Oort, Jan Hendrik (ōrt) [key], 1900–1992, Dutch astronomer. He confirmed (1927) Bertil Lindblad's theory of the Milky Way galaxy's rotation. In the 1950s he and his colleagues used radio astronomical means to map the spiral-arm structure of the galaxy. Oort proposed (1950) that comets originate in a cloud of material (the Oort cloud ) orbiting the sun at great distance and that they are occasionally deflected into the inner solar system by gravitational perturbation from the passing of nearby stars.
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