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Oort, Jan Hendrik


(b. Franeker, Netherlands, 28 April 1900; d. Leiden, Netherlands, 5 November 1992)

astronomy, astrophysics, galactic system, radio astronomy.

Oort is generally regarded as one of the leading astronomers of the twentieth century. He performed very important researches in a number of areas, most notably on the structure and dynamics of the galactic system. Oort also played a crucial part in the development of radio astronomy, in the founding of what became the European Southern Observatory, and in advancing the place of Holland in European and world astronomy.

Early Years . Oort was born in the small town of Franeker in the province of Friesland in the north of the Netherlands. He was one of five children of Abraham Hendrikus Oort and Hannah Faber. Abraham was a psychiatrist and the family left Franeker shortly after Oort’s birth when his father became the director of a psychiatric clinic in Oegstgeest near Leiden. Oort went to primary school in Oegstgeest and then to Leiden for secondary school. Encouraged by his parents to follow his interests, Oort decided to study physics at the University of Groningen, which he entered in 1917. After attending lectures by the famous astronomer Jacobus Cornelius Kapteyn, Oort was inspired to switch to astronomy. He later recalled that the most important lessons that Kapteyn taught him as an undergraduate were to link interpretations directly to observations and to be very skeptical about speculations and hypotheses. He put these lessons into effect throughout his career.

Oort took his doctoral examination in 1921. This marked the completion of coursework for the degree of doctor of science, but attaining the higher degree would require several more years of study. After the examination Oort became the assistant to Kapteyn’s collaborator, Pieter Johannes Van Rhijn, at Groningen. But after a year Oort left to gain experience in the United States. From 1922 to 1924, Oort was an assistant to Frank Schlesinger at the Yale Observatory. Schlesinger was a great authority on fundamental positional astronomy. Oort was trained in astrometry, in particular the use of a zenith telescope to secure very accurate positions of stars. He later judged the experience at Yale as useful, but it did not mesh well with his own aspirations. Hence when Willem de Sitter, director of the observatory at Leiden, offered him a position there, Oort quickly accepted. At first a research assistant, he held various positions until he became a “Professor Extraordinary” and vice director of the Observatory in 1935.

On arriving at Leiden in 1924, Oort tackled a problem he had encountered while at Yale: the properties of the so-called high velocity stars. He found that these stars displayed a puzzling and unexplained asymmetry as they were traveling toward one hemisphere of the sky. This asymmetry became apparent when the velocity of the stars was greater than about 65 kilometers per second relative to the Sun; slower moving stars did not display the same behavior. The high velocity stars formed the subject of Oort’s doctoral thesis, which he defended at Groningen in May 1926.

In the next year he married Mieke Graadt van Roggen, whom he had first met at a university celebration. The couple had three children between 1928 and 1934, Coenraad, Abraham, and Marijke, and their happy marriage lasted until Oort’s death sixty-five years later.

The Galactic System . Kapteyn had died in 1922, but his researches on the structure of the galactic system continued to be extremely influential, particularly among Dutch astronomers. Over the course of his career, Kapteyn had elaborated a model that became known as the Kapteyn universe or Kapteyn system, as well as a set of approaches for working out the structure of our stellar system based on detailed counts of stars. In a paper written at the end of his life, Kapteyn had calculated the limits of the galactic system. Taking the limits of the ellipsoid-shaped system where the density of stars sank to a value of one-hundredth that in the neighborhood of our Sun, then the distance from the center to the edge along the galactic plane was about 25,000 light-years. If measured from the center, it extended for about 5,000 light-years at right angles to that plane. The Sun was roughly 2,000 light-years from the center. Kapteyn had suspected that the high-velocity stars were interlopers that moved through the galactic system, but were not related to it.

The Swedish astronomer Bertil Lindblad treated the high velocity stars in a very different way. By 1926 he had developed a model in which these stars are an integral part of the galactic system, which he divided up into several subsystems. All of the subsystems rotate about a common axis and center. The different subsystems have different degrees of flattening depending on the speeds of rotation of the subsystem. A few years earlier, at the Mount Wilson Observatory in California, Harlow Shapley had advanced the radical idea that the galaxy is much larger than Kapteyn had allowed. In Shapley’s scheme, the Sun was far distant from the center of the galactic system, which had a diameter of around 300,000 light-years. For Shapley, the galaxy was framed by the collection of globular star clusters with the center of the system of globular clusters coinciding with the center of the galactic system. But Shapley’s vision of the galaxy was controversial and to many astronomers it seemed to rest on dubious assumptions. Lindblad, however, argued that the direction of the center of the galactic system coincided with the direction of the center of system of globular clusters as defined by Shapley.

