1900- . Dutch astronomer. In 1927, he
calculated the mass and size of our Galaxy,
the Milky Way, and the Sun's distance from
its centre, from the observed movements of
stars around the Galaxy's centre. In 1950
Oort proposed that comets exist in a vast
swarm, now called the Oort Cloud, at the edge
of the solar system. In 1944 Oort's student
Hendrik van de Hulst calculated that hydrogen
in space would emit radio waves at 21 cm/8.3
in wavelength, and in the 1950s Oort's team
mapped the spiral structure of the Milky Way
from the radio waves given out by
interstellar hydrogen.