Pieter Theodorus Oosterhoff (30 March 1904,

Leeuwarden Leeuwarden (; fy, Ljouwert, longname=yes /; Town Frisian: ''Liwwadden''; Leeuwarder dialect: ''Leewarden'') is a city and municipality in Friesland, Netherlands, with a population of 123,107 (2019). It is the provincial capital and seat of ...

- 14 March 1978,

Leiden Leiden (; in English and archaic Dutch also Leyden) is a city and municipality in the province of South Holland, Netherlands. The municipality of Leiden has a population of 119,713, but the city forms one densely connected agglomeration wit ...

) was a Dutch

astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, moons, comets and galaxies – in either o ...

. He was the co-administrator, along with

Jan Oort Jan Hendrik Oort ( or ; 28 April 1900 – 5 November 1992) was a Dutch astronomer who made significant contributions to the understanding of the Milky Way and who was a pioneer in the field of radio astronomy. His ''New York Times'' obituary ...

, of the

Leiden Observatory Leiden Observatory ( nl, Sterrewacht Leiden) is an astronomical institute of Leiden University, in the Netherlands. Established in 1633 to house the quadrant of Rudolph Snellius, it is the oldest operating university observatory in the world, wit ...

in the Netherlands. His published papers are primarily in regard to

variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as e ...

s and

photometry Photometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical object's electrom ...

. He is most noted for his 1939 observation that there appear to be two populations of

globular clusters A globular cluster is a spheroidal conglomeration of stars. Globular clusters are bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of membe ...

based on the periodicities of their

RR Lyrae variable RR Lyrae variables are periodic variable stars, commonly found in globular clusters. They are used as standard candles to measure (extra) galactic distances, assisting with the cosmic distance ladder. This class is named after the prototype ...

stars. These two populations would come to be known as ''Oosterhoff groups'' after him. Between 1951 and 1952 he served as assistant general secretary of the

International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach ...

, and served as general secretary from 1952 until 1958. In 1954 he was one of twelve European astronomers who drafted a statement that would lead to the formation of the

ESO The European Organisation for Astronomical Research in the Southern Hemisphere, commonly referred to as the European Southern Observatory (ESO), is an intergovernmental research organisation made up of 16 member states for ground-based ast ...

. The

asteroid An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic o ...

1738 Oosterhoff is named after him.

References

External links

* * 1904 births 1978 deaths 20th-century Dutch astronomers People from Leeuwarden {{Europe-astronomer-stub