VAN DE HULST, HENDRIK CHRISTOFFEL

(b. Utrecht, Netherlands, 19 November 1918;

d. Leiden, Netherlands, 31 July 2000), astrophysics, physical optics, radio astronomy, space research.

Van de Hulst was an astrophysicist with a wide range of interests, who did outstanding work in very diverse fields. As a student, he predicted that interstellar hydrogen must emit a spectral line at the 21-centimeter wavelength, a discovery that revolutionized the study of the Milky Way and of external galaxies. He also made fundamental contributions to the theory of light scattering by small particles, culminating in two books that were—and still are— widely used in various fields of science and technology. For more than twenty years he played a leading role in space research, in Europe and worldwide. He was a member of the Royal Netherlands Academy of Sciences, the American Academy of Arts and Sciences, the U.S. National Academy of Sciences, and the Royal Society. He received prestigious medals from the Royal Astronomical Society, the U.S. National Academy of Sciences, the Royal Society, the Astronomical Society of the Pacific, and the Astronomische Gesellschaft.

Early Life and Study. H. C. (Henk) van de Hulst was a son of Willem G. van de Hulst and Jeannette Maan. He had three sisters and two brothers. Together they formed a warm, balanced family. Van de Hulst’s mother was a cheerful, hospitable woman. His father was headmaster of a primary school and an excellent storyteller; he had written a large number of good children’s books with a religious slant, including a children’s bible and a book of bible stories for youngsters. In Van de Hulst’s own, often witty, scientific writings one can trace some of his father’s talent and style.

Van de Hulst was a bright youngster, with broad interests; a love for nature, books, and puzzles; and an exceptional ability in mathematics. He entered Utrecht University in 1936 as a student of physics, where he was fascinated by Leonard Salomon Ornstein’s lectures on physical optics. But soon the newly appointed astronomy professor, Marcel Gilles Jozef Minnaert, caught Van de Hulst’s interest with his brilliant lectures and his novel scheme of practical exercises, including observations with naked eye and telescopes, and mental and slide rule calculations; so Van de Hulst chose astronomy for his major. In 1939 Minnaert completed his monumental Photometric Atlas of the Solar Spectrum, with Jakob Houtgast and Gerardus Franciscus Wilhelmus Mulders as coauthors; astronomy students were asked to analyze various aspects of the spectrum. Minnaert invited Van de Hulst to measure the profiles of terrestrial atmospheric lines and derive the instrumental profile of the Solar Atlas. Van de Hulst solved this tricky deconvolution problem, and fifteen years later returned to a similar problem in the analysis of 21-centimeter line profiles.

On the roof of the observatory Van de Hulst met a younger fellow student, Wilhelmina Mengerink; the 1943 Easter full moon played a role in their courtship, and they were married in 1946; they had two sons and two daughters.

Van de Hulst was an excellent student and completed the work for his master’s degree early in 1943. But in the spring of 1942, Minnaert had been taken hostage by the German occupying forces. Van de Hulst then delayed his master’s exam till after the war (September 1945), and continued his work as an assistant at the observatory. However, in 1941 Minnaert had drawn his attention to a prize contest about interstellar particles, announced by Leiden University. Thus, Van de Hulst’s research turned away from the Sun to interstellar physics.

Interstellar Dust and the 21-centimeter Line. In 1930 Robert Trumpler of Lick Observatory in California had demonstrated that light suffers a strong general extinction in interstellar space, but the nature of the absorbing particles remained unknown. The prize essay contest announced by Leiden University asked for a discussion of the origin and growth of solid particles in interstellar space. Van de Hulst decided to meet the challenge. He made a thorough study of the physical and chemical aspects of the problem, scanned dozens of books in chemical libraries, and picked up a variety of details on nucleation and coagulation. The jury did not award a prize, but the essay submitted by Van de Hulst in 1942 received “honorable mention,” because of its excellent review of the literature, displaying a “mature scientific spirit.” Van de Hulst presented his work at a colloquium organized by the Nederlandse Astronomen Club (NAC) on 9 January 1943; the other speakers were the other two competitors: Dirk ter Haar and Adrianus Jan Jasper van Woerkom, plus the two key members of the jury, professors Hendrik Anthony Kramers (theoretical physics) and Jan Hendrik Oort (astronomy).

