Astronomer Biography: Jacobus Kapteyn

Jacobus Cornelius Kapteyn was born on January 19, 1851 in Barneveld, Gelderland. In 1868, at the age of 17, he attended the University of Utrecht, where he studied physics and mathematics. He finished his thesis in 1875 and then took a job at the Leiden Observatory, where he worked for three years. He then moved on the University of Groningen as the first Professor of Astronomy and Theoretical Mechanics. He remained in this position until his retirement in 1921.

            So in the period from about 1896 to 1900, Kapteyn really wanted to work on observing astronomical objects, but he lacked an observatory to work from. A position opened up at the Royal Observatory at the Cape of Good Hope to measure photographic plates taken by David Gill, and Kapteyn decided to volunteer for it. Gill was conducting a photographic survey of southern hemisphere stars, and Kapteyn was eager to get in on this project. This apparently served both Kapteyn and Gill well, because their collaboration resulted in the publication of a catalog called Cape Photographic Durchmusterung, which listed magnitudes and positions of approximately 454,875 stars.

            While working on this project, Kapteyn actually discovered his own star (aptly named “Kapteyn’s Star”) in 1897. The significance of this star was that it had the highest proper motion of any star known at the time. This must have intrigued Kapteyn, because he went on to study the proper motions of stars; in 1904 he concluded that the proper motions of stars were not in fact random, which was the common belief at the time. He also determined that that stars could be separated into two different streams that moved in almost opposite directions. Unnoticed at the time, this data actually ended up being the first evidence of the rotation of our Galaxy (later followed by the actual discovery of galactic rotation by Jan Oort and Bertil Lindblad). In 1906 Kapteyn decided to take this proper motion study even further—he commenced a proposal for the significant study of the distribution of stars in the Galaxy, measuring spectral type, apparent magnitude, proper motion, and radial velocity of stars in two hundred and six different zones and directions. This was a pretty big project, and since it involved the cooperation of more than forty different observatories, it was the first coordinated statistical analysis in astronomy.

            In 1922, Kapteyn published basically all of the information he had gathered throughout his life, titling it First Attempt at a Theory of the Arrangement and Motion of the Sidereal System. In this work, he discussed an “island universe” shaped like a lens, in which the density decreased moving away from the center. This model of the universe is now known as the Kapteyn Universe: our galaxy was approximated to be 40,000 light years in size, with the Sun about 2,000 light years in proximity to the center. (Later, long after Kapteyn’s death, an astronomer by the name of Robert Trumpler realized that the assumed amount of interstellar reddening had been a great underestimation, and increased the estimate of the size of the galaxy to 100,000 light years, placing the Sun at a distance of 30,000 light years from the center.)

            Jacobus Kapteyn won several awards for his work: the Gold Medal of the Royal Astronomical Society in 1902, the James Craig Watson Medal in 1913, and the Bruce Medal in 1913. He also had several astronomical things named after him: the Kapteyn crater on the Moon; Asteroid 818 Kapteynia; Kapteyn’s Star, of course; an institute was named in his honor at the university he taught at-- the Kapteyn Astronomical Institute at the University of Groningen; a telescope at La Palma, one of the Canary Islands—the Jacobus Kapteyn Telescope; and finally the Kapteyn Package, an astronomical package for Python.

            Jacobus Cornelius Kapteyn died in Amsterdam on June 18, 1922, at age 71. He left behind a legacy that forever changed astronomy and the way we view the galaxy today.