Young Dutch astronomers at important positions abroad

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Young Dutch astronomers at important positions abroad

J.H. Oort

Originally published in Dutch in Hemel & Dampkring, 39, 355–363 (1941)

Recently we received the news that Dr. W. H. van den Bos, ‘chief-assistant’ at the Observatory of the Union of South Africa in Johannesburg, has been appointed as successor of Dr. H. E. Wood as director of this Observatory. Almost simultane- ously it became known that a second Dutch astronomer, Dr. D. Brouwer, hitherto associate professor at Yale University in New Haven, Connecticut, has been ap- pointed ordinary professor and also director of the so important Yale Observatory.

Dr. van den Bos studied astronomy in Leiden under Prof. de Sitter and Prof.

Hertzsprung. Guided by the enthusiasm of the latter he already became a passion- ate observer of binary stars during his studies in Leiden. ‘Binary star observers are not made but born’, and van den Bos was certainly such a born binary star observer. During the years 1920–1925 he did more than 3000 measurements with the 10112inch telescope at Leiden, which were published in the Annalen der Lei- dsche Sterrewacht (Leiden Observatory). After his PhD in 1925 on a thesis titled

‘Micrometer measurements of binary stars’ he left in the same year as Leiden observer for Johannesburg.

In 1923 the two directors at that time, Innes and de Sitter, had concluded an agreement between the Union Sterrewag in Johannesburg and the Leiden Obser- vatory, according to which Leiden Observers obtained the right to use the beautiful instruments there, in the climate of Johannesburg, which is so ideally suited for astronomical observations. As a result of this agreement Prof. Hertzsprung had started as a guest at the Union Sterrewag from 1923 to 1925, and his work now has been continuing in Leiden and Johannesburg for almost 20 years by a large

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number of researchers. Van den Bos, who went to Johannesburg as the second Leiden astronomer, strengthened the foundations of this collaboration, which was later continued by van Gent, A. de Sitter and Martin as Leiden observers. His appointment as director justifies the hope that in the future this cooperation will be continued in the same spirit and with the same important results as it developed under Innes and Wood.

When van den Bos arrived in Johannesburg, the new visual 2612inch telescope had just been put into use there. The telescope had been purchased by Innes with the aim to bring the southern observations of binary stars – like all the southern observations far behind the northern ones – to a higher level. Van den Bos devoted himself with great enthusiasm to this task, first for 212years as Leiden observer, and from 1 January 1928 as ‘chief-assistant’ at the Unie Sterrewag. After some time he had the privilege to have the telescope almost exclusively at his disposal. It has not only been his great merit by systematically organizing his observations into a material that lends itself for statistical research, but he has also, with a great sense of responsibility, carefully ensured that all binary stars that were in a critical part of their orbit or of which observations were necessary for other reasons, were also measured in time. The ‘Binary Star Survey’, a healthy continuation of Aitken’s and Hussey’s ‘Lick Survey’, in which all stars brighter than 9m. 0 in the Cape Pho- tographic Durchmusterung would be examined for duplicity, was largely carried out by van den Bos, and uncovered a few thousands new and largely narrow bi- nary stars. If the large backlog of southern regions is now largely made up, this is mainly due to the work of van den Bos that was made possible by Innes’ design.

It is not surprising that he became one of the very first authorities in this field and he did not lack recognition of his merits; in 1928 he received the Gold Medal of the Danish Academy of Sciences for a treatise on ξ Ursae Majoris, and in 1938 he was appointed chairman of the binary star commission of the International As- tronomical Union.

Dr. Brouwer also studied in Leiden, where he chose a completely different direction from the beginning, and under the supervision of Prof. de Sitter and Dr.

Woltjer he devoted himself especially to celestial mechanics. In his thesis ‘Dis- cussion of the observations of the satellites I, II and III of Jupiter, performed on Johannesburg by Dr. R.T.A. Innes in de jaren 1908–1925’ (in 1928 more exten- sively published in the Leiden Annals) he completed a part of the new reduction of observations of Jupiter’s satellites undertaken by the Sitter. Shortly after his PhD defense in 1927 he went as ‘fellow of the International Education Board’ to the Yale Observatory, where he soon became a very well-regarded collaborator;

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in particular he worked there with Prof. E.W. Brown on the study of the lunar motion. After the death of Prof. Brown in 1938, he took over his teaching duties completely, which he had already taken over for a large part for a long time. At the same time Brouwer continued to devote himself with full energy to research, the work of the Observatory.