Oort and Van de Hulst extended the study presented by the latter in his prize essay, and in 1946 published it as a paper on “Gas and Smoke in Interstellar Space.” Van de Hulst had used the word “smoke” rather than “dust” for the interstellar solid particles, because it appeared likely that these particles formed in a similar manner as smoke.

Following the NAC colloquium, Oort invited Van de Hulst to spend a few months at Leiden Observatory. The visit took place in January–April 1944. Oort, the leading expert in Galactic structure, had read with great interest Grote Reber’s 1940 paper “Cosmic Static” (Astrophys. J. 91, 621), which showed radio radiation emitted by the Milky Way at a 1.87-meter wavelength, with a maximum in the region of the Galactic center; Reber’s work was the first study of Galactic radio emission after the initial discovery by Karl Jansky in 1931. Oort realized that radio waves would not be absorbed by the micrometer-sized interstellar particles. In January 1944 he asked Van de Hulst to investigate whether there might be any measurable spectral lines at radio wavelengths. If so, Doppler shifts of the lines might betray the motions of the emitters, allow measurements of Galactic rotation and hence determination of the structure of the Galactic System. Oort had, in 1927, demonstrated the effects of differential Galactic rotation in the relative motions of stars in the solar neighborhood, but the interstellar extinction of light had limited his studies to distances of a few kpc, whereas the distribution of globular clusters indicated a system size of twenty or thirty kpc; hence, the structure of the Galactic System remained essentially unknown.

At a colloquium of the NAC on 15 April 1944, Van de Hulst reviewed the measurements of Reber, discussed the possible nature of the radio radiation, and presented the results of his search for radio spectral lines. From an analysis of the literature on atomic physics, he had found that the ground state of the hydrogen atom must show hyperfine splitting; the upper and lower sublevels correspond to, respectively, parallel and antiparallel orientations of the proton spin and electron spin. The minute energy difference corresponds to a wavelength of 21.1 centimeters. The importance of this result was very great indeed: The most abundant element in interstellar space, in its dominant ground state, would emit or absorb a spectral line at a radio wavelength to which both the Earth’s atmosphere and the interstellar medium would be transparent. Hydrogen emission had so far only been observed from regions where the gas is predominantly ionized (now called HII regions), at optical wavelengths susceptible to interstellar extinction. Although the expected 21-centimeter line strength remained uncertain, the prospects for unraveling Galactic structure appeared bright.

Detection of the hydrogen-line radiation would require a large radio telescope and a sensitive receiver. Oort immediately started attempts to obtain these, but even after the end of World War II in 1945, lack of money, equipment, and experience led to many years of delay. The first detections of the 21-centimeter line occurred in March and May 1951, and soon the hopes held since 1944 were fulfilled (see below).

Meanwhile, Van de Hulst had continued working on his doctor’s thesis, which was planned to compare observational data on the wavelength dependence of interstellar extinction with calculations made by Jesse Greenstein and Carl Schalen from Gustav Mie’s theory of light scattering. At Oort’s suggestion, Van de Hulst decided to study the Mie theory in detail, and in the end the thesis became an extensive development of scattering theory. Van de Hulst presented his thesis at Utrecht on 17 June 1946 and received the rare distinction “cum laude.” Within two weeks, he married Wil Mengerink and together they sailed for America.

Postdoc at Yerkes. In June 1945, a month after the liberation of the Netherlands, Gerard P. Kuiper from Yerkes Observatory (University of Chicago), who had obtained a PhD at Leiden in 1933, visited the Dutch astronomical institutes as an officer in the U.S. Army. This visit resulted in an offer to Van de Hulst of a postdoctoral position at Yerkes, at the time one of the world’s best astronomical institutes; the faculty included, among others, Subrahmanyan Chandrasekhar, Jesse Greenstein, Louis Henyey, Gerhard Herzberg, William Wilson Morgan, Bengt Strömgren, Otto Struve, and Kuiper himself.

The two postdoc years (1946–1948) proved extremely stimulating and fruitful. Van de Hulst demonstrated a link between the outer parts of the Corona, the hot outer atmosphere of the Sun, and the zodiacal light, the light scattered by a cloud of particles extending from the Sun to beyond the Earth’s orbit. This study evolved into an authoritative chapter, “The Chromosphere and the Corona,” in Kuiper’s book The Sun (1953). At the request of Kuiper, who was working on infrared spectra of planets, Van de Hulst also made a study of multiple light scattering in planetary atmospheres, a subject recurring in his later work. In this research, Van de Hulst crossed paths with Chandrasekhar, who was an authority on radiative transfer. Van de Hulst was able to strike a fruitful balance between Chandra’s rigorous mathematical analysis and the physical intuition that he had inherited from Minnaert. When his postdoc position ended in 1948, Van de Hulst accepted an offer by Oort of a faculty position at Leiden, in spite of various attractive possibilities in the United States.

