Jacobus Cornelius Kapteyn was born in Barneveld, the Netherlands on 19th January 1851. He studied at the University of Utrecht from 1868. He moved to the Leiden astronomical observatory in 1875, and three years later became the first Professor of Astronomy and Theoretical Mechanics at the University of Groningen.
Kapteyn's initial work was analysing photographs of Southern hemisphere stars taken at the Cape Town Observatory in South Africa. This resulted in the publication between 1896 and 1900 of the Cape Photographic Durchmusterung, listing positions and magnitudes for nearly 455,000 stars.
His subsequent work, published in 1904, used the technique of trigonometric parallax to measure the proper motion of stars. He found the first evidence that proper motions were not random, but that stars could be divided into two streams apparently moving in opposite directions. This represented the first evidence for the rotation of our galaxy.
In 1906 Kapteyn started a major study of the distribution of stars in the galaxy, using counts of stars in different directions. This involved measuring the magnitude, spectral type, radial velocity and proper motion of stars in 206 zones. It represented the first major international collaborative project in astronomy, involving over 40 observatories.
The conclusions were published in 1922, and described a lens-shaped island universe, whose density decreased away from the centre. This galaxy was thought to be 40,000 light-years in size, the sun being relatively close (2,000 light-years) to the centre. The discovery of interstellar extinction after Kapteyn's death resuled in the estimated size of the galaxy being increased to 100,000 light-years, and the sun being relegated to a distance of 30,000 light years from its centre.
Kapteyn was a member of the French Academy of Science, a Fellow of the Royal Society and a founder member of the International Astronomical Union. He died in Amsterdam on 18th June 1922.
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