Jan Hendrik Oort

1900-1992

Dutch Astronomer

Jan Oort is popularly known for his description of the Oort Cloud that now bears his name. This cloud is located far outside of our solar system and is the source of the comets that orbit our sun. However, among professional astronomers, Oort is best known for his many discoveries concerning the structure of the Milky Way and other galaxies.

Oort was born in the Netherlands in 1900. During his lifetime he saw our view of the cosmos transformed from a single galaxy to an entire universe filled with galaxies. Many of his discoveries drastically changed our perceptions of our home galaxy, the Milky Way.

Oort's first major discovery came in 1927. His analysis of the motions of stars in our galaxy provided support for Bertil Lindblad's 1926 theory that our galaxy was rotating. Oort showed that the motion of different streams of stars could be explained easily if the galaxy was rotating. He also showed that it would take our Sun 200 million years to orbit the center. This was a revolutionary discovery, since it had only been recently discovered that the Milky Way did not comprise the entire universe. Oort's discovery was also one of the first demonstrations that gravity didn't act only in our solar system or in nearby binary solar systems, but that the force was felt throughout the entire galaxy. Oort also calculated that most of the galaxy's mass would be concentrated toward its center.

Oort began his studies of comets in the late 1940s when he began supervising a Ph.D. student whose advisor had died. At that time many researchers were grappling with the origin of comets. Measurements of a typical comet's path show that it orbits our Sun and is not merely passing through our solar system a single time. But they seemed to originate from far outside the solar system. Secondly, comets seem to enter our solar system from all directions. Thirdly, comets lose a large percentage of their mass each time they pass near the Sun, which heats and boils them away. Therefore comets have a relatively short lifetime. Given that we still see them today, how is the supply of comets being replenished?

Oort proposed that comets are debris left over from the initial formation of our solar system. Icy clouds of dust and water that were over 30 times the distance of the Sun to the Earth (1 astronomical unit or AU) were not absorbed into the planets. Oort suggested that these clouds of debris merged to form comets and drifted away until they were 30,000-100,000 AU away from the Sun. Chance collisions and gravitational interactions with passing stars would occasionally nudge a comet into motion towards our Sun. This cloud of debris, which surrounds our solar system like a giant ring, is now named the Oort Cloud in his honor.

Oort was also an active leader of the International Astronomical Union. He served as General Secretary and President. As General Secretary during and after World War II Oort was often in the position of mediator between scientists whose countries had formerly been enemies. He was also one of the guiding forces behind the European Southern Observatory. He first attempted organizing this collaboration between many of the nations of Europe in 1954. By 1962 these nations had agreed to begin construction of an optical telescope in the Southern Hemisphere.

Oort was one of the first astronomers to recognize the effect of the interstellar medium on the structure of galaxies. He was also instrumental in advancing radio astronomy in the Netherlands. He used radio measurements to confirm our galaxy's spiral structure, map its center, and track the motion of vast clouds of gas inside and outside our galaxy. After his retirement from active research in 1970 he continued to publish many important review papers on the structure of galaxies, galactic nuclei, and the organization of superclusters of galaxies throughout the universe.

PHILIP DOWNEY