Professor Cornelis (“Kees”) de Jager, world-renowned astronomer, former General Secretary of the IAU (1970–1973), pioneer of Dutch and European space research, inspiring teacher, great populariser of science and fighter against pseudoscience, died on Thursday 27 May 2021 at the age of 100 years. He also was a talented science diplomat and marathon runner.

Kees de Jager was born on 29 April 1921 in den Burg on the Dutch island of Texel, where he lived for the last 18 years and where he has now passed away. He spent his childhood in the Dutch East Indies (now Indonesia), where his father was principal of a primary school, first in North Celebes (now Sulawesi), where Kees attended primary school, and later on Java, where Kees completed his secondary education (HBS) in Surabaya. The beautiful starry sky of Celebes made a deep impression on him. His father had already pointed out the different colours of the stars and told him that this meant that they had different temperatures, which greatly interested Kees.

In 1939 he traveled by boat back to the Netherlands to study physics at Utrecht University. There, Marcel Minnaert's astronomy lectures impressed him so much that he decided to study astronomy, much against the wishes of his parents. During the war, which started in the Netherlands in 1940, he continued his studies in hiding (he had refused to sign the declaration of loyalty to the occupying forces and should actually have gone to a forced labour camp in Germany). In 1946 he graduated and, as Minnaert’s assistant, began a doctoral research project on the spectrum of the Sun. His thesis, in which he derived the temperature and pressure distribution in the Sun’s atmosphere from a precise study of the shape of the hydrogen lines in the solar spectrum, made a great impression internationally and led to the offer of a position at Princeton University, which he was unable to accept as the US refused his visa because of his left-wing sympathies. But thanks to the work of Minnaert and de Jager, Utrecht subsequently became an important international centre of solar research in the 1950s and 1960s. Kees remained active in this field until the end of his life: only last year he and his collaborators published a book on the influence of solar activity on our climate. 

In 1957 he was appointed lecturer and in 1960 professor at Utrecht University. In 1963 he succeeded Minnaert as director of the Utrecht Observatory, a position he held till his retirement in 1986. 

In the meantime, de Jager had seen the great new opportunities for astronomy offered by space research. In 1961 he co-founded the Utrecht Laboratory for Space Research (now part of the Institute SRON of the Netherlands research council NWO), which within a short time grew to become one of the foremost laboratories in this field in Europe. He led this institute until 1983. He also participated in the establishment of the European space research organisation ESRO (now ESA). In his laboratory, instruments for satellites were built for the measurement of ultraviolet and X-ray radiation from the Sun and stars. 

One of the great successes of his laboratory was an instrument built for NASA’s Solar Maximum Mission satellite, which discovered in 1980 that during large eruptions on the Sun, so-called “solar flares”, the gas in the solar atmosphere is heated to more than 100 million degrees. Building on the experience gained from this instrument, de Jager, at the invitation of the Soviet Space Agency, built a successful instrument for the Russian space station MIR for the measurement of to measure X-rays from stars. This instrument made the world press since, after an instrument failure, Russian astronauts managed to repair it during a spacewalk of many hours outside the space station. 

He also founded the international scientific journals Space Science Reviews (1962) and Solar Physics (1967), of which he was editor for many years. Around 1980, De Jager decided to shift his research to stars other than the Sun, and he chose to study the most massive and luminous stars. He wrote the book The Brightest Stars and did important research in this area for decades with a group of co-workers, focusing in particular on the effects of mass loss from stars. 

In addition to his scientific work, he devoted much a great deal of to popularising science. For example, he organised eclipse expeditions with groups of amateur astronomers, held numerous lectures for laymen, and wrote articles and several popular books. Thanks to his efforts, the former Utrecht Observatory was transformed into what is now the Museum Observatory Sonnenborgh, where visitors can learn everything about astronomy and the Universe. 

De Jager also proved also to be a gifted diplomat of science. From 1970 to 1973 he was General Secretary of the IAU. 1973 was an interesting and exceptional year for the IAU: in addition to the regular GA, held that year in Sydney (the first GA outside Europe or North America), a second extraordinary GA in Poland was organised to mark the 500th anniversary of the birth of Nicolaus Copernicus. 

As well as being General Secretary of the IAU, de Jager held several other high-level science-policy position. With the support of both the United States and the Soviet Union, he was twice elected President of the world space research organisation COSPAR (1972–1978 and 1982–1986). Also, from 1978 he was for several years president of the International Council of Scientific Unions (ICSU). During his tenure in these positions, he brought several delicate problems between countries to a satisfactory solution. He himself attributed his diplomatic gifts to his contact in his youth with Indonesian culture, with its refined manners.

De Jager was also the co-founder and chairman of the Netherlands Skeptical Society, and president of the council of European skeptical societies, dedicated to fighting pseudoscience and quackery, to which he devoted much attention after his official retirement as a professor in 1986.

He was also, since his childhood on Java, very interested in athletics, especially long-distance running. Even at the age of 75 he still ran the entire New York marathon.

For his scientific work de Jager received numerous national and international distinctions, including two honorary doctorates and the Gold Medal of the Royal Astronomical Society, arguably one of the highest international awards in astronomy. 

Just 4 weeks ago, Kees celebrated his 100th birthday with an online symposium. In late 2019, at the Gala of Dutch Astronomy celebrating IAU 100 years, Kees was honoured for his accomplishments in front of a large audience by IAU President Ewine van Dishoeck.

More information

The IAU is the international astronomical organisation that brings together more than 12 000 active professional astronomers from more than 100 countries worldwide. Its mission is to promote and safeguard astronomy in all its aspects, including research, communication, education and development, through international cooperation. The IAU also serves as the internationally recognised authority for assigning designations to celestial bodies and the surface features on them. Founded in 1919, the IAU is the world's largest professional body for astronomers.

With the death of Kees de Jager, the world has lost an exceptionally inspiring scientist.

Links

• Full obituary by Ed van den Heuvel

Contacts

Ewine van Dishoeck
President of the IAU
Email: ewine@strw.leidenuniv.nl 

Lars Lindberg Christensen
IAU Press Officer
Cell: +49 173 38 72 621
Email: lars@eso.org