Jan Hendrik Oort

Dutch astronomer

• Born: April 28, 1900; Franeker, Netherlands
• Died: November 5, 1992; Leiden, Netherlands

Dutch astronomer Jan Hendrik Oort postulated the existence of the vast swarm of comets known subsequently as the Oort cloud. One of the pioneers of radio astronomy, he also proposed the existence of dark matter and helped establish the structure of the Milky Way Galaxy.

Primary field: Astronomy

Specialties: Observational astronomy; theoretical astronomy

Early Life

Jan Hendrik Oort was the son of Abraham Hermanus Oort, a physician, and Ruth Hannah Faber. Following his graduation from the Leiden gymnasium (secondary school) in 1917, Oort enrolled in the University of Groningen, where he studied under the prominent astronomer Jacobus Kapteyn, one of the pioneers in mapping the Milky Way Galaxy.

After sharing the Bachiene Foundation Prize in 1920 for his paper on stars of the F, G, K, and M types, Oort turned his attention to Kapteyn’s research. He began studying stars of high velocity, which had proved difficult to reconcile with a galactic model that had the sun near its center. Oort spent the years 1922 to 1924 in the United States as a research assistant at Yale University in New Haven, Connecticut, during which time he became familiar with Harvard University astronomer Harlow Shapley’s study of the Milky Way Galaxy. Oort’s analysis of high-velocity stars and his close observation of Shapley’s work during his research assistantship convinced Oort that the galaxy was much larger than Kapteyn had assumed.

On his return to the Netherlands, Oort began working at the University of Leiden, an association that would last throughout his career. He became an instructor in 1926, after receiving his PhD from the University of Groningen with a dissertation on high-velocity stars. Oort married Johanna Maria Graadt van Roggen in 1927.

Life’s Work

Two papers presented in 1927 established Oort’s reputation as a major astronomical scholar. In them, Oort wrote that the Milky Way Galaxy rotates not as a unit but more like the solar system, with those stars far from the center moving slower and those closer moving faster. Through elaborate calculations involving rotational velocities of various stars, Oort introduced the Oort constants, A and B, which helped him determine galactic rotation. He was then able to estimate the galactic mass, the number of stars, and the dimensions of the Milky Way. Oort also established Oort’s limit, which is the value of density near the Milky Way Galaxy’s midpoint. In 1932, he proposed the existence of dark matter, which is believed to make up most of the mass of the universe.

Oort was promoted to professor of astronomy in 1935. During the next decade, he turned his attention to the distribution of stars in the Milky Way Galaxy, realizing that Kapteyn’s problem in creating a galactic model had been the interstellar dust that obscures the view of the distant parts of the galaxy. Although Oort’s theory about this dust was accepted by the scientific community, it was not possible to test his hypothesis until the development of radio astronomy in the 1940s and 1950s.

Oort’s student Hendrik van de Hulst theorized in 1944 that clouds of hydrogen should emit radio waves at a constant wavelength of twenty-one centimeters, and that measuring this spectral line could help astronomers determine the structure of the Milky Way. Along with Dutch astronomer Alex Muller, Oort and van de Hulst constructed a twenty-one-centimeter receiver that enabled them to prove van de Hulst’s hypothesis in 1951; the twenty-one-centimeter waves pass through dust and gas, allowing astronomers to “see” as they cannot with optical telescopes. This and the construction of large radio telescopes led to the establishment of radio astronomy.

In 1950, Oort proposed the existence of what became known as the Oort cloud. His observations of the actions of nineteen long-period comets led him to the conclusion that there exists a vast body of trillions of comets consisting largely of debris from the disintegration of a planet. As a result of gravitational forces from various stars, comets are periodically thrust into orbits that bring them into our solar system. Oort theorized that there must be a swarm of these comets extending almost to the nearest stars.

