physicists Edward Purcell
and Harold Ewen (1922–1997), the Dutch team of van der Hulst, Jan
Oort, and C. Alex Muller began mapping the
distribution of neutral hydrogen clouds in the Milky Way and delineating
its spiral structure. Subsequently, van de Hulst made studies of interstellar
grains and their interaction with electromagnetic radiation, investigated
the solar corona, and wrote important books
on light scattering and radio astronomy.
Based at the University of Leiden from 1948 until his retirement in 1984,
he was a leader in several international organizations and in the development
of Dutch and European space research.