Jan Oort

Jan Oort was a Dutch astronomer who made fundamental discoveries about the structure and dynamics of the Milky Way and helped pioneer radio astronomy. He demonstrated that the galaxy rotates differentially rather than as a rigid body, overturning the earlier assumption that the Sun sat near the galactic center, and calculated the Sun's distance from the galactic core and its orbital period. In 1932 he inferred from the motions of stars that considerably more mass must be present than could be seen, an early argument for what is now called dark matter. He also proposed the existence of a vast, roughly spherical shell of comets far beyond Neptune, now called the Oort cloud, to explain the origin of long period comets. Oort studied under Jacobus Kapteyn at Groningen, earning his doctorate in 1926, joined Leiden University in 1924 and became a full professor there in 1945, led Dutch radio astronomy efforts after the Second World War, collaborated with Hendrik van de Hulst on the detection of the 21 centimeter hydrogen line, and served as General Secretary of the International Astronomical Union from 1935 to 1948.