Professor at Leiden. In 1948 Van de Hulst was appointed associate professor (“lector”) for theoretical astronomy at Leiden; in 1952 he was promoted to professor. He held this position till his retirement in 1984 but he remained active in research until the end of his life. During his tenure at Leiden, he held visiting appointments at Harvard (1951), at the California Institute of Technology (Caltech, 1954), and at Columbia University and the Goddard Institute of Space Science in New York (1962–1963). Over the years, he received many enticing offers, but he remained at Leiden. Even after his highly stimulating stay at Yerkes, he found the “presence of many different challenges at Leiden invigorating” (1998, p. 9), and he worked on a great variety of subjects: interstellar physics and gas dynamics, radio astronomy and Galactic structure, light scattering, and dynamics of the Solar System and of galaxies, indeed “Roaming through Astrophysics” (1998, p. 1). The following sections discuss some of these subjects in detail. Throughout his work, Van de Hulst emphasized fundamental issues, and he often developed new methods.

Interstellar Physics. In his first year at Leiden, Van de Hulst became involved in the organization, by Oort and Johannes Martinus Burgers, of a conference on cosmical aerodynamics (Paris, 1949), which brought together top specialists from a variety of fields. The 1948 discovery of interstellar polarization by William Albert Hiltner and John S. Hall had demonstrated the importance of interstellar magnetic fields. Van de Hulst reviewed this subject, and together with Burgers he edited the proceedings of the conference. Related conferences (IAU Symposia 2 and 8) were held in 1953 (Cambridge, U.K.) and 1957 (Cambridge, Massachusetts), again with major involvement of Van de Hulst, so that in those years he came to consider gas dynamics his chief field of specialization. In this period he also served as president of Commission 34 (Interstellar Matter) of the International Astronomical Union (1952–1958).

Major progress in surveys of the polarization of radio radiation led in 1967 to reviews by Van de Hulst of the Galactic magnetic field. In the 1970s he was instrumental in the formation of an institute for laboratory astrophysics at Leiden, with J. Mayo Greenberg as director; its aims included simulations and studies of interstellar processes under laboratory conditions.

Radio Astronomy. The 21-centimeter line emitted by interstellar hydrogen atoms, predicted by Van de Hulst in 1944, was first observed at Harvard in March 1951 by Harold I. “Doc” Ewen and Edward Mills Purcell, and at Kootwijk (Netherlands) by Christiaan Alexander Muller and Oort in May 1951. At the time, Van de Hulst was spending a few months at Harvard, where he gave a lecture course on radio astronomy, a course given in Leiden in the fall of 1950, and written up in mimeographed form as the first-ever book on the subject. Frequent correspondence between Van de Hulst at Harvard and Oort at Leiden provided strong liaison between the two groups searching for the 21-centimeter line, which fostered rapid progress. After the detection, this fruitful exchange of information culminated in the simultaneous publication in Nature of the first 21-centimeter line observations by Ewen and Purcell and by Muller and Oort.

The first major survey of neutral hydrogen along the Galactic equator was started by Muller at Kootwijk in May 1952. In the spring of 1953, the line profiles (intensity versus line-of-sight velocity), which had been obtained at fifty-three positions, were measured and reduced by a large team of students and observatory personnel working under Van de Hulst’s guidance. With a preliminary estimate of the Galactic rotation law, locations of hydrogen concentrations could be derived from the velocities measured, and a first map of the hydrogen distribution over part of the Galactic plane derived. Van de Hulst presented this map in his Halley Lecture for the Royal Astronomical Society, held at Oxford on 13 May 1953; the lecture (1953) included a thorough introduction about the differences between optical and radio astronomy, and showed a clear spiral pattern in the Galaxy. (Short sections of spiral arms had been found already in 1951 by Morgan, Stewart Sharpless, and Donald Osterbrock, from the positions and distances of regions of ionized hydrogen.) A thorough discussion of this first survey by Van de Hulst, Muller, and Oort appeared in 1954.