In 1935 he took the initiative to implement a plan to improve the fundamental positions of fixed stars with the help of asteroids, an idea which – in a slightly different form – had also been worked out by the Russian astronomer Numerov, who was killed shortly afterwards. One of the most difficult tasks in astronomy is to measure the positions of the stars with as much accuracy as is necessary for the determination of the distances of the stars from their proper motions and for the determination of their systematic motions. The astronomer is not satisfied with the so-called ‘relative’ positions and motions – which for these purposes can be recorded with sufficient accuracy with respect to other surrounding stars – but he also needs ‘absolute’ positions and ‘absolute’ motions. The latter must be mea- sured relative to a rest frame of coordinates. It is obvious that this is an extraor- dinarily difficult task, and we are still a long way from a satisfactory solution.

In Brouwer’s plan the question of the determination of these absolute positions is reduced to a measurement of relative positions, i.e. positions of stars relative with respect to sixteen especially suitable asteroids. The rest frame of coordinates hereby – however paradoxical this may sound – is recorded with the use of these asteroids.

Nowhere could this plan have found a better environment than at the Yale Ob- servatory, whose director, Dr. Schlesinger, was the first to improve the accuracy of relative photographic positions to such an extent that parallaxes of stars could be measured on a large scale, and who had also been the pioneer in the determina- tion of star positions by means of large field cameras. The implementation of the plan could therefore be done both practically and theoretically in Yale. Practical:

with the large parallax telescope of the southern station of the Yale Observatory in Johannesburg, while with the large field camera there the positions of the compar- ison stars used in the large telescope could be connected to fundamental stars; for the northern positions: observations with the wide-angle camera in New Haven were supplemented with plates recorded with the large photographic telescope of the Allegheny Observatory, while also Leiden, where Dr. van Herk regularly is taking plates of Brouwer’s asteroids with the 1312inch telescope, has taken a part in the program. Brouwer obtained the cooperation of Dr. W.J. Eckert (then as- sociated with Columbia University in New York, now director of the American Ephemeris), and of the Astronomical Hollerith Computing Bureau, led by Eckert.

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The work on the asteroids thus became a bridge between the theoretical work of Prof. Brouwer and the practical astrometric work that forms such a large part of the program of the observatory he will now lead.

The appointments of these astronomers, who grew up in the Netherlands, as directors of two important astronomical institutes abroad, are not the only ones of their kind. No less than five directorships of foreign observatories are held by ‘younger’ astronomers trained at Dutch universities, while in 1942 another sixth, which I have to leave unnamed for the moment, will take up this position at an important American astronomical institute. The directorate of the Bosscha Observatory in Lembang, filled by Dr. A. de Sitter, is not taken into consideration because this is an institute belonging to the Dutch state. Something about the three others follows here about their work and careers.

Dr. J. Schilt has been director of the Rutherfurd Observatory since 1931 in New York and is a professor at Columbia University there. Schilt studied as- tronomy mainly in Groningen, then was assistant to the Leiden Observatory from 1922 to 1925; in 1924 he obtained his doctorate in Groningen with a thesis enti- tled ‘On a thermoelectric method of measuring photographic magnitudes’. With this for the first time an electrical method to measure size and blackening of stars on photographic plates was made suitable for practical use; the thermoelectric photometers constructed under his supervision, first in Groningen, later in Lei- den, soon found their way into many observatories, given the now so well-known name of Schilt photometer.

A year after Schilt left for the Mount Wilson Observatory in 1925 with a Rock- efeller scholarship, he was appointed research assistant at the Yale Observatory. In 1931 he became director of the Columbia University Observatory. Next to exten- sive photometric work he did there with the 13 inch photographic telescope built and completed in 1869 by L.M. Rutherfurd (the ‘inventor’ of the photographic method of determining star positions) new repeat observations of the important series of photographs of the Pleiades and Praesepe. With the help of these plates, still taken on so-called ‘wet’ plates at the time, probably the oldest star exposures that are still of practical importance today, Schilt and his collaborator Titus man- aged to determine proper motions with average errors of no more than00.0005 and to determine internal motions in both star clusters.

