Jan Hendrik Oort ForMemRS[1] (/ˈɔːrt/ or /ˈʊərt/;[2] 28 April 1900 – 5 November 1992) was a Dutch astronomer who made significant contributions to the understanding of the Milky Way and who was a pioneer in the field of radio astronomy.

[8] Additionally Oort is responsible for a number of important insights about comets, including the realization that their orbits "implied there was a lot more solar system than the region occupied by the planets.

[3] His one hesitation about studying pure science was the concern that it "might alienate one a bit from people in general", as a result of which "one might not develop the human factor sufficiently."

But he overcame this concern and ended up discovering that his later academic positions, which involved considerable administrative responsibilities, afforded a good deal of opportunity for social contact.

He later considered his experience at Yale useful as he became interested in "problems of fundamental astronomy that [he] felt was capitalized on later, and which certainly influenced [his] future lectures in Leiden."

"[10] In 1924, Oort returned to the Netherlands to work at Leiden University, where he served as a research assistant, becoming Conservator in 1926, Lecturer in 1930, and Professor Extraordinary in 1935.

[4] Oort "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", according to the Oxford Dictionary of Scientists.

[11] He "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.

"[12] These early discoveries by Oort about the Milky Way overthrew the Kapteyn system, named after his mentor, which had envisioned a galaxy that was symmetrical around the Sun.

"[10] As the European Space Agency states, Oort "sh[ook] the scientific world by demonstrating that the Milky Way rotates like a giant 'Catherine Wheel'."

In the early 1930s he received job offers from Harvard and Columbia University, but chose to stay at Leiden, although he did spend half of 1932 at the Perkins Observatory, in Delaware, Ohio.

[4] In 1934, Oort became assistant to the director of Leiden Observatory; the next year he became General Secretary of the International Astronomical Union (IAU), a post he held until 1948; in 1937 he was elected to the Royal Academy.

In 1939, he spent half a year in the U.S., and became interested in the Crab Nebula, concluding in a paper, written with American astronomer Nicholas Mayall, that it was the result of a supernova explosion.

The speech by Rudolph Cleveringa, the dean of the faculty of Law and former graduate student of professor Meijers, was widely circulated during the rest of the war by the resistance groups.

Resigning from the Royal Academy, from his professorial post at Leiden, and from his position at the Observatory, Oort took his family to Hulshorst, a quiet village in the province of Gelderland, where they sat out the war.

Oort and his colleagues also made the first investigation of the central region of the Galaxy, and discovered that "the 21-centimeter radio emission passed un-absorbed 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 was helped in this project by the Dutch telecommunications company, PTT, which, he later explained, "had under their care all the radar equipment that was left behind by the Germans on the coast of Holland.

This radar equipment consisted in part of reflecting telescopes of 7 1/2 meter aperture.... Our radio astronomy was really started with the aid of one of these instruments… it was in Kootwijk that the first map of the Galaxy was made.

[4] Oort also investigated the source of the light from the Crab Nebula, finding that it was polarized, and probably produced by synchrotron radiation, confirming a hypothesis by Iosif Shklovsky.

After his retirement, he wrote comprehensive articles on the galactic center and on superclusters and published several papers on the quasar absorption lines, supporting Yakov Zel'dovich's pancake model of the universe.

[4] An incomplete list: Awards Named after him Memberships Upon his death, Nobel Prize winning astrophysicist Subrahmanyan Chandrasekhar remarked, "The great oak of Astronomy has been felled, and we are lost without its shadow.