Oort paid very close attention to Lindblad’s researches. Bart Bok, who later became a prominent astronomer in his own right, would later remember that in 1927 Oort was delivering a series of seminars for Bok and three other students at Leiden. Oort, however, skipped two of the seminars after telling the students that he had become bogged down in Lindblad’s complex calculations. By the next seminar, Oort had mastered these and had derived his own relatively simple formulas to explain the motions of the stars in terms of a differential galactic rotation. That is, he argued that the galactic system does not rotate as a solid body, but the angular velocity of the stars changes with distance from the center. In his analysis he employed two constants, A and B, which later became known as the Oort constants. Oort now wrote a paper on “Observational Evidence Confirming Lindblad’s Hypothesis of a Rotation of the Galactic System.” There were, nevertheless, differences to begin with between Lindblad’s system and Oort's. In particular, Oort reckoned that the systematic effects he had found in 1927 revealed a large concentration of mass in the central region of the galaxy, whereas Lindblad contended that the greater part of the galactic system is formed by an ellipsoid of constant density.

Other astronomers provided evidence generally interpreted as buttressing Oort’s arguments. Indeed, astronomers soon regarded Lindblad and Oort as the codiscoverers of galactic rotation with Oort seen as having furnished its observational proof. There was, however, still a discrepancy between the distances to the center of the galactic system as determined by Shapley’s and Oort’s estimates from the motions of the stars, 65,000 light-years versus about 20,000 light-years. Shapley had calculated the scale of the galaxy assuming there was negligible interstellar absorption and, except for isolated dark clouds, its effects could effectively be ignored. Kapteyn had worried about the possible effects of absorption. In fact, the possible effects of such absorption were the great unknowns in studies of galactic structure in the first three decades of the century. But in constructing the Kapteyn universe, he came round to the view that it was not significant. In the opinion of many astronomers, however, this assumption was undermined in the early 1930s, with the most compelling evidence being provided by Robert Julius Trumpler at Lick when he measured the distances to open star clusters using two methods. He exploited both the sizes of the clusters and the colors of the stars within them in his distance estimates but found discrepancies in the answers from the two tools. Trumpler calculated that if interstellar absorption was changing the colors of the stars, then the discrepancies were resolved. The amount of interstellar absorption completely undercut the results of Kapteyn’s attacks on the structure of the galactic system if not the methods he had fashioned. Shapley’s model of the galaxy also had to shrink in size, with the result that Oort’s and Shapley’s estimates of the distance to the galactic center came into line.

Within just a few years of completing his dissertation, Oort had propelled himself into the front rank of younger astronomers. Job offers followed. In 1930 Harvard invited him to become the Willson Professor of Astronomy and in 1932 Columbia University wanted him as the director of its astronomy department. He considered both offers seriously but in the end he chose to stay at Leiden, where he remained for the rest of his career.

Leiden, however, did have its drawbacks. There were no large optical telescopes there (the biggest were a 26-cm visual refractor and a 33-cm astrometric telescope, and both dated from the nineteenth century), and anyway Holland was a poor location from which to pursue optical astronomy. In 1923 an agreement was reached whereby Leiden astronomers had access to the Union Observatory in Johannesburg, South Africa. But the instruments available to Oort there, as at Leiden, were much less powerful than the best of the telescopes in the United States. Hence Oort made a half-year visit in 1932 to the Perkins Observatory in Delaware, Ohio. There he photographed galaxies with a 60-inch reflector in order to measure their luminosity distributions. His goals were to investigate the bulges in spiral galaxies and the forces operating within elliptical galaxies so as to better understand the development and maintenance of spiral structures. But he had very little experience of this sort of observing and overall he was not especially successful.

