Jan Oort

Dutch astronomer (1900–1992)

The son of a physician from Franeker in the Netherlands, Oort(ohrt) was educated at the University of Gröningen where he worked under Jacobus Kapteyn and gained his PhD in 1926. After a short period at Yale University in America he was appointed to the staff of the University of Leiden where he was made professor of astronomy in 1935 and from 1945 to 1970 served as director of the Leiden Observatory. He also served as director of the Netherlands Radio Observatory.

Oort's main interest was in the structure and dynamics of our Galaxy. In 1927 he succeeded in confirming the hypothesis of galactic rotation proposed by Bertil Lindblad. He argued that just as the outer planets appear to us to be overtaken and passed by the less distant ones in the solar system, so too with the stars if the Galaxy really rotated. It should then be possible to observe distant stars appearing to lag behind and be overtaken by nearer ones. Extensive observation and statistical analysis of the results would thus not only establish the fact of galactic rotation but also allow something of the structure and mass of the Galaxy to be deduced.

Oort was finally able to calculate, on the basis of the various stellar motions, that the Sun was some 30,000 light-years from the center of the Galaxy and took about 225 million years to complete its orbit. He also showed that stars lying in the outer regions of the galactic disk rotated more slowly than those nearer the center. The Galaxy does not therefore rotate as a uniform whole but exhibits what is known as ‘differential rotation’.

Oort was also one of the earliest of the established astronomers to see the potential of the newly emerging discipline of the 1940s, radio astronomy. As one of the few scientists free to do pure research in the war years, he interested Hendrik van de Hulst in the work that finally led to the discovery in 1951 of the 21-centimeter radio emission from neutral interstellar hydrogen.

By measuring the distribution of this radiation and thus of the gas clouds Oort and his Leiden colleagues lost little time in tracing the spiral structure of the galactic arms and made substantial improvements to the earlier work of William Morgan. They were also able to make the first investigation of the central region of the Galaxy: the 21-centimeter radio emission passed unabsorbed through the gas clouds that had hidden the center from optical observation. They found a huge concentration of mass there, later identified as mainly stars, and also discovered that much of the gas in the region was moving rapidly outward away from the center.

Oort made major contributions to two other fields of astronomy. In 1950 he proposed that a huge swarm of comets surrounded the solar system at an immense distance and acted as a cometary reservoir. A comet could be perturbed out of this Oort cloud by a star and move into an orbit taking it toward the Sun. In 1956, working with Theodore Walraven, he studied the light emitted from the Crab nebula, a supernova remnant. The light was found to be very strongly polarized and must therefore be synchrotron radiation produced by electrons moving at very great speed in a magnetic field.

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).

Jan Oort
Commemorative plaque at the house where Jan Hendrik Oort was born, Franeker, Netherlands
Born	28 April 1900(1900-04-28) Franeker, Friesland
Died	5 November 1992 (aged 92) Leiden
Nationality	Dutch
Fields	astronomy
Doctoral advisor	Jacobus Cornelius Kapteyn
Known for	Oort cloud

Jan Hendrik Oort (28 April 1900 – 5 November 1992) was a Dutch astronomer. He was a pioneer in the field of radio astronomy. The Oort cloud of comets bears his name.

Oort was born in Franeker, Friesland and studied in Groningen with Jacobus Cornelius Kapteyn. His Ph.D thesis was titled The stars of high velocity. In 1927 he confirmed Bertil Lindblad's theory that the Milky Way galaxy rotates, by analyzing the movements of stars.^[1] In 1935 he became professor at the observatory of the University of Leiden, where Ejnar Hertzsprung was the director.

Oort was fascinated by radio waves from the universe. After the Second World War he began work in the new field of radio astronomy, using an old radar antenna from the Germans.

In the 1950s he raised funds for a new radio telescope in Dwingeloo, in the east part of the Netherlands, to research the center of the galaxy. In 1970 a bigger telescope (the Westerbork Synthesis Radio Telescope) was built in Westerbork, near the old one. It consisted of twelve smaller telescopes working together to perform radio interferometry observations, a technique which had been previously suggested by Oort, but which was first tested experimentally in Cambridge by Martin Ryle and in Sydney by Joseph Pawsey.

His hypothesis that the comets have a common origin, postulated in 1950, was later proven to be incorrect in detail, though correct in principle. That is, different types of comets have origins in different regions of the outer solar system. For more, see Oort Cloud, Hills Cloud, and Kuiper Belt. Another contribution Oort made was to demonstrate that the light from the Crab nebula was polarized.

Oort died on 5 November 1992 in Leiden.

Contents 1 A few of Oort's discoveries 2 Honors 3 References 3.1 Notes 3.2 Literature 4 Online exhibition

A few of Oort's discoveries

• In 1924, Oort discovered the galactic halo, a group of stars orbiting the Milky Way but outside the main disk.^[2]
• In 1927, he calculated that the center of the Milky Way was 5,900 parsecs (19,200 light years) from the Earth in the direction of the constellation Sagittarius.^[1]
• He showed that the Milky Way had a mass 100 billion times that of the Sun.
• In 1950 he suggested that comets came from a common region of the Solar System (now called the Oort cloud).
• He found that the light from the Crab Nebula was polarized, and produced by synchrotron emission.

Honors

Awards

• Bruce Medal of the Astronomical Society of the Pacific in 1942
• Gold Medal of the Royal Astronomical Society in 1946
• Henry Norris Russell Lectureship of the American Astronomical Society in 1951
• Gouden Ganzeveer in 1960
• Karl Schwarzschild Medal of the Astronomische Gesellschaft in 1972
• Balzan Prize for Astrophysics in 1984

Named after him

• Asteroid 1691 Oort
• Oort Cloud
• Oort constants of galactic structure

Upon his death, Subrahmanyan Chandrasekhar remarked, "The great oak of Astronomy has been felled, and we are lost without its shadow."^{[citation needed]}

References

Notes

1. ^ ^{a b} J. H. Oort (1927-04-14). "Observational evidence confirming Lindblad's hypothesis of a rotation of the galactic system". Bulletin of the Astronomical Institutes of the Netherlands 3 (120): 275–282. 
2. ^ J. H. Oort (June 1924). "On a Possible Relation between Globular Clusters and Stars of High Velocity". Proc Natl Acad Sci U S A. 10 (6): 256–260. doi:10.1073/pnas.10.6.256. PMID 1085635. http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1085635. 

Literature

• Oort (1970), "Galaxies and the Universe: Properties of the universe are revealed by the rotation of galaxies and their distribution in space", Science 170 (3965): 1363–1370, 1970 Dec 25, doi:10.1126/science.170.3965.1363, PMID 17817459 
• Rougoor, G W; Oort, J H (1960), "Distribution And Motion Of Interstellar Hydrogen In The Galactic System With Particular Reference To The Region Within 3 Kiloparsecs Of The Center", Proc. Natl. Acad. Sci. U.S.A. 46 (1): 1–13, 1960 Jan, doi:10.1073/pnas.46.1.1, PMID 16590580 
• Oort, J H (1924), "Note on the Difference in Velocity between Absolutely Bright and Faint Stars", Proc. Natl. Acad. Sci. U.S.A. 10 (6): 253–6, 1924 Jun, doi:10.1073/pnas.10.6.253, PMID 16586937 

Online exhibition

Jan Oort, astronomer (Leiden University Library, April-May 2000) [1]

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