In Memoriam Kees de Jager, Astronomer (1921-2021) (Astronomy)

On Thursday 27 May 2021, Professor Kees de Jager, world-renowned astronomer, pioneer of Dutch and European space research, inspiring teacher, great popularizer of science and fighter against pseudoscience, passed away. Also talented science diplomat, marathon runner and writer. And also a very amiable and social person. He is also the man who added the word ‘Oerknal’ to the Dutch language. 

Kees de Jager was born on April 29, 1921 in den Burg on Texel, the place where he lived again for the last 18 years and has now passed away. He spent his childhood in the Dutch East Indies (now Indonesia), where his father was a head teacher, first in North Celebes (now Sulawesi), where Kees attended primary school, and later in Java, where Kees finished secondary school in Surabaya ( HBS) and also met his future wife Doetie. The beautiful starry sky on Celebes made a big impression on him. His father had already pointed out the different colors of the stars and told him that this means that they have different temperatures, which Kees found very interesting.

In 1939 he left for the Netherlands by boat to study physics in Utrecht. During the boat trip, in the Suez Canal, word came that the Second World War had broken out, and he arrived in the Netherlands with much delay. Professor Minnaert’s astronomy lecture in Utrecht impressed him so much that he decided to study astronomy, much against the wishes of his parents, who had come to the Netherlands on leave a little later in 1939. During the war, he continued his studies as a person in hiding (he had refused to sign the declaration of loyalty to the occupier and should actually have started working as a forced laborer in Germany). He graduated in 1946 and, as Minnaert’s assistant, began his doctoral research on the spectrum of the Sun. His dissertation, in which he had managed to deduce the course of the temperature and pressure in the atmosphere of the Sun from the shape of the hydrogen lines in the solar spectrum, made a great international impression. Thanks to the work of Minnaert and de Jager, Utrecht became one of the most important international centers in the field of solar research from the 1950s onwards. A number of the solar researchers trained here between 1960 and 1980 became professors or directors of solar observatories in other countries, particularly in the United States. Kees remained active in this field until the end of his life: last year he published an important book with an employee about the influence of the Sun on our climate. made a big impression internationally. Thanks to the work of Minnaert and de Jager, Utrecht became one of the most important international centers in the field of solar research from the 1950s onwards. A number of the solar researchers trained here between 1960 and 1980 became professors or directors of solar observatories in other countries, particularly in the United States. Kees remained active in this field until the end of his life: last year he and an employee published an important book about the influence of the Sun on our climate. made a big impression internationally. Thanks to the work of Minnaert and de Jager, Utrecht became one of the most important international centers in the field of solar research from the 1950s onwards. A number of the solar researchers trained here between 1960 and 1980 became professors or directors of solar observatories in other countries, particularly in the United States. Kees remained active in this field until the end of his life: last year he published an important book with an employee about the influence of the Sun on our climate. A number of the solar researchers trained here between 1960 and 1980 became professors or directors of solar observatories in other countries, particularly in the United States. Kees remained active in this field until the end of his life: last year he published an important book with an employee about the influence of the Sun on our climate. A number of the solar researchers trained here between 1960 and 1980 became professors or directors of solar observatories in other countries, particularly in the United States. Kees remained active in this field until the end of his life: last year he published an important book with an employee about the influence of the Sun on our climate.

In 1957 