During the 1930s Oort continued to work principally at problems of galactic structure and dynamics. In 1932, for example, as Kapteyn had a decade earlier, he investigated the forces exerted by the stellar system perpendicular to the galactic plane and the density of stars in this direction. Critical to all aspects of Oort’s project was the effort to better determine the effects of interstellar absorption on the brightness and colors of stars, effects that as noted had crippled Kapteyn’s investigations. In 1938 Oort wrote on “Absorption and Density Distribution in the Galactic System.” This time he performed counts of faint stars at moderate and high galactic latitudes and combined these with data on interstellar absorption drawn from counts of distant galaxies by Edwin Hubble. Among other things, Oort concluded that the obscuring material is confined to a relatively thin layer in the galactic plane.

The style of this 1938 paper was in many respects typical of Oort’s scientific writings before the end of World War II and the start of his active involvement in radio astronomy. He was the sole author and it was based on a mastery of the available literature and a very careful compilation and painstaking analysis of a large body of generally available observational evidence. As usual Oort did not employ highly advanced mathematical methods and avoided far-reaching speculations. He closed the paper, for example, by warning that in terms of comparing his results for high and moderate galactic latitudes with data at low latitudes, it might prove advisable to wait for better and more extensive data.

With Oort’s growing seniority he assumed extra administrative tasks. In 1934 Willem de Sitter died and Ejnar Hertzsprung became director of the Leiden Observatory. Oort became Hertzsprung’s deputy. Like his mentor Kapteyn, Oort was a convinced internationalist when it came to science, and in 1935 his standing in the wider international astronomical community was also underlined when he became general secretary to the International Astronomical Union, the leading international organization of astronomers. Oort did not relinquish this position until 1948, although much of the administrative burden in the war years was carried by Walter Adams at the Mount Wilson Observatory in the United States because of the disruption of communications in war-torn Holland. Oort continued to play a leading part in the running of the union and he would serve as its president from 1958 to 1961.

The War Years and Radio Astronomy . Oort spent part of 1939 in the United States. He arrived back in the Netherlands days before the outbreak of World War II. With the German occupation of the country in 1940, conditions became at best difficult for astronomical research and communications with astronomers outside the Netherlands slowed to a trickle by late 1941.

Oort did continue some research however, including on the Crab Nebula, generally identified as the result of a stellar explosion. During his 1939 U.S. visit, Oort and Nicholas Ulrich Mayall at the Lick Observatory had discussed data on the Crab. These discussions led to papers in 1942, although the final manuscript of one listing Mayall and Oort as coauthors never reached Oort. Oort, however, had authorized Mayall to publish it if this happened. Oort was aided in his research by Jan Julius Lodewijk Duyvendak, a Sinologist at Leiden who had searched Chinese and Japanese records for original observations of the “nova” and produced convincing evidence that the Crab Nebula resulted from a nova viewed from Earth in 1054 CE. In part due to the work of Duyvendak, Mayall, and Oort, Walter Baade confidently identified the Crab Nebula as the result of a Type 1 supernova.

In 1942 Jewish professors were dismissed from the University of Leiden. Oort was a member of a group of professors who met regularly to discuss the problems of

the occupation and the Nazification of the university. After a number of these professors were put into detention camps, Oort, to avoid the same fate, decided to leave for the small village of Hulshorst some 100 kilometers from Leiden (later, in 1944, he moved to Nunspeet). He also resigned from his posts at the university. In effect, he disappeared from view, at least as far as the Germans were concerned.

Oort, nevertheless, stayed in touch with the observatory and was able to continue some astronomical research. On occasion he cycled back to Leiden. He continued to organize work and to deliver unauthorized lectures. Oort also attended scientific meetings of the Nederlandse Astronomenclub.