A second survey, by Muller, Gart Westerhout, and Maarten Schmidt, was carried out in 1953–1955, and yielded the distribution of hydrogen in three dimensions, for those parts of the Galaxy accessible from the Netherlands. (Observations by Frank J. Kerr and associates at Sydney, Australia, filled in the missing parts, and led to a paper “The Galactic System as a Spiral Nebula” by Oort, Kerr, and Westerhout [1958] in the Monthly Notices of the Royal Astronomical Society.) For the analysis of these surveys, Alexander Ollongren and Van de Hulst (1957) had developed a method to correct the line profiles for the smearing effects of limited spectral resolution—a sequel to Van de Hulst’s early work on the solar spectrum. Van de Hulst reviewed these groundbreaking results at various international conferences; in fact, he gave major papers at six of the first nine IAU symposia.

When the 25-meter (82-foot) radio telescope at Dwingeloo became available, Van de Hulst took part in the first makeshift observations in November 1955 of the occultation of the Crab Nebula by the Moon, and after an observing night walked 15 kilometers (9 miles) to Hoogeveen railway station, to catch the early morning train to Leiden.

In October 1956, a 21-centimeter line receiver built by Muller became available at Dwingeloo. Van de Hulst had calculated that interstellar hydrogen in the Andromeda Nebula (M31) should be measurable with this instrument. Early attempts by Westerhout and Ernst Raimond failed, because the receiver noise varied too strongly with frequency. But Van de Hulst came for a visit and invented the “on-off” method, in which at each frequency the signal from the Nebula was compared to that several degrees away. With this method, later called the “sky reference,” the distribution of hydrogen in M31 could be determined—the first study of neutral hydrogen outside the Galaxy and its satellites. Van de Hulst again developed the methods to derive the distribution from the observations; the angular resolution (0.6 degrees) was a significant fraction of the diameter of M31 (only 3 degrees on photographs).

After 1957, Van de Hulst had to drop his radio-astronomical research; he became too heavily occupied in other parts of the spectrum. The work at Kootwijk and Dwingeloo, and later (after 1970) that with the Synthesis Radio Telescope at Westerbork, was done under the aegis of the Stichting Radiostraling van Zon en Melkweg (Netherlands Foundation for Radio Astronomy). For forty years (1948–1988), longer than anyone else, Van de Hulst served on the executive committee of this foundation; he chaired it from 1970 to 1975.

Light Scattering. After his 1946 doctoral thesis “Optics of Spherical Particles,” Van de Hulst continued and extended his work on the scattering of light. In 1957 he published a monograph on Light Scattering by Small Particles. This book soon became a classic, and was republished by Dover Publications in 1981. The obituaries by Harm Habing and by Joop Hovenier describe in some detail the great merits of this book: It starts with a general solution of a simple problem, and proceeds to problems of greater complexity; its illustrations are clear and efficient; the style is lucid, and it combines arguments based on physical intuition with mathematical derivations. The book was widely consulted, also outside astrophysical contexts. As Van de Hulst stated in his Karl Schwarzschild Lecture about scaling laws in multiple light scattering, “it gave me peculiar satisfaction when one day … I learned that the computations, which I had initiated for the Milky Way, were now being applied to real milk [in the optical process to monitor the homogenization]” (1996, p. 4). It is striking that this book was completed at a time (1957) when its author was heavily involved in problems of interstellar physics and gas dynamics, and in groundbreaking radio-astronomical work.

A second monograph, Multiple Light Scattering: Tables, Formulas, and Applications, was published in 1980. The book deals mainly with scattering in plane-parallel atmospheres. The basis for this book was laid in work with Kenneth Grossman, at the Goddard Institute for Space Physics in 1962–1963, and even already at Yerkes in 1948, in a study of scattering in planetary atmospheres.

Space Research. Van de Hulst’s life took a totally unexpected turn in the fall of 1958, as described in his 1998 autobiography (pp. 10–11). At the daily observatory morning coffee on 15 November, Oort told him: “The IAU has asked me to be their representative at a meeting in London. But I have little time. Would you like to go?” Van de Hulst asked what it would involve; and Oort replied: “Not much, unless, of course, you would become a member of the Board.” When Van de Hulst returned after ten days, he had missed his daughter’s fourth birthday, and his own fortieth, and he had become president of COSPAR, the Committee on Space Research. The International Council of Scientific Unions (ICSU) had established COSPAR to promote international cooperation in space research. At the time, the United Nations was considering proposals to form a new specialized agency dealing with all space matters. ICSU feared that space would become subject to the full hassle of the Cold War, and that research efforts would be pushed into second place. Several international scientific unions had managed to maintain cooperation between Western countries and the Eastern bloc. ICSU hoped to prove through COSPAR that they could foster East-West cooperation in space research informally and efficiently.

