Editing Jan Oort (section)

=== Oort's radio astronomy ===
[[File:JanOort.jpg|thumb|Oort by an image of the galaxy [[Messier 81]].]]
Before the war was over, he initiated, in collaboration with a [[Utrecht University]] student, [[Hendrik van de Hulst]], a project that eventually succeeded, in 1951, in detecting the 21-centimeter radio emission from interstellar hydrogen spectral line at radio frequencies. 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."<ref name= OX2 /> In June 1945, after the end of the war, Oort returned to Leiden, took over as director of the Observatory, and became Full Professor of Astronomy.<ref name= NYT /> During this immediate postwar period, he led the Dutch group that built radio telescopes at [[Radio Kootwijk]], Dwingeloo, and Westerbork and used the 21-centimeter line to map the Milky Way, including the large-scale spiral structure, the [[Galactic Center]], and gas cloud motions. 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."<ref name= OHT /> For a brief period, before the completion of the Jodrell Bank telescope, the Dwingeloo instrument was the largest of its kind on Earth.

It has been written that "Oort was probably the first astronomer to realize the importance" of radio astronomy.<ref name= NYT /> "In the days before radio telescopes," one source notes, "Oort was one of the few scientists to realise the potential significance of using radio waves to search the heavens. His theoretical research suggested that vast clouds of hydrogen lingered in the spiral arms of the Galaxy. These molecular clouds, he predicted, were the birthplaces of stars."<ref name= ESA /> These predictions were confirmed by measurements made at the new radio observatories at [[Dwingeloo Radio Observatory|Dwingeloo]] and [[Westerbork Synthesis Radio Telescope|Westerbork]]. Oort later said that "it was [[Grote Reber]]'s work which first impressed me and convinced me of the unique importance of radio observations for surveying the galaxy."<ref name= OHT /> Just before the war, Reber had published a study of galactic radio emissions. Oort later commented, "The work of Grote Reber made it quite clear [radio astronomy] would be a very important tool for investigating the Galaxy, just because it could investigate the whole disc of the galactic system unimpeded by absorption."<ref name= OHT /> Oort's work in radio astronomy is credited by colleagues with putting the Netherlands in the forefront of postwar astronomy.<ref name= NYT /> Oort also investigated the source of the light from the Crab Nebula, finding that it was [[Polarization (waves)|polarized]], and probably produced by [[synchrotron radiation]], confirming a hypothesis by [[Iosif Shklovsky]].<ref name= oort-crab>{{cite book |title=A short biography of Jan Hendrik Oort: 7. Crab Nebula |publisher=Leiden University Library |first1=J. |last1=Katgert-Merkelijn |first2=J. |last2=Damen |chapter=Jan Oort, astronomer : Catalogue of an exhibition in Leiden University library, April 20 - May 27, 2000 |date=2000 |df=dmy-all |hdl = 1887/77628}}</ref>