Despite the isolation of Holland from the outside astronomical world, Oort had come across a paper by an American engineer, Grote Reber, in a copy of the Astrophysical Journal smuggled into the country. Here Reber reported on his investigations of radio emissions from the Milky Way. The great majority of astronomers had paid little heed to Reber’s researches, but Oort found this news intriguing. He wondered if radio waves, which would not be impeded by interstellar absorption in the same way as starlight, could be exploited to survey the galaxy. In 1944 Henk C. van de Hulst, a graduate student at Utrecht who had come to work at Leiden, told Oort about a prize that had been offered for an essay on the “Formation of Solid Particles and Condensation in Interstellar Space.” Van de Hulst received an honorable mention for his essay, and the interaction between gas and dust was a topic that he and Oort would later pursue further, addressing questions about the heating and evaporation of dust due to nearby stars, for example. But at this time in 1944 Oort gave him another assignment: to review the existing theories on the sources of cosmic radio waves and calculate if spectral lines might exist in the radio spectrum. Oort had grasped that if there were detectable lines in the radio spectrum (as there were absorption and emission lines in the optical section of the electromagnetic spectrum), these could be used, through Doppler shifts of the lines, to investigate the location and rotation of interstellar gas throughout the galactic system, not just the very limited region of several thousand light years to which optical observers were restricted by the effects of interstellar absorption. The lines in the radio spectrum might therefore be extremely powerful probes of galactic structure.

Van de Hulst provided his answers at a meeting of the Nederlandse Astronomenclub at Leiden on 15 April 1944. He had calculated that there should be a spectral line at a wavelength of 21 centimeters due to a switch in the spin of the electron in a hydrogen atom, a so-called hyperfine line. As there was so much hydrogen in the galaxy, van de Hulst reckoned it might even be possible to detect this line, although he was unsure if it would be in emission or absorption. Van de Hulst’s results were published in 1945, and, as we shall see shortly, were to prove to be enormously important.

The end of the war meant a rapid end to Hertzsprung’s directorship of the Leiden Observatory. Oort succeeded him and also became a full professor. Much of Oort’s time was now devoted to improving conditions at the observatory and attending to administrative tasks. But he was also itching to press forward with the radio researches he had started in the war.

Van de Hulst joined the Yerkes Observatory in the United States on a postdoctoral position at the conclusion of the war. He visited Grote Reber and encouraged him to search for the 21-centimeter line, but while Reber later started to build a spectrometer to try to detect the line he did not complete the project. Back at Leiden, Oort stressed the importance of radio observations for studies of the galactic system and interstellar matter. He wanted to build a large dish and radio receiver and sought information from assorted experts on how this might be done. He even wrote to Reber for advice. Oort made his first proposal for a radio telescope to the Dutch Academy of Sciences in late 1945, which then forwarded it to the government. But unlike other nations that had been actively engaged in the development of radar in World War II, the Netherlands was way behind the state of the art, nor were large sums of money available for research expenditures.

Despite the consequent slow start, Oort now showed himself to be an effective political insider with many useful contacts. He was friendly and courteous in his personal dealings although some students found him intimidating as he continually urged them to probe more deeply into a problem. As director at Leiden and the leader of Dutch astronomy, he displayed tenacity in pursuing institutional goals and demonstrated the knack of being able to persuade others of the worth of projects in which he believed. In 1948 Oort helped establish the Stichting voor Radios-traling van Zon en Melkweg (Foundation for radio radiation from the Sun and the Milky Way, also often also known as the Netherlands Foundation for Radio Astronomy). This became the organization that would put radio astronomy onto a secure footing in Holland. Observations got underway in 1948 at Kootwijk (the site of the Dutch Postal and Telephone Company’s research station) with the aid of a salvaged antenna originally used in the war by the Germans. But matters moved achingly slowly. The result was that the detection of the 21-centimeter line was made first at Harvard by Harold I. Ewen and Edward M. Purcell in 1951. Shortly after, the line was detected in Holland, and additional confirmation came about a month later from Wilbur Norman Christiansen and J. V. Hindman in Australia. Oort was soon busy searching for an interpretation of the growing number of observations. He reckoned that some of the gas being detected in the plane of the galaxy was at a distance of about 25,000 light-years, far beyond the reach of optical observations to that date.

With the successful detection of the 21-centimeter line, Oort’s arguments in favor of a larger instrument to investigate the line grew in force. In 1956 a radio telescope with a dish of diameter 25 meters (it was the biggest in the world for about a year) was built at Dwingeloo. With this instrument in operation, he immediately advocated a still more powerful one. Oort’s drive was to be crucial in the establishment in 1970 of the Westerbork Synthesis Radio Telescope, a collection of twelve 25-meter dishes arranged in an east-west line 1.5 kilometers long.