Dr. W. J. Luyten also distinguished himself, although in a completely different way, in the field of proper motions. Already in the time when he worked as a

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young astronomer at the Lick Observatory, he dedicated himself in particular to the study of stars with large proper motion (Lick Bull. No. 344 was published in 1923, in which 749 stars with proper motions of more than00.500 per year was cataloged.) His greatest work in this field was only completed during the ten years or so that he has now been director of the observatory of the University of Minnesota. Using plates recorded with the 24-inch Bruce telescope of the southern station of the Harvard Observatory, he searched the whole of the southern sky for stars with considerable proper motion; the research included almost all stars brighter than 14m. 5 photographically and a large number of fainter ones. He found about one hundred thousand proper motions (most larger than 00.050), for which an estimated ten million stars had to be observed! Among these proper motions, more than 500 hitherto unknown motions of more than 00.500 per year were of particular importance for our understanding of intrinsically faint stars.

Luyten completed his astronomical studies in Leiden, where he arrived in 1918, after having studied for several years for his candidate [bachelor] degree in Amsterdam, and in 1921 he obtained his PhD with Prof. Hertzsprung on ‘Ob- servations of variable stars’; the detailed results of his observations, which he had already started at the age of 16, appeared in the Leiden Annales. With his innate sense of adventure he left for America in 1921 at his own risk, where he was soon able to continue his astronomical work with a ‘fellowship’ at the Lick Observa- tory. From 1923 to 1931 Luyten was affiliated with the Harvard Observatory.

Also Dr. P. van de Kamp, who since 1937 is director of the Sproul Obser- vatory at Quaker College in Swarthmore, near Philadelphia, became well known for his work on proper motions, albeit in a completely different area than Luyten.

I will only mention here the great study during his long stay at the McCormick Observatory in Virginia together with Vyssotsky, a study of the proper motions of faint stars, initially inspired by a suggestion published by Kapteyn shortly before his death. The faint stars measured by van de Kamp and Vyssotsky occurred on plates taken in order to determine the parallax of bright stars; by determining the relative positions of the faint stars with respect to the artificially attenuated im- age of the parallax star and by repeating this on plates recorded later, the absolute proper motions of the stars could be determined, because they were known with respect to the bright star. Using these proper motions the authors were able to de- rive improved values for the rotation of the Milky Way Galaxy and for the average parallaxes of faint stars.

Van de Kamp had studied in Utrecht; he subsequently became assistant to van Rhijn in Groningen from 1922 to 1923, and it seems to me that it was especially

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his relationship with the Groningen Astronomical Laboratory that determined the direction of his further astronomical work; in 1926 he returned to Groningen to obtain a PhD. In 1923 he had gone to America, initially as an assistant for a year at the McCormick Observatory, where he soon became part of the permanent scientific staff, as well as taking a share in astronomical teaching at the University of Virginia.

It would be a very arbitrary one-sided thing to limit myself to director positions in this discussion. Two other younger Dutch astronomers, Dr. G.P. Kuiper and Dr.

B.J. Bok occupy positions that are at least as important.

Kuiper, who studied in Leiden from 1924 to 1933 and worked there as an as- tronomer, soon showed himself to be a very skillful observer, and on top of that he had a talent for obtaining observations in such a way that they led to especially in- teresting results. Characteristic for his work is the conscientious way with which he has always set up his programs for an even more far-reaching purpose. In his time in Leiden his interest was mainly focused on binary stars, of which he made positional measurements and in particular brightness measurements. The ideal that he had in mind at that time was the statistics of binary stars, on which he ob- tained his doctorate with Prof. Hertzsprung in 1933. In the same year he went to California as a binary star astronomer, where he had been appointed ‘Martin Kel- log fellow’ at the Lick Observatory. Another ideal, which occupied him next to the binary statistics, was to obtain better data bearing on the distribution of the stars in the Hertzsprung-Russell diagram, and to penetrate into its deeper significance. His work in this field led, among other things, to an enormous increase in our knowl- edge of the white dwarfs – that remarkable family of stars of which only three or four members were known before Kuiper’s discoveries; it also led to a discussion about the probable connection between the character of the Hertzsprung-Russell diagram in star clusters with the hydrogen content of the stars belonging to such a cluster, and recently to the discovery of the so-called ‘sub-dwarfs’.

