PIETER JOHANNES VAN RHIJN, KAPTEYN'S ASTRONOMICAL LABORATORY AND THE <italic>PLAN OF SELECTED AREAS</italic>

Abstract

In this paper I discuss the Kapteyn Astronomical Laboratory during the period of Pieter Johannes van Rhijn's Directorate, which lasted from 1921 to 1957. It had developed under the founder Jacobus Cornelius Kapteyn into one of the leading astronomical research institutes in the world. When van Rhijn took over at the retirement of Kapteyn, it was in the process of coordinating Kapteyn's <italic>Plan of Selected Areas</italic> with the aim of determining the structure of the Sidereal System. Kapteyn had just, with the help of van Rhijn, presented a first model of the distribution of stars in space from existing observations, and on this basis he constructed a 'first attempt' model of the Stellar System including dynamics, i.e. deriving the gravitational field from the distribution of the stars and showing how the random and systematic motions of the stars, the kinematics, could provide equilibrium and stability to the system. Under van Rhijn, the work on the <italic>Selected Areas</italic> progressed well, but the Groningen Laboratorium went into decline, loosing much of its status and prestige. I conclude the following. There was very little choice for Kapteyn's successor and van Rhijn was appointed effectively by default. Kapteyn himself must have seen that the future for astronomy in the Netherlands lay with Leiden Observatory, where already during his lifetime two of his protégées, Willem de Sitter and Ejnar Hertzsprung, were in charge, and to which a third one was added not much later in the person of Jan Oort. Kapteyn had his <italic>Plan of Selected Areas</italic> in addition to the scientific case set up as a way of ensuring the future of the Groningen Laboratorium at least for the time of this effort, arranging it to be supplied with plate material to complete the provision of the data for the final attack on the Sidereal problem. Van Rhijn's research was solid and professional, but in his papers he invariably stopped before discussing how his findings fitted into the larger emerging picture of the structure of the Galaxy. Comparison of his work in the 1930s on the distribution of stars as a function of spectral types and Jan Hendrik Oort's determination of his 1938 crosscut through the Galaxy illustrates this. While this work by Oort constituted the second attempt following on Kapteyn's first, and may be seen as the actual completion of the research for which the <italic>Plan of Selected Areas</italic> was designed, van Rhijn continued adding more and more data in the context of the Plan without redefining the aims in the light of developments. The <italic>Spectral Durchmusterung</italic> carried out in collaboration with the Bergedorf-Hamburg Sternwarte and Harvard College Observatory was an enormous drain on the Groningen resources. Van Rhijn's successful attempt to obtain his own telescope resulted in very relevant and correct determinations of the wavelength dependence of interstellar extinction from photographic spectra, but these results only became available in the 1950s when the field had turned to photoelectric methods. Kapteyn's concept of an astronomical laboratory lost its viability in van Rhijn's time when observa-tories were no longer interested in providing observational material for others to do research. Kapteyn's model on the other hand of a laboratory-like institution, where research is based on material obtained elsewhere on Earth or in space, is now the normal operating mode in astronomical research. The only difference is access to observing facilities primarily, which is now structured and guaranteed (subject to proposal acceptance of course), through national and international organizations rather than access on the basis of one's fame or stature. The cause of the loss of the Kapteyn Laboratorium of its prominent place in the international context was largely van Rhijn's unwavering dedication to complete Kapteyn's work. The <italic>Plan of Selected Areas</italic> had been important for the progress of astronomy, but effectively reached the goals Kapteyn had in mind for it already in the 1930s with Oort's 1932 work on the vertical force field and local 'Oort limit' and his 1938 crosscut through the Galaxy. Van Rhijn was unfortunately hampered throughout almost his entire Directorate by factors that severely limited his attempts to obtain more funding in spite of local support from his university. These were of course in the first place the Great Depression of the 1930s and World War II and its aftermath, while during most of the 1940s he suffered from tuberculosis. But, also, the remote location of Groningen compared to Leiden, where a major infrastructure led by three important protégées of Kapteyn was in place, and the Governmental bias towards support for Leiden over Groningen was an important factor. Finally, I examine the developments in the 1950s and the circumstances that made Adriaan Blaauw accept his appointment as van Rhijn's successor in 1957 and initiate the beginnings of the revival of the Kapteyn Laboratorium under his leadership.