As Reimar Lüst mentioned at a memorial symposium held at Noordwijk in the Netherlands on 6 November 2000, Van de Hulst brought several merits to his new job: he had a very strong international reputation as a scientist; he belonged to a younger generation; he came from a small European country, rather than one of the big powers; and he could convince people that he was serving peaceful, scientific interests only. In his Karl Schwarzschild Lecture, Van de Hulst (1996, p. 4) mentions a highlight of his COSPAR years: “the first meeting of a Russian cosmonaut, Titov, with an American cosmonaut, Glenn. At that occasion I delivered the shortest official speech I ever made. I gave each of the two gentlemen one of a pair of wooden shoes, which I had bought in my home town, adding that these were cut from the same tree and were meant to be used together.” The deep meaning of this speech was not lost on anyone.

Van de Hulst’s autobiography mentions many examples of the diplomatic work involved in his three years (1959–1962) as COSPAR president. Thereafter, although returning to science, he remained active in European and national space research organizations. He fulfilled several key functions in the European Space Research Organization (ESRO) in 1960–1975, and in its successor, the European Space Agency (ESA), in 1975–1986; he was chairman of the ESRO council during the critical years 1968–1970. Lüst recalls that he was an excellent moderator, free of the suspicion of serving national interests; patient and stubborn, and often able to resolve difficult discussions by the use of a simple metaphor. Also at the Noordwijk memorial symposium, Hermann Bondi cited his honesty and calming influence, based on knowledge and insight and a good judgment of the moods of people. Livio Scarsi emphasized his role in the realization of the gamma-ray satellite COS-B, the first ESA satellite, launched in 1975. Van de Hulst also played a major role in the establishment of ESTEC, the European Space Research and Technology Center, at Noordwijk, a village on the seashore close to Leiden.

When ESA joined NASA with a 15 percent share in the preparation of the Hubble Space Telescope, a weighty contribution was needed in dealing with the major partner. ESA undertook to build the Faint Object Camera (FOC), the most sensitive instrument for the Space Telescope; and with his keen insight and wide influence, Van de Hulst was appointed to the chair of the FOC Instrument Definition Team. As Malcolm Longair recalled at the Noordwijk memorial, Van de Hulst played a diplomatic role in the Space Telescope Institute Council. He also served on ESA’s Space Telescope Working Group (STWG), and in 1981 chaired the STWG selection panel for the Space Telescope European Coordinating Facility, which eventually was formed by ESA together with the European Southern Observatory at ESO’s headquarters near Munich.

In 1959 the Royal Netherlands Academy of Sciences formed a Committee for Geophysics and Space Research (GROC). Van de Hulst held the chair of GROC from 1959 to 1984, when GROC was succeeded by a new foundation, the Space Research Organization of the Netherlands (SRON), supported by NWO, the Netherlands Organization for Scientific Research. Through GROC, the Netherlands succeeded in making major contributions to space research, including the Astronomical Netherlands Satellite (ANS); a large (42.5-percent) share in the Infrared Astronomical Satellite (IRAS); and an important share in the gamma-ray satellite COS-B, for which Van de Hulst led a large team of brilliant engineers recruited from Delft Technical University. As emphasized by several speakers at the Noordwijk memorial symposium, Van de Hulst demonstrated and personified the notion that scientific quality is vital to success in space research.

Religion and Philosophy. As mentioned earlier, Van de Hulst grew up in a strongly religious family. As students, he and his later wife Wil were active members of the Nederlandse Christen-Studenten Vereniging (NCSV), the Student Christian Movement in the Netherlands affiliated with the World’s Student Christian Federation (WSCF) based in Geneva; Willem “Wim” Visser‘t Hooft, in 1948 the founder of the World Council of Churches, had earlier been a member of the NCSV, and president of the WSCF. Van de Hulst’s strong religious belief is obvious from several sentences in the preface of his 1946 doctor’s thesis, including: “May God give me strength and insight to follow the right path. My wife will be at my side in this. In Thee, Lord, we have trusted; never let us be shamed!”

