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Jan Hendrik Oort

Born: April 28, 1900, in Franeker, Netherlands
Died: November 5, 1992, in Leiden, Netherlands

After graduating from Groningen University, Jan Hendrik Oort was appointed astronomer to Leiden Observatory in 1924. The following year, Swedish astronomer Bertil Lindblad theorized that the Milky Way rotates in its own plane around the centre of the galaxy. In 1927, Oort was able to confirm this hypothesis through his own observations of star velocities in the galaxy. In his subsequent work, he focused his efforts on testing and strengthening the Lindblad-Oort theory.

In 1935, Oort became a professor of Leiden University. In the same year, using radio astronomy, he was able to determine that the Sun is 30,000 lightyears from the Milky Way's centre and takes 225 million years to make a complete orbit around it.

Oort was promoted to director of Leiden Observatory in 1945, and held that post until 1970.

In 1950, Oort advanced the theory that comets originate from a vast cloud made up of small bodies composed of rock and ice, orbiting the Sun at a distance of about one lightyear. He postulated that when the orbits of these bodies are disturbed, for example by the influence of approaching stars, they are sometimes nudged into trajectories that take them close to the Sun. Over time, astronomers have come to accept the existence of this region and have named it the Oort Cloud in his honour.

The discovery of interstellar hydrodrogen's 21cm emission line* in 1951 (by American physicists Harold Ewen and Edward Purcell), was a boon for astronomers. Using this new knowledge as a tool, Oort was able to map the Milky Way's spiral structure in 1952.

Jan Hendrik Oort

Among his other achievements, Oort served as general-secretary of the International Astronomical Union from 1935 to 1948, and as president of the organization from 1958 to 1961.

*The hydrogen 21cm emission line refers to the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation is at the precise frequency of 1402.4 MHz, which is equivalent to 21.1 cm in free space. This wave length, or frequency, falls within the microwave radio region of the electrmagnetic spectrum, and it is used frequently in radio astronomy, since those radio waves can penetrate the large clouds of interstellar dust which are opaque to visible light.