28 April 1900|
|Died||5 November 1992
|Doctoral advisor||Jacobus Cornelius Kapteyn|
|Known for||Oort cloud|
Jan Hendrik Oort ForMemRS (Franeker, 28 April 1900 – Leiden, 5 November 1992) was a prolific Dutch astronomer. He made many important contributions in the field of astronomy and was a pioneer in the field of radio astronomy. In 1932 he became the first person to discover evidence of dark matter. First proposed by Swiss astronomer Fritz Zwicky in the 1930s, at nearly the same time, Jan Oort in the Netherlands discovered that the density of matter near the Sun was nearly twice what could be explained by the presence of stars and gas alone. 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. 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.
In 1928, his son Coen Oort was born, who later became an important Dutch economist and public official and who in 1990 headed the Oort Commission, which was responsible for a major overturn of Dutch tax law.
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.
- 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.
- In 1932, by measuring the motions of stars in the Milky Way he was the first to find evidence for dark matter, when he found the mass of the galactic plane must be more than the mass of the material that can be seen.
- 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.
- 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
- Van De Hulst, H. C. (1994). "Jan Hendrik Oort. 28 April 1900-5 November 1992". Biographical Memoirs of Fellows of the Royal Society 40: 320–326. doi:10.1098/rsbm.1994.0042.
- 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, Bibcode:1927BAN.....3..275O.
- J. H. Oort; Arias, B; Rojo, M; Massa, M (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, Bibcode:1924PNAS...10..256O, doi:10.1073/pnas.10.6.256, PMC 1085635, PMID 16586938.
- Ken Freeman, Geoff McNamara (2006). In Search of Dark Matter. Springer. ISBN 978-0-387-27616-8. "the story of the emergence of the dark matter problem, from the initial 'discovery' of dark matter by Jan Oort"
- van de Hulst, H. C. (1994), "Jan Hendrik Oort (1900–1992)", Quarterly Journal of the Royal Astronomical Society 35 (2): 237–242, Bibcode:1994QJRAS..35..237V.
- Oort, J. H. (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. 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". Proceedings of the National Academy of Sciences 46: 1. doi:10.1073/pnas.46.1.1.
- Oort, J. H. (1924). "Note on the Difference in Velocity between Absolutely Bright and Faint Stars". Proceedings of the National Academy of Sciences 10 (6): 253. doi:10.1073/pnas.10.6.253.
Online exhibition 
Jan Oort, astronomer (Leiden University Library, April–May 2000)