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Oort at Leiden University, in 1976

Jan Hendrik Oort (April 28, 1900, Franeker – November 5, 1992, Leiden) was an internationally famous Dutch astronomer. He stimulated radio astronomy. The Oort cloud of comets bears his name.

Oort was born in 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 was a pioneer 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 correct. Another contribution of Oort was that he was able to demonstrate that the light from the Crab nebula was polarized.

• 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.

Awards

• Bruce Medal in 1942
• Gold Medal of the Royal Astronomical Society in 1946
• Henry Norris Russell Lectureship in 1951
• Karl-Schwarzschild-Medaille of the Astronomische Gesellschaft in 1972

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", reflecting the high regard in which Oort was held by his colleagues.

1. ^ ^{a b} J. H. Oort (1927 April 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. PMID 1085635. 

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