Kuiper is now associate professor and a member of the scientific staff of the Yerkes Observatory and the closely associated McDonald Observatory in Texas.

Dr. Bok, who is an ordinary professor at Harvard University and leader of a team for research of the Galaxy at the Harvard Observatory, initially studied in Leiden; a year after his candidate (bachelor) exam he became van Rhijn’s assis- tant at the Kapteyn Laboratory in Groningen, where he continued his studies from 1927 to 1929. In this last year he went to America, first as a temporary, later as a permanent employee at the Harvard Observatory. His gift to teach and to lead

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students flourished at Harvard, where in those very years the teaching of astron- omy underwent a strong improvement; an important number of young American astronomers owe the direction of their work to Bok. In the later years this work was especially aimed at the study the detailed structure of the Galaxy by means of star counts, for which a large number of plates were available at Harvard, and with the many instruments present at Cambridge, Oakridge and the southern Harvard station at Bloemfontein all the new material needed could easily be collected.

During a leave of absence in the Netherlands in 1932 Bok received his doc- torate from Prof. van Rhijn on a dissertation entitled ‘A study of the η Carinae region’.

Among the other young Dutch astronomers currently working abroad I would also like to mention Dr. H. Zanstra, although he does not occupy a permanent position – as far as is known. Zanstra was educated at the Technical Highschool in Delft, and only later became an astronomer, largely through studies at his own initiative. He worked a lot abroad, a.o. as a mathematics professor in Seattle, from where he occasionally crossed to Victoria to make observations of gas neb- ulae with the large reflector. For his astronomical development he owed a lot to Dr. Baade in Bergedorf; however, his most important work was more theoretical:

the composition and the structure of planetary nebulae, and research on cosmic rays, on which he worked for some years as an assistant in Amsterdam with Prof.

Clay. In order to be able to work again in the astronomical field and especially to continue his studies on gaseous nebulae, he left Amsterdam in 1937 and accepted an appointment as ‘Radcliffe Traveling Fellow’; as such he worked for some years in Oxford and now at the Radcliffe Observatory in Pretoria.

It might be good to emphasize that in the above, by no means all Dutch as- tronomers abroad have been discussed. For example, the limitation to one or more subsequent generations was responsible for not mentioning the staff member of many years years at the Mount Wilson Observatory, Dr. A. van Maanen, who, despite working at Mt. Wilson for almost thirty years, continued to feel bound to his homeland with so many ties. As a result of this, also no mention was made of Dr. J. Stein S. J., the highly esteemed and warm friend of many astronomers, to whom, on the occasion of his seventieth birthday, the notes of friendship were recently sent to the Vatican State, where he is director of the now so beautifully equipped Papal Observatory at Castel Gandolfo since 1930.

The extent in which Dutch astronomers are working in the most prominent po- sitions abroad highlights in a gratifying way the spirit of international cooperation

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which astronomers have always enjoyed in such a special way, while on the other hand one sees something reflected in it of the great flourishing that Dutch astron- omy has enjoyed in the last decades. If one wonders why the Netherlands could play this privileged role, then our thoughts automatically turn to three very differ- ent, but in their own way great figures, who have inspired so much the younger generation in our country: J.C. Kapteyn, the first modern explorer of the Milky Way; W. de Sitter, the so pre-eminently ‘practical’ theorist and reorganizer of the Observatory and of astronomical training in Leiden; and Ejnar Hertzsprung, the original and ardent enthusiastic compatriot of Brahe, who raised the observing in our time to such exceptional perfection as the latter did in his time more than 300 years ago.

In addition to a well-deserved congratulation to those whose appointment gave rise to this article, I would like to end by expressing the wish that after them other young people, who feel the vocation to devote their energy and imagination to the exploration of the stars and the universe, may find an equally beautiful way to follow this vocation. There is no reason to doubt that, in the near future, not only America will be the land of unlimited possibilities for the realization of their ideals.

Figure 1: Three mentors: Jacobus Cornelius Kapteyn, 19 January 1851–18 June 1922; Willem de Sitter, 6 May 1872–20 November 1934; Ejnar Hertzsprung, born 8 October 1873

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