With his involvement in radio astronomy, Oort became much more of an observational astronomer than he had ever been before. In writing on radio astronomy topics, he now also generally coauthored papers with one or two colleagues. But he maintained his focus on the structure and dynamics of the galactic system. Oort had long suspected that the galaxy is a form of spiral. Proving this, however, was very challenging due to the obstacles thrown up by interstellar absorption and the Sun’s position within the galactic system. However, in 1951 the American astronomer William W. Morgan, aided by Donald Osterbrock and Stewart Sharpless, presented what other astronomers regarded as compelling evidence of spiral arms within the galactic system by charting the positions of clouds of ionized hydrogen, HII regions. The solution to the long-standing question of the existence of spiral arms had not come, then, from the sorts of sophisticated star-counting techniques pioneered in large part by Kapteyn and other Dutch astronomers. But the future would lie with radio techniques and Oort, in a joint paper with van de Hulst and C. A. Muller, quickly helped to both confirm and extend Morgan’s arguments on spiral arms though radio observations. A few years later Oort coauthored with Frank J. Kerr and Gart Westerhout “The Galactic System as a Spiral Nebula.” This paper contained what became a very famous representation of the distribution of neutral hydrogen in the plane of the galaxy, and Oort and his colleagues argued that the Sun is on the inner section of an arm of neutral hydrogen. As Oort had anticipated over a decade earlier, radio observations had indeed made it possible for astronomers to probe the most distant regions of the galactic system.

The European Southern Observatory . In addition to his efforts in establishing the Netherlands at the forefront of radio astronomy by securing powerful radio telescopes, Oort was also eager to secure access to large optical telescopes for Dutch astronomers. The idea for what became the European Southern Observatory was sparked by discussions between Walter Baade, one of the outstanding observational astronomers of his time and very experienced in the use of large telescopes, and Oort. Baade, although based at Mount Wilson in California, very much viewed himself as a European, and he often gave advice to Oort as Oort worked hard and adroitly with various international partners to fashion an observatory to be funded by the governments of a number of European nations. Planning got seriously underway after Oort called a meeting of leading European astronomers in Groningen in 1953. It still took over a decade of demanding diplomacy to reach an agreement between the Netherlands, France, the Federal Republic of Germany, and Sweden (Belgium and Denmark would join a few years later) to establish the European Southern Observatory. The observatory was dedicated on La Silla in Chile in 1969 and went into operation, initially with middle-sized telescopes but larger ones followed.

Comets . In comparison with his studies in galactic and extragalactic astronomy, Oort spent little time on solar system astronomy. However he did perform very important researches on comets. This line of study grew out of the researches of a student at Leiden, A. J. J. van Woerkom, who worked on comets’ orbits. His interest piqued by van Woerkom’s dissertation, Oort in short order developed his own theory of a cloud of protocomets that surround the solar system. He argued that when the solar system was formed, very many small bodies orbited between Mars and Jupiter. Over time, Jupiter’s gravitational force acted to expel many of these objects from the solar system, but some were carried into highly elongated orbits that took them out from the Sun to distances between 50,000 astronomical units and 200,000 astronomical units. Oort further calculated that passing stars could redirect some of these bodies into the inner regions of the solar system, thereby producing a “new” comet. While Oort termed his scheme “speculative” when he advanced it in 1950, it had become widely accepted by the late twentieth century and his proposed cloud of comets has become known as the Oort cloud.

Galaxies and Retirement . In his inaugural lecture as a professor at Leiden in the 1935, Oort had discussed the origin and evolution of the universe and galaxies. He was also deeply interested throughout his career in the clues offered by observations of other galaxies in interpreting our own galaxy. But he began to regularly write scientific papers on other galaxies only in the 1950s. Of particular interest to him were issues of structure that might be significant on a cosmological scale, especially the development of superclusters of galaxies. When he retired in 1970, he chose to speak at his seventieth-birthday symposium on “Galaxies and the Universe.”

With retirement he was freed of administrative duties but he continued to be very active in research until well into his eighties, and the last piece he wrote for an astronomical journal was published in the year he died. He won numerous prestigious prizes and awards, as well as honorary doctorates from Brussels, Cambridge, Harvard, Oxford, Turin, and others.


The collections of the Center for the History of Physics of the American Institute of Physics contain an important oral history with Oort by David DeVorkin taken in 1977.


“The Stars of High Velocity.” Thesis, Groningen University. Publications of the Kapteyn Astronomical Laboratory at Groningen 40 (1926): 1–75.

