Annual Report 2000

Onderzoeksinstituut Research Institute Sterrewacht Leiden Leiden Observatory Annual Report 2000 Sterrewacht Leiden Faculty of Mathematics and Natural Sciences University of Leiden Niels Bohrweg 2 2333 CA Leiden The Netherlands ¡ ¢ ¤£¦¥¨§¢©¡ ¤£ ¡¡¤£ In memoriam Hendrik van de Hulst November 18, 1918 – July 31, 2000 Hendrik van de Hulst Hendrik van de Hulst died in Leiden on July 31st, 2000, at the age of 81. He was one of the greatest Dutch astronomers of the past 150 years. In 1944 he had predicted that the amount of neutral atomic hydrogen in interstellar space would be so great as to produce a measurable signal at the radio wavelength of 21 centimeters. This prediction led to a breakthrough in astronomical research. It was a strikingly origi- nal contribution to the spectacular blossoming of astronomy which began after the second world war and which continues unabated. Henk van de Hulst was born in Utrecht, The Netherlands, on November 19, 1918. He was one of the six children born to W.G. van de Hulst, a well-known writer of children's books in Calvinist religious settings. Henk was not the only one of the children to be highly gifted; one of his brothers became a respected painter. Henk remembered growing up in a happy family atmosphere. The belief in the teachings of the Dutch Reformed Church were followed seriously at home and ev- idently the younger Henk adhered to them too: in his Ph.D. thesis one finds the pious dedication “. to Him who steers everything”. Later in his life his religious beliefs loosened, although his knowledge of the Bible became a familiar signature in his discussions. At the most unexpected moments, and to his listeners's general surprise, he could cite an entirely appropriate verse from the Bible in order to put the topic under discussion directly in accurate perspective. Henk van de Hulst's father was the principal of the elementary school where Henk received his first education. Henk was in rather poor health as a child, and frequently was confined to his house: this situation is difficult to assimilate with the physically powerful man which he later became. But his health problems did not interfere at all with his educational progress. He described how he did all of the problems in the arithmetic books which he was to have in his high school Gymna- sium curriculum, during the summer vacation of 1930, before he was even to en- ter high school. The aptitudes and interests which were evident before he entered his teens made it obvious that he was well-suited to a university education with a strong mathematical emphasis. He was, however, the first in his family to receive a university education, since universities at that time were still largely citadels for the privileged. It was not immediately obvious that Henk would specialize in astronomy. Dur- ing his high school years astronomy was just one of his numerous hobbies. He con- sidered attending the Institute of Technology in Delft, but he had been dissuaded from this choice by an engineer who had advised Henk that his talents were too strictly theoretical for a purely technical education. Henk followed this advice and remained grateful for it, yet one can question if his talents did not in fact also embrace mechanical matters. During his entire life he enjoyed finding practical solutions to all sorts of technical problems: he improved the sagging foundation of his daughter's house in Amsterdam, he designed and installed the protective bar- rier on the dike guarding his vacation lake-house in Friesland, and he prepared an good model for the size and shape of the European instrument that was installed in the Hubble Space Telescope. The decision to become an astronomer was taken in Henk van de Hulst's second year at the University of Utrecht, and was strongly influenced by the lectures given by M. Minnaert, well-known for his educational talents. But Henk's studies were in- terrupted in 1939, when he was drafted into military service following the general mobilization shortly before the invasion of The Netherlands and the beginning of the second world war. He was not involved in overt military operations while he was in the army, but upon his discharge the University was not effectively func- tioning: Minnaert, together with many other well-known Dutch, had been jailed as a hostage in St. Michielsgestel. Henk attempted to study on his own and to take ex- aminations under these circumstances, but Minnaert was constrained by the reg- ulations of his detention to only answer with a few lines. Nevertheless, shortly before he had been taken hostage Minnaert had introduced an important direction in Henk's career, by pointing out a Prize Competition which had been announced by the University of Leiden in 1941. This competition centered on the small particles of dust which had been discovered