In 1951 the largest Protestant church in the Netherlands, the Nederlandse Hervormde Kerk, through its Council for the Affairs of Church and Theology, formed a Study Committee on Faith and Physical Science, to discuss problems occurring in this area. Several prominent scientists, philosophers, and theologians joined this committee. Van de Hulst was a member from the start; he became its chairman in 1955 and kept the Committee on track, and steered it toward completion of its work in 1963. The final report of the committee took the form of a book Geloof en Natuurwetenschap (Faith and Physical Science) in two volumes. Together with the physicist Jan Volger, Van de Hulst contributed a chapter to this book.

In 1953, together with Cornelis Anthonie (Kees) van Peursen, professor of philosophy at Leiden, Van de Hulst published a book Phaenomenologie en Natuurwetenschap, in which the relationships of philosophy and the sciences are discussed. Habing writes in his obituary (2001): “about the same time Henk dissociated himself from his earlier religious beliefs.” My personal impressions, from close contacts with Van de Hulst in astronomy and in the committee just mentioned, and with his family, do not confirm this, but the dissociation may well have occurred in later years.

After her marriage to Henk, Wil van de Hulst did not continue her astronomical studies. Instead, she moved to a study of psychology, and later started practicing psychotherapy. Habing notes that, after about 1980, Van de Hulst and his wife visited annual psychotherapy workshops, and he found great satisfaction in these. And undoubtedly, these shared events further strengthened their happy, stable marriage.

Although often ill in his childhood, Van de Hulst had developed into a strong and healthy man. But around 1995 his health weakened, and in 1999 an inoperable lung cancer was discovered. He lived through his final months with great mental strength, aware that his life had been richly blessed.

Teacher, Guide and Colleague. Van de Hulst served as thesis adviser on twenty-eight PhD projects, covering a wide range of subjects, from Cepheid atmospheres to space research, radio sources and Galactic dynamics. He was an outstanding counsellor: accessible and sensitive, open and direct, and rich in practical advice. In his courses and research, he concentrated on fundamental matters. He emphasized insight rather than facts. Throughout his life, he gave many excellent review talks. The “look-back” papers which he wrote in his last few years make fascinating reading; they are enlightening and full of wit.

After Van de Hulst’s appointment at Leiden in 1948, he stayed there forever. Several prominent Leiden graduates, having returned after a foreign postdoc period, left after a few years and made their career elsewhere. Not so Van de Hulst. What then was his relationship with Jan Oort, the powerful, world-renowned Leiden Observatory director?

Oort had put him on the search for a radio spectral line, which resulted in the 1944 prediction of the 21-cm line. Oort had suggested that he make a detailed study of the Mie theory of scattering, which led to his 1946 doctoral thesis. In 1948 Oort offered him a position at Leiden and involved him in cosmical aerodynamics. In 1951, during his stay at Harvard, they exchanged many letters— about the 21-cm line and about general Observatory affairs. In 1952-1954, Van de Hulst was responsible for the reduction and analysis of the first Galactic hydrogen-line survey, and first author on the paper presenting the results. In 1958 Oort asked him to go to London, and Van de Hulst was (as he later put it) “launched into a space career”; Oort of course understood the critical importance of the COSPAR job, but he knew he could not take it on himself.

They worked closely together for many years, but they published few joint papers; both had their separate areas and responsibilities. To quote Blaauw’s obituary, “The outstanding but ambitious directorship of Oort left little room for competing initiatives from his—devoted— staff. Van de Hulst’s many talents, however, enabled him to stake out his special prominent place beside Oort at Leiden Observatory.” I concur with Harm Habing’s analysis: that Van de Hulst “had been raised in a religious tradition that emphasizes humility.… He had a strong sense of the relativity of all things. He was a man of great talents, but without a mission. He labored where he considered himself able to contribute, but had no explicit need to achieve great things. In that respect his personality was different from that of Jan Oort, his immediate colleague and paragon.” Hovenier writes: “Henk … seemed to consider his fame more like a burden than something of which to be proud. He considered other things far more important, such as authenticity, sincerity and simplicity.”

Van de Hulst himself wrote (1998, p. 15): “What I enjoyed most in my research were not the big successes, but rather the little discoveries when a sudden insight revealed a surprising connection between bits of information that had seemed to be far apart.” Though modest indeed, he clearly was a leader, who took decisions where required, after listening carefully to everyone concerned; but he acted as a guide rather than a chief.

