Jan Hendrick Oort | Biography

Jan Oort received his higher education at the University of Groningen where he obtained a Ph.D. in 1926. He began working at the Leiden Observatory in 1924, and became its director in 1945.

In 1927, Oort had been able to demonstrate that the Milky Way galaxy was rotating. Since a galaxy is not a solid object, but comprised of billions of stars, it does not rotate as a single solid object. Instead, stars near the center of the galaxy move faster, while those farther move slower. By studying the motion of the stars in our vicinity, Oort was able to determine that the center of the galaxy lay in the same direction as the constellation of Sagittarius. That was in agreement with the observations of American astronomer Harlow Shapley. Shapley, however, had placed the galactic center at a distance of 50,000 light-years. Oort, taking into account the discovery of dark dust clouds that obscured the light of distant stars, thought the galactic center was not that far away. He suggested a distance of 30,000 light-years, a distance that is accepted today.

Karl Jansky had discovered radio emissions from outer space in the early 1930s, and Grote Reber's 1940s map of their intensity showed a peak toward Sagittarius. Radio waves could penetrate the dust that blocked visual light, making radio telescopes ideally suited for the task of examining the galactic center. However, World War II was in progress, and only theoretical work could be carried out. An associate of Oort's, Hendrik van de Hulst, made calculations which indicated that it should be possible to detect radio noise at the 21 cm wavelength. This would be produced by the hydrogen gas in interstellar space.

Once the war was over, Oort and his group set out to perform the search. They were successful in 1951. Because the hydrogen gas is more concentrated in the arms of the Milky Way galaxy, they were able to use radio telescopes to trace out the spiral structure.

Despite all his work on the structure of the galaxy, Oort is best known for a unique theory to account for the origin of comets. He suggested that there was a great shell of cometary material encircling the solar system at a distance of one light-year. Occasionally objects leave the so-called Oort cloud, for reasons not entriely understood, and head into the inner solar system. As such objects encounter the Sun's radiation, it warms and the gases and ice comprising it develop into the long tails characteristic of a comet.

Jan Oort died on November 12, 1992, his wide-ranging research interests and signal achievements having established him as one of the eminent astronomers of the twentieth century.