some ten years earlier in interstellar space but which were still largely a mystery. Scattering of light by these particles of “interstellar smoke” determines much of the appearance of the Milky Way, which had made a deep impression on Henk during the years of the war, when city lights were extinguished. The subject of the prize competition challenged Henk, and he submitted his entry in April, 1942. The jury, which included among others J.H. Oort and H.A. Kramers, did not award the prize but instead offered two honorable men- tions, one for Henk van de Hulst (citing “. a mature scientific spirit . ”) and one for the entry submitted by D. ter Haar. The Prize Competition had two particularly important consequences. Henk became acquainted with Jan Oort, and he became deeply interested in the general problem of light scattering in an astronomical context. This interest was soon re- flected in his Doctor's thesis, which carried title “Optics of spherical particles”. He was awarded the Ph.D., cum laude, in June, 1946, with Minnaert as his supervisor. The subject of light scattering remained central to his interests for Henk's entire life. He wrote two monographs on the subject. The first of these - “Light Scatter- ing by Small Particles” - was published in 1957 and was immediately recognized to be a classic; it was republished by Dover in 1981. This book illustrates Henk's best scientific talents. He begins with a very simple statement of the problem, and solves this problem in the most general manner. In the following chapters he con- tinually expands the statement of the problem, offering solutions of increasing, but still straightforward, mathematical complexity, until he arrives at the Mie Theory. But the reader is gradually introduced to the general subject and becomes con- vinced that the constituent problems can be simply stated and simply solved. The presentation is elegant and clear; the illustrations are so efficient as to be almost self-explanatory; the historical background is given all appropriate attention. For these reasons the book was widely consulted, beyond the confines of its obvious use in an astrophysical context. Van de Hulst himself proudly stated that the book “. was written to explain the light of the Milky Way, but was in fact also used in dairy factories to measure the size of particles of fat in milk”. Henk's scientific interest in light scattering led to the establishment of a laboratory in Leiden in the 1970's, where J. M. Greenberg replicated interstellar scattering processes with con- siderable success. We return to the year 1944. At that time scientific news from the United States had not reached Europe for several years. But news had arrived earlier which was to fundamental change astronomy. An American engineer, and radio amateur, Grote Reber, had proceeded from the discovery made several years earlier by Karl Jan- sky, in Jansky's investigation of the source of interference which disturbed long- distance communication systems. Reber had himself constructed a steerable an- tenna and with this instrument had made a map of cosmic radio emission. The emission was particularly strong in the general direction of the center of the Milky Way. Reber's map, together with the news of an interpretation by two American astronomers that the emission could not be explained in terms of known mecha- nisms, reached Oort in Leiden. Oort immediately saw the challenge of a new subject of astronomical research, and he also realized that radiation at radio wavelengths would penetrate the cloudy Dutch sky. At a meeting of the Nederlandse Astronomen Club held at the Leiden Obser- vatory on April 15, 1944, the possibilities and prospects of radio astronomy were discussed for the first time. Oort had asked Henk van de Hulst to consider the pos- sibility of observing a spectral line in the radio regime. At the meeting, Henk gave a talk in which he suggested that neutral atomic hydrogen, which in its hyperfine transition radiates and absorbs at a wavelength of 21 cm, might be expected to oc- cur at such high column densities as to provide a spectral line sufficiently strong as to be measurable. Shortly after the end of the war several groups set about to test this prediction. The 21-cm line of atomic hydrogen was detected in 1951, first at Harvard University followed within a few weeks by Dutch observations made in Kootwijk and by Australian ones made near Sydney. The discovery demonstrated that astronomical research, which at that time was based on observations of con- ventional light, could be complemented with observations at other wavelengths, revealing a range of new physical processes. Today, in addition to optical and radio astronomy, observations embrace X-rays, gamma rays, and the infrared and ultra- violet regime.

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