January 19th marked the 175th birthday of Dutch astronomer Jacobus Kapteyn. His work and legacy had a profound yet subtle impact on the astronomical community and our understanding of the cosmos. His lasting contributions and methodologies continue to be refined and provide the foundation for ongoing astronomical research and discoveries. After he completed his studies, Kapteyn worked at Leiden Observatory for three years beginning in 1875. He then became the very first professor of Astronomy and Theoretical Mechanics at the University of Groningen. His research expanded humanity’s understanding of our place in the cosmos by orders of magnitude, which is why this blog is dedicated to his 175th birthday!
His most significant studies focused on the proper motion of stars. Proper motion of the stars is the slow but steady change in their position relative to each other within the night sky. This motion is responsible for how constellations change their shape over millenia. The Big Dipper, part of Ursa Major, would have looked very different 50,000 years ago than it does now, and it will look very different 50,000 years in the future. Its signature “dipper” shape is just a fleeting organisation that we have the pleasure to observe in the ever-changing landscape of the cosmos. The movement of the stars themselves was postulated by the ancients, but it was not proven until 1718 by Edmund Halley. Kapteyn expanded upon our knowledge of the proper motion of the stars, with implications that even he did not foresee.
In 1906, Kapteyn organised the first-ever statistical analysis in astronomy and included forty different observatories to gather data on this proper motion. When the data was analysed, he found that the motion of the stars was not random, nor were all stars in general moving in just one direction across the sky in relation to one another. What he found was that all of the stars in the sky generally move in two distinct directions. This data and Kapteyn’s findings would later be reinterpreted as our cosmological understanding was continuing to be refined.
To appreciate the significance of Kapteyn’s analysis, one must consider its historical context: his comprehensive survey and conclusions were published just one year after Einstein’s Special Theory of Relativity. At the time of Kapteyn’s most influential work, the scientific community had yet to reach a consensus on the existence of anything beyond the Milky Way. Objects now recognised as distinct, distant galaxies, such as nebulae and globular clusters, were categorised as local phenomena within our own system. In this era, the Milky Way was effectively synonymous with the universe itself. While Kapteyn’s research did not provide the empirical data to prove the existence of other galaxies, his contributions remained foundational long after our cosmological horizons expanded.
It wasn’t until 1921, a full 15 years after his survey, that Galaxies were recognised as distinct objects of their own, well beyond the distances of the stars that we observe and are familiar with. This radically changed and extended our view of the Universe, and recognised the distinct stars we see as being within our own Galaxy under this new classification: our own Milky Way. With this new understanding accepted by the astronomical community, Kapteyn’s data and findings of the two-directional proper movement of our own Galaxy’s stars over time would help us understand the rotational motion of the Milky Way itself. His research also led to the island universe model, in which the density of our galaxy decreases from the center. Although the Sun’s distance from the galactic center has been more accurately determined since Kapteyn’s era, the finding of a dense core and a diminishing stellar density toward the periphery remains a universal characteristic of galactic structures.
Over the past century, the statistical models and sensitivity of our observations of proper motion have been improved. But the basic method of measuring just how stars move over time continues to provide scientific insights and will continue to be a significant tool in astronomy in the future. Proper motion of the stars in our Milky Way was used to infer the existence of the supermassive black hole at its center, known as Sagittarius A. This has since been directly observationally proven to be true. In addition, proper motion measurement is used to be able to measure the distances of other galaxies away from our own, including their direction of movement in the cosmos. This is how we know how far away the Andromeda Galaxy is from us, as well as, though uncertainties arose lately, that a collision with our galaxy could happen!
In addition to the direct impact of his research, part of his legacy includes the teaching and mentorship of his students, who had a lasting impact on astronomical research. Among his students was Jan Oort, who postulated the existence of a shell of icy bodies at the very outermost edge of our solar system, which contains the long-period comets. The aptly named Oort Cloud is widely accepted to be the very definition of the edge of our solar system, and crossing that threshold is the beginning of true interstellar space. Willem de Sitter was also amongst his students, and is credited with deriving the cosmological constant, and one of the primary driving forces in the initial models that the universe itself is expanding.
While Jacobus Kapteyn completed his earthly journey on June 18, 1922, his remarkable spirit of discovery continues to shine brightly. Today, his name is proudly carried forward by the Kapteyn Astronomical Institute at the University of Groningen and the Jacobus Kapteyn Telescope in the Canary Islands. This ensures that his passion for the heavens inspires every new generation of stargazers.