Oort and van de Hulst completed a map of the spiral structure of the Milky Way Galaxy’s outer region, based on their radio-wave analysis, in 1954. Oort also showed that the part of the galaxy that is visible is far too small to account for its gravitational effect on distant stars, which later led to evidence of a black hole in the Milky Way. In 1956, Oort and van de Hulst published their discovery of radio signals produced by ionized hydrogen in space. Further investigation showed that these emissions occur across the Milky Way. Using the twenty-one-centimeter wavelength, they established the rotational period of the galaxy to be 225 million years.

Also in 1956, Oort engaged in the first extensive examination of the light polarization in the Crab Nebula, a gaseous mass assumed to be the result of the fabled supernova of the year 1054. In a 1957 article in Scientific American, Oort stated that the light of this nebula consists of synchrotron radiation produced by high-velocity electrons spiraling about large magnetic fields. This phenomenon is believed to be responsible for the significant radio-wave radiation from the Crab Nebula and other radio sources in space.

Along with Australian astronomer Frank Kerr, Oort discovered in the late 1950s that the galactic core of the Milky Way consists of “turbulent” hydrogen that is constantly expanding outward, only to be replaced by other hydrogen falling back into the center from the top and bottom of the central bulge. Oort’s observations suggest that a massive explosion in the core of the disk, perhaps ten million years ago, produced this turbulence. Further radio analysis allowed him to establish the Milky Way Galaxy as a rapidly whirling disk with a jagged edge, produced by the juxtaposition of rapidly rotating material on the inside and material of slow rotation on the outside. In the 1960s, Oort and his colleague G. W. Rougoor discovered a ring of neutral hydrogen three thousand parsecs from the galactic center. These gases, which form the spiral arms of the Milky Way, are expanding rapidly outward and also support the hypothesis of a major explosion.

Oort then began studying the space through which the Milky Way Galaxy moves, a nearly limitless cloud of hydrogen, both on its own and as part of the cluster of galaxies to which it belongs. Given the fact that the densest part of this hydrogen corona is above, he hypothesized that the galaxy is moving upward. He also theorized that while some gas is absorbed and other gas is moving outward, the fact that some is moving inward will double the mass of the galaxy every three billion years and cause it to change shape.

In addition to his duties in Leiden, Oort served from 1935 to 1948 as general secretary of the International Astronomical Union and from 1958 to 1961 as its president. He received numerous honorary degrees and other awards, including the 1942 Bruce Medal from the Astronomical Society of the Pacific, the Gold Medal of the Royal Astronomical Society in 1946, and the Vetlesen Prize—established to supplement the Nobel Prizes in areas where they are not awarded—in 1966. Oort died in Leiden on November 5, 1992.

Impact

Oort’s name and reputation are remembered through the Oort cloud, the Oort constants, and Oort’s limit. Despite the public recognition inherent in these names, Oort’s greatest achievements lie in his contributions to an understanding of the structure and function of the Milky Way Galaxy. He was able to blend physics, mathematics, and astronomy to provide evidence for a rotating spiral galaxy. His confirmation of galactic rotation encouraged other astronomers to search for further evidence and to continue efforts to determine the size of the galaxy.

While more sophisticated methods of modern astronomy have modified or enhanced some of Oort’s theoretical work, his methodology, research, and arguments convinced the scientific community to accept his theories even when no direct visual evidence existed. Thus, Oort’s pioneering work in the field of radio astronomy and other areas research of research helped advance knowledge of our galaxy. His work gave impetus to improvements in radio astronomy, and he also stimulated the development of faster photographic film and improved photographic equipment and techniques.

Bibliography

Burke, Bernard F., and Francis Graham-Smith. An Introduction to Radio Astronomy. 3rd ed. New York: Cambridge UP, 2009. Print. Describes radio telescopes and the techniques for using them. Reviews radio observations of the universe, including the work of Oort.

Mo, Houjun, Frank van den Bosch, and Simon White. Galaxy Formation and Evolution. New York: Cambridge UP, 2010. Print. Covers such topics as the universe’s radiation content and dark matter. Includes the Oort constant.

Sagan, Carl, and Ann Druyan. Comet. 1985. New York: Random House, 1997. Print. The study of comets from both a scientific and a cultural perspective. Discusses Oort and the Oort cloud.