BIBLIOGRAPHY

A fairly (but not fully) complete bibliography is available from http://adsabs.harvard.edu/abstract_service.html.

WORKS BY VAN DE HULST

“Radiostraling uit het wereldruim. II. Herkomst der radiogolven.” Nederlandsch Tijdschrift voor Natuurkunde 11 (1945): 210–221. The 1944 NAC presentation predicting the existence of the 21-cm line of neutral atomic hydrogen. Translated as “Paper 34” in Classics in Radio Astronomy, edited by Woodruff Turner Sullivan III, pp. 302–316. Dordrecht: Reidel, 1982.

“Optics of Spherical Particles.” PhD thesis, University of Utrecht, 1946.

With Jan Hendrik Oort. “Gas and Smoke in Interstellar Space.” Bulletin of the Astronomical Institutes of The Netherlands 10 (1946): 187–204. Based on the 1942 prize essay by Van de Hulst.

With Johannes Martinus Burgers, eds. Problems of Cosmical Aerodynamics: Proceedings of the Symposium on the Motion of Gaseous Masses of Cosmical Dimensions Held at Paris, August 16–19, 1949. Dayton, OH: Central Air Documents Office, 1951.

“The Chromosphere and the Corona.” In The Solar System: 1, The Sun, edited by Gerard P. Kuiper, pp. 207–321. Chicago: University of Chicago Press, 1953. Standard reference.

With Cornelis Anthonie van Peursen. Phaenomenologie en Natuurwetenschap: Bezinning op het wereldbeeld. Utrecht: Erven J. Bijleveld, 1953. Discussion of relationships between philosophy and the sciences.

“The Galaxy Explored by Radio Waves.” The Observatory 73 (1953): 129–139. The Halley Lecture for 1953, written for nonspecialists. First map of the Galaxy’s spiral pattern.

With Christiaan Alexander Muller and Jan Hendrik Oort. “The Spiral Structure of the Outer Part of the Galactic System Derived from the Hydrogen Emission at 21 cm Wavelength.” Bulletin of the Astronomical Institutes of The Netherlands 12 (1954): 117–149. First survey of Galactic atomic hydrogen: full discussion of instrument, observations, analysis, and interpretation.

Light Scattering by Small Particles. New York: Wiley, 1957. Reprint, New York: Dover, 1981. A classic text, with wide applications.

With Ernst Raimond and Hugo van Woerden. “Rotation and Density Distribution of the Andromeda Nebula Derived from Observations of the 21-cm Line.” Bulletin of the Astronomical Institutes of The Netherlands 14 (1957): 1–16. First hydrogen-line study of an external galaxy beyond the Magellanic Clouds.

Multiple Light Scattering: Tables, Formulas, and Applications. 2 vols. New York: Academic, 1980. Major sequel to 1957 text.

“Space Science beyond the Solar System.” Space Science Reviews 65 (1993): 201–219. “Distinguished Lecture” presented to the World Space Congress, Washington, DC, on 1 September 1992. Fascinating.

“Scaling Laws in Multiple Light Scattering under Very Small Angles.” Reviews in Modern Astronomy 9 (1996): 1–16. The Karl Schwarzschild Lecture for the Astronomische Gesellschaft, delivered at Bonn, 19 September 1995. Very readable.

“Roaming through Astrophysics.” Annual Review of Astronomy and Astrophysics 36 (1998): 1–16. Invited autobiography, giving valuable insight into life and work.

OTHER SOURCES

Blaauw, Adriaan. “Hendrik Christoffel van de Hulst.”Proceedings of the American Philosophical Society 146 (2002): 420–423. Biography by a long-time colleague. Detailed mention of many honors and distinctions.

Habing, Harm J. “Obituaries: Hendrik Christoffel van de Hulst, 1918–2000.” Astronomy and Geophysics 42 (2001):1.33–1.37. Excellent obituary by a close, younger colleague. Appeared first in August 2000 on Leiden Observatory Web page.

Hovenier, Joop W. “Obituary H. C. van de Hulst (19 November 1918–31 July 2000).” Journal of Quantitative Spectroscopy and Radiative Transfer 68 (2001): iii–v; reprinted, vol. 27 (2007): e1–e3. Written by one of Van de Hulst’s former students, a specialist in scattering in planetary atmospheres.

Hugo van Woerden