Jan Hendrik Oort
The Netherlands
1984 Balzan Prize for Astrophysics
For having exerted a profound influence on 20th century astronomy. Jan Hendrik Oort is acknowledged all over the world as a leader in this field because of his pioneering investigations on galactic rotation, on the distribution of hydrogen in the galaxies and on gaseous clouds in intergalactic space; he has played a leading role in introducing and using radioastronomy as a tool in studying cosmic physics.
Professor Jan Hendrik Oort (*1900 - †1992) of Leiden University is internationally regarded as an outstanding leader in the field of astronomy. He initiated his great work on the composition and dynamics of galaxies by discovering (1926-1927) with Professor Bertil Lindblad (Stockholm Observatory) the rotation of our galaxy, now a fundamental astronomical concept.
This phenomenon deduced from the rational interpretation of the apparent star movements, is due to the complex dynamics of the galaxy in its own gravitational field.
Oort found that the galaxy doesn’t rotate like a stiff wheel, but has a rotating motion similar to the revolving of planets round the sun. Indeed, in a wheel the particles have the same period of revolution, while in the solar system, the planets nearest to the sun have a shorter period. This is a characteristic of the circular motion round a centre of gravity, in fact most of the galaxy mass lies in central area.
In 1944 Oort entered upon his second period of research, radioastronomy. He brought radioastronomy to a level that opened wholly new vistas onto celestial space. In particular his laboratory managed to record radiation from galaxies more distant than any previously observed. Helped by his pupil Van de Hulst, who showed that a 21 cm radiowave emanates from interstellar hydrogen clouds, Oort succeeded in mapping the spiral arms of a galaxy; consequently, he could study, for the first time, the whole galactic system.
The most favoured description of the origin of comets in the solar system is also due to Oort’s study. In this picture there is an Oort cloud of comets orbiting a very large distance from the sun, but occasionally perturbations by passing stars cause the comets to be put into orbits which pass close to the sun, making them observable.
Since 1970 Oort has been studying the structure and the dynamics of external galaxies trying to verify, in their central areas, the existence of great structures in expansion.

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