Dutch astronomer who interpreted Kapteyn's two streams of stars as evidence that the Milky Way Galaxy was rotating. With C. A. Muller, Oort confirmed Purcell and Ewen's detection of the theoretically predicted 21 centimeter radiation. Oort proposed that comets come from a huge spherical shell surrounding the solar system, now known as the Oort cloud.

Additional biographies: Bruce Medalists, Bonn

Kapteyn, Purcell