“Observational Evidence Confirming Lindblad’s Hypothesis of a Rotation of the Galactic System.” Bulletin of the Astronomical Institute of the Netherlands3 (1927): 275–282.

“The Force Exerted by the Stellar System in the Direction Perpendicular to the Galactic Plane and Some Related Problems.” Bulletin of the Astronomical Institute of the Netherlands6 (1932): 249–287.

“Absorption and Density Distribution in the Galactic System.” Bulletin of the Astronomical Institute of the Netherlands8 (1938): 233–264.

With Nicholas Ulrich Mayall. “Further Data Bearing on the Identification of the Crab Nebula with the Supernova of 1054 A.D.: Part II: The Astronomical Aspects.” Publications of the Astronomical Society of the Pacific54 (1942): 95–104.

“Some Phenomena Connected with Interstellar Matter.” Monthly Notices of the Royal Astronomical Society 106 (1946): 159–179. George Darwin Lecture.

With C. A. Muller. “The Interstellar Hydrogen Line at 1.420 Mc/sec, and an Estimate of Galactic Rotation.” Nature 168 (1951): 357–358.

“Origin and Development of Comets.” Observatory 71 (1951): 129–144. Halley Lecture.

With Th. Walraven. “Polarization and Composition of the Crab Nebula.” Bulletin of the Astronomical Institute of the Netherlands 12 (1956): 285–308.

With Frank J. Kerr and Gart Westerhout. “The Galactic System as a Spiral Nebula.” Monthly Notices of the Royal Astronomical Society 118 (1958): 379–389.

“Galaxies and the Universe.” Science 170 (1970): 1363–1370.

“The Development of Our Insight into the Structure of the Galaxy between 1920 and 1940.” Annals of the New York Academy of Sciences 198 (1972): 255–266.

“The Galactic Center.” Annual Review of Astronomy and Astrophysics15 (1977): 295–362.

“Superclusters.” Annual Review of Astronomy and Astrophysics21 (1983): 373–428.

“The Origin and Dissolution of Comets,” Observatory106 (1986): 186–193. Halley Lecture.


Blaauw, Adriaan. ESO’s Early History: The European Southern Observatory from Concept to Reality. Garching, Germany: European Southern Observatory, 1991. Details Oort’s role in establishing the European Southern Observatory.

_____. History of the IAU: The Birth and First Half-Century of the International Astronomical Union. Dordrecht, Netherlands: Kluwer, 1994.

Katgert-Merkelijn, J. K. The Letters and Papers of Jan Hendrik Oort. Dordrecht, Netherlands: Kluwer, 1997. Contains a bibliography of Oort’s papers.

Kruit, P. C. van der, and Klaass van Berkel. The Legacy of J. C. Kapteyn: Studies on Kapteyn and the Development of Modern Astronomy. Dordrecht, Netherlands: Kluwer, 2000. Especially significant for Oort are the chapters by Klaass van Berkel, David H. DeVorkin, and Woodruff T. Sullivan III.

Osterbrock, Donald E. Walter Baade: A Life in Astrophysics. Princeton, NJ: Princeton University Press, 2001.

Paul, Erich Robert. “The Death of a Research Programme: Kapteyn and the Dutch Astronomical Community.” Journal for the History of Astronomy 12 (1981): 77–94.

Smith, Robert W. The Expanding Universe: Astronomy’s Great Debate 1900–1931. Cambridge, U.K.: Cambridge University Press, 1982.

Woerden, Hugo van, Willem N. Brouw, and Henk C. van de Hulst, eds. Oort and the Universe: A Sketch of Oort’s Research and Person. Dordrecht, Netherlands: D. Reidel, 1980. Important collection of papers detailing various aspects of Oort’s career presented on the occasion of Oort’s eightieth birthday.

Robert W. Smith

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Jan Hendrik Oort

Jan Hendrik Oort

The Dutch astronomer Jan Hendrik Oort (1900-1992) overturned the idea that our sun is at the center of the Milky Way galaxy. He contributed greatly to knowledge about the structure and evolution of our galaxy, and also discovered the place of origin of most comets, the Oort Cloud.

