BornBarneveld, the Netherlands, 19 January 1851
Dutch astronomer Jacobus Kapteyn made his most important contributions to the study of stellar statistics, i.e., the determination of the numbers and types of stars in different parts of space and their motions. His name is attached to the Kapteyn selected areas (particular directions in the sky that are informative for studying stellar statistics) and to the so-called Kapteyn universe (his reconstruction of the stellar distribution, which put the Sun very near the center of a rather small galaxy). The former are still used.
Kapteyn was the son of Gerrit J. and Elisabeth C. (née Koomans) Kapteyn, who conducted a school for boys. Jacobus married Catharina Elise Kalshoven. They had a son and two daughters, one of whom married Ejnar Hertzsprung . At the age of 16, Kapteyn passed the entrance examination for the University of Utrecht. There he studied mathematics and physics, receiving a PhD (magna cum laude) in 1875 with a thesis on the vibration of a membrane.
Kapteyn accepted a staff position at Leiden Observatory in 1875. As a result, he made astronomy a career; he was appointed in 1878 to the newly instituted professorship of astronomy and theoretical mechanics at the University of Groningen. Kapteyn created, and became director of, the Astronomical Laboratory at Groningen in 1896 and held both positions until his retirement in 1921.
Kapteyn’s major contributions were in the domain of galactic research. His work presented the first major step after those of William and John Herschel . At the time that Kapteyn initiated his ambitious, systematic program, the execution of which would become his life’s work, the problem of the space distribution of the stars was still tantamount to the problem of the structure of the Universe. It was not known yet that the galaxy was only one of the countless stellar systems that populate the universe. Milestones in Kapteyn’s research were the discovery, in 1904, of the so-called star streams, the determination of the stellar luminosity function, the study of isolated, loose groups of massive, hot B-stars, and the model of the Galaxy presented in his article “First Attempt at a Theory of the Arrangement and Motion of the Sidereal System” (published in the Astrophysical Journal of May 1922).
Even before he made his discovery of the star streams, Kapteyn had accomplished a major reference work known as the Cape Photographic Durchmusterung [CPD] in collaboration with David Gill , director of the Royal Observatory in Cape Town, South Africa. Since the University of Groningen (in spite of Kapteyn’s request) could not provide him with a telescope, he offered to Gill to undertake at Groningen the measurement of stellar positions on photographic plates taken by Gill. Their purpose was to provide for the southern sky the data on stellar positions and brightness, which for the northern sky had been measured by visual – not photographic – means several decades earlier by Friedrich Argelander at Bonn Observatory and known as the Bonner Durchmusterung. For these measurements, Kapteyn devised an unconventional method using a theodolite, thus obtaining equatorial coordinates directly and skipping the intermediate phase of rectangular coordinates. The CPD, published in three volumes in the years 1896–1900 after 13 years of collaboration, contains 454,875 stars between the South Celestial Pole and the declination −18°. This project may be regarded as the first step toward the establishment of Kapteyn’s unique astronomical laboratory that soon would gain international fame.
As a first step in the estimation of the distances of the stars, a conventional method used the stars’ proper motions, i.e., their displacements on the sky. A large proper motion is a strong indicator of proximity of the star to the Earth; small proper motions generally indicate remoteness. Kapteyn applied this method using improved proper motions partly measured at his laboratory. This led to a major discovery: It had been assumed, more or less tacitly, by earlier investigators that stellar motions are similar to molecular motions in that they show no preferential direction. Kapteyn discovered that this is not so: A preferential direction exists which he interpreted as evidence for relative motion between two intermingled stellar populations. The full understanding of this phenomenon came in the 1920s in the context of the dynamical theory of the notation of the galaxy.
For the exploration of the structure and dimensions of the galaxy, Kapteyn devised statistical methods using large numbers of stars with known apparent magnitudes, colors, proper motions, and trigonometric parallaxes. In order to arrive at an unbiased yet sufficiently limited sample, he proposed a scheme called The Plan of Selected Areas, according to which these data would be assembled for all stars within the limits of observation in 206 small areas evenly distributed on the sky. The proposal met with considerable response, so that eventually 43 observatories collaborated in one way or another. After Kapteyn’s death, Commission 32 of the International Astronomical Union was created for the supervision and extension of the project.
Kapteyn’s ultimate aim was the determination of the stellar density distribution in the galaxy. Observational data required were the numbers of stars at different apparent magnitudes in different directions, combined with the distribution of their proper motions. The approach was essentially numerical; no model was presupposed. An important intermediate quantity to be determined was the “Luminosity Function,” which describes the distribution of the intrinsic luminosities of the stars contained within a given volume of space. It has proven to be a most important piece of information for the study of the so-called Initial Luminosity Function, the distribution of stellar luminosities, and hence of stellar masses – at the time of their birth. According to Kapteyn’s model, arrived at around the year 1921, the Galaxy showed a disk-like structure with the Sun located close to the center. Its greatest extension was in the direction of the Milky Way (about 30,000 light years). Its smallest dimension (about 5,000 light years) was in the directions of the galactic poles. The latter result, which might be called the “thickness” of the galaxy, has been confirmed and refined by later authors including Kapteyn’s pupil Jan Oort . However, Kapteyn’s results for the position of the Sun and the extent of the system in the directions perpendicular to the pole have been found to be spurious because he neglected the absorption of light by interstellar matter. Kapteyn was aware of the problem of the possible existence of such matter and vigorously pursued methods to identify it through its reddening effect on the colors of distant stars but without conclusive results.
Kapteyn received numerous honors from scientific societies and universities all over the world. He was a celebrated lecturer to audiences of all kinds.
At the invitation of George Hale , founder of the Mount Wilson Observatory, Kapteyn paid annual visits of several months duration to Mount Wilson until these were interrupted by World War I. He firmly believed it to be the duty of scientists to bridge gaps caused by political developments and was deeply shocked when, upon termination of the war, the Central Powers were excluded from newly created international organizations.
The archives of the Kapteyn Institute of Groningen University contain notebooks used by Kapteyn in the years 1907–1922, in which he jotted down quick calculations and drafts for articles and letters. Also kept here are copies of the correspondence of Kapteyn with leading astronomers all over the world.