He studied with J. C. Kapteyn, whose work on star streams he continued, finding an overall net motion of the Sun with respect to other stars. In 1927 he showed that high-velocity stars appeared to be rotating about the galactic centre, and went on to estimate the Sun’s distance from the centre and also the Galaxy’s diameter and mass. He also suggested that the Galaxy had missing mass. In the 1950s he and others (including B. J. Bok and H. van de Hulst) used the 21-cm line of interstellar hydrogen to map the Galaxy, revealing its spiral structure. In 1950 he suggested that comets originate in a region now called the Oort Cloud.