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Oort, Jan Hendrik

galaxy sun mass stars

[oh®t] (1900–92) Dutch astronomer: detected the rotation of the Galaxy and proposed theory for origin of comets.

Oort worked mainly in Leiden, as director of the Observatory from 1945–70. In 1927 he proved, by extensive observation of the proper motion of stars, that our Galaxy is rotating, with the nearer stars to the centre having higher angular velocities than more distant ones (recalling that the inner planets of the Solar System move more rapidly than the outer ones). He established that the Sun is about 30 000 light years from the centre of the Galaxy and that it completes an orbit in about 225 million years, moving at 220 km s –1 . The Galaxy has a mass about 10 10 times that of the Sun. He was influential in the discovery in 1951 of the 21 cm radio emission from interstellar hydrogen, which has allowed the distribution of interstellar gas clouds to be mapped. This technique has also revealed a large ‘hidden’ mass of stars at the centre of the Galaxy. In 1956 Oort observed that light from the Crab supernova remnant was strongly polarized, implying that it was synchrotron radiation produced by electrons moving at relativistic velocities through a magnetic field.

In 1950 Oort suggested the existence of a sphere of incipient cometary material surrounding the Solar System, at a distance of about 50 000 AU and with a total mass of perhaps 10–100 times that of the Earth. He proposed that comets occasionally detached themselves from this Oort cloud and went into orbits about the Sun. Because the cloud is spherical comets can approach the Sun at any angle, and not just in the plane of the ecliptic.

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