Jacobus Cornelius Kapteyn
(1851-1922)

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.

Apart from being an innovative astrophysicist, Jacobus Kapteyn was a considerable figure in the promotion of international cooperation in astronomy, and one of Holland's foremost men of science.

The abiding interest of his life was the investigation of the structure of the Universe and the list of his achievements reflects this. A doctor of physics, he only entered the astronomical field when aged 24; his inaugural address on stellar parallax marked the keystone of his career.

The University of Groningen could boast no observatory at the time Kapteyn was appointed and, failing to obtain proper funding for observatory buildings, he established an astrometrical laboratory where he hoped to exploit the full potential of photography in astronomy, then in its early days. His association with David Gill at the Royal Observatory, Cape of Good Hope developed from his offer to measure and reduce plates taken there. Thirteen years of work yielded the Annals of the Cape Observatory, a catalogue of nearly half a million stars between 18°S declination and the Pole, work done with help only from unskilled assistants.

During the years of the measurement and reduction of the Cape photographic plates Kapteyn was also active in other fields, particularly the photographic determination of parallaxes. In 1882 the parallaxes of only 34 stars were known: by the end of his life the Groningen Publications had carried papers giving more than 10,000 parallax measures.

All this work tended to the general aim of improving our knowledge of the stellar system. Growing naturally from his work on annual parallax, the study of proper motion next claimed his attention. The determination of absolute proper motions of neighbouring stars will yield the Sun's motion towards the solar apex and in turn will give an ever-increasing baseline for the determination of secular parallaxes. This work directly led to the greatest single discovery of his life, that of 'star streaming'. It had previously been assumed that the proper motions of stars would be random. By analysis of proper motions in selected areas of the sky, Kapteyn showed that stars were moving in two streams 140° apart. This is today recognized as direct evidence of the rotation of our Galaxy, though the full significance of the discovery was not realized until after Kapteyn's death and the demonstration that some nebulae were extragalactic, being indeed galaxies in their own right.

Kapteyn proposed international collaboration in the analysis of the properties of the stars. Their magnitude, proper motion, parallax, spectral class and radial velocities were to be determined in measurements carried out according to his 'Plan of Selected Areas'. He selected 206 areas distributed uniformly over the whole of the sky and 46 areas near the galactic plane, and the results from all these zones were to be treated statistically to form a picture for the whole sky. This big effort represented the first major international collaborative project in astronomy, involving over 40 observatories.

Only weeks before his death, some of the results of his 'Plan' were published under the title 'First Attempt at a Theory of the Arrangement and Motion of Sidereal System' in the Astrophysical Journal. The conclusions 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 resulted 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. Though the general scheme of our Galaxy which Kapteyn outlined was shown to be erroneous by the work of Leavitt, Shapley and Hubble, the importance of the huge amount of work done by him was in the solid foundation of astrometric data and analysis on which later generations have built with confidence.

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.