Yan Oort Dutch astronomer Date of Birth: 28.04.1900 Country: Netherlands

Biography of Jan Oort

Jan Oort was a Dutch astronomer and a member of the Royal Netherlands Academy of Arts and Sciences. He completed his studies at the University of Groningen, where he was mentored by J.C. Kapteyn. From 1921 to 1922, he worked at the same university, and from 1922 to 1924, he worked at the Yale Observatory in the United States.

Career at Leiden Observatory and Leiden University
From 1924 to 1970, Oort worked at the Leiden Observatory, serving as its director from 1945 onwards. He also taught at Leiden University from 1926 to 1970, becoming a professor in 1945. His main scientific research focused on the structure and dynamics of the Galaxy and cosmogony.

Contributions to Galactic Dynamics
In 1927, Oort provided further evidence for Bengt Lindblad's hypothesis on the rotation of the Galaxy by statistically studying radial velocities and proper motions of stars. He discovered that the Galaxy does not rotate as a solid body, with the inner parts rotating faster than the outer parts. He determined the magnitude of the differential rotation effect (known as the Oort constant), the velocity of galactic rotation (220 km/s near the Sun), and the period of rotation (220 million years near the Sun). Oort's work laid the foundation for the study of Galactic dynamics. He also extensively studied the role of interstellar matter in the kinematics and dynamics of the Galaxy.

Radiophysics and Stellar Density
With the advent of radio astronomy, Oort continued his studies of the Galaxy using radio astronomical methods. He participated in establishing the large-scale structure of the Galaxy and investigating interstellar gas clouds. In 1932, he estimated the density of diffuse interstellar matter using the z-component of stellar velocities perpendicular to the Galactic plane and found its limit to be 3 x 10^24 g/cm³. In 1938, he showed that most of the absorbing matter in the Galaxy is concentrated in a layer about 200 parsecs thick on both sides of the Galactic plane. He also demonstrated that stellar density increases towards the Galactic center and that the Sun is located in an area with lower stellar density.

Contributions to Cometary Science
Oort is known as the author of the theory of the extended cometary cloud, which is the source of observable comets. This cloud extends up to 150,000 astronomical units from the Sun, and comets spend most of their time far away from the Sun, making them invisible. However, under the perturbations of nearby stars, the velocities of individual comets can change to the point where they approach the vicinity of the Sun, becoming visible. In this region, planetary perturbations can alter their orbits, turning them into periodic comets. Oort also proposed, together with L. Spitzer, the mechanism for the formation of protostars in interstellar clouds (compression of gas due to the radiation pressure from previously formed hot stars). Additionally, he, along with H.C. van de Hulst, developed the theory of the formation of interstellar dust particles through the accretion of interstellar gas. He discovered that the radiation from the Crab Nebula is polarized and has a synchrotron nature.

Contributions to Radio Astronomy and International Cooperation
Oort played a significant role in the development of radio astronomy in Europe. He contributed to the construction of the Westerbork Radio Telescope and the South European Observatory. From 1958 to 1961, he served as the President of the International Astronomical Union.

Awards and Recognitions
Jan Oort was a member of numerous academies, including being a foreign member of the USSR Academy of Sciences since 1966. He received the Bruce Medal from the Astronomical Society of the Pacific in 1942 and the Gold Medal from the Royal Astronomical Society in London in 1946. He was also honored with the Henry Norris Russell Lectureship Award in 1951.