Jan Oort was born on April 28, 1900, in the farming village of Franeker in Holland. At the age of 17 he entered the University of Groningen and earned his doctoral degree in 1926. He received the Bachiene Foundation Prize (1920), undertook research at the Leiden Observatory (1924), and lived abroad as a research associate at the Yale University Observatory (1924-1926).

In 1926 Oort became an instructor at the University of Leiden, and the following year he married Johanna M. Graadt van Roggen. They had three children, sons Coenraad and Abraham and a daughter, Marijke. Oort became a professor of astronomy (1935) and director of the observatory (1945) at the University of Leiden. In his career he was elected leader of several international astronomical groups. He received numerous awards, including the important Vetlesen Prize in 1966 from Columbia University.

Oort's early studies, under his teacher Jacobus Kapteyn, made him familiar with Kapteyn's celestial model, which placed the sun at the center of a relatively small galaxy. In 1917, however, Harlow Shapley challenged Kapteyn's model, proposing a far bigger one. Oort's first major scientific achievement was to provide observational evidence that confirmed the main features of Shapley's model. Shortly after he joined the Leiden faculty in 1926, Oort found that stars with velocities greater than about 65 kilometers per second move predominantly toward one hemisphere of the night sky. That is consistent with the theory that our solar system rotates around the distant center of our galaxy and that other solar systems move around the same center. It was the first direct evidence of the Milky Way's rotation.

From his observations and calculations, Oort was able to show that our galaxy was much bigger than previously thought and that it contained many more stars. Oort also determined that the sun was not even close to the galaxy's center. "Like a modern Copernicus, Oort showed that our position in nature's grand scheme was not so special," said Seth Shostak, a U.S. astronomer.

After World War II Oort and his associates at Leiden built a huge radio telescope to detect radio waves in hydrogen and made far-reaching discoveries on the evolution and structure of our galaxy. They found evidence that supported the hypothesis that stars are formed out of hydrogen and dust clouds; they proved the spiral structure of our galaxy and found its period of rotation to be over 200 million years; and they located and investigated the processes occurring in the galactic core and the vast corona of hydrogen encircling the galaxy. They also investigated the origin of radio signal sources, including the group of stars known as the Crab Nebula, which they demonstrated to be a remnant of the supernova that appeared in 1054. Oort was credited with promoting radio astronomy in its early years and with putting the Netherlands in the forefront of postwar astronomy.

Oort's observations showed that there is much more mass in the universe than can be detected visually. This was a pioneering recognition of the undetected "missing mass" or "dark matter" that is believed to make up more than 90 percent of the universe.

Oort is best known to casual students of astronomy for his discoveries in what to him was a sideline, the study of comets. By plotting their trajectories, Oort traced comets back to a region on the outskirts of the solar system. He theorized that in the distant past a planet that occupied a position between Mars and Jupiter exploded, sending most of its material into interstellar space, but a small percentage of the material became trapped in a region roughly 4,000 times as far away from our sun as Pluto. Fragments of this material are occasionally pulled by the gravity of the outer planets or a passing star into an orbit around the sun. The region that is the birthplace of comets became known as the Oort Cloud.

Further Reading

For a general account of Oort's work in a broader context see Otto Struve and Velta Zebergs, Astronomy of the 20th Century (1962). An appreciation of Oort by B. Strömgren is in Lodewijk Woltjer, ed., Galaxies and the Universe (1968). He is profiled in Asimov's Biographical Encyclopedia of Science and Technology (1976). □

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Oort, Jan Hendrik

Jan Hendrik Oort (ōrt), 1900–1992, Dutch astronomer. He confirmed (1927) Bertil Lindblad's theory of the Milky Way galaxy's rotation. In the 1950s he and his colleagues used radio astronomical means to map the spiral-arm structure of the galaxy. Oort proposed (1950) that comets originate in a cloud of material (the Oort cloud) orbiting the sun at great distance and that they are occasionally deflected into the inner solar system by gravitational perturbation from the passing of nearby stars.

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Oort, Jan Hendrik

Oort, Jan Hendrik (1900–92) Dutch astronomer. He carried out research on the structure and dynamics of stellar systems, especially our galaxy, whose rotation he confirmed in 1927. In 1950, he proposed the existence of what has come to be called the Oort cloud – a spherical region of space surrounding the Solar System in which comets are thought to reside.

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