Kapteyn , Jacobus Cornelius

(1851–1922) Dutch astronomer

Born at Barneveld in the Netherlands, Kapteyn studied at Utrecht University and became professor of astronomy at the University of Groningen in 1878. He was a very careful stellar observer and using David Gill's photographs of the southern hemisphere skies, he published in 1904 a catalog of over 450,000 stars within 19 degrees of the south celestial pole. He repeated William Herschel's count of the stars by sampling various parts of the heavens and supported Herschel's view that the Galaxy was lens-shaped with the Sun near the center; but his estimate of its size was different from Herschel's – 55,000 light-years long and 11,000 light-years thick. He pioneered new methods for investigating the distribution of stars in space.

Kapteyn also discovered the star, now called Kapteyn's star, with the second greatest proper motion – 8.73 seconds annual motion compared to the 10''.3 of Barnard's star. He found this as part of a wider study of the general distribution of the motions of stars in the sky. To his surprise he found, in 1904, that they could be divided into two clear streams: about 3/5 of all stars seem to be heading in one direction and the other 2/5 in the opposite direction. The first stream is directed toward Orion and the second to Scutum, and a line joining them would be parallel to the Milky Way. Kapteyn was unable to explain this phenomenon; it was left to his pupil Jan Oort to point out that this is a straightforward consequence of galactic rotation.