means a 20th-century dutch astronomer who mapped the structure and rotation of the milky way and proposed the vast shell of icy debris now named after him.

from born 1900 in franeker, netherlands; in 1927 he confirmed the galaxy rotates around a distant center by tracking star motions, and in 1950 he theorized a spherical reservoir of comets far beyond pluto to explain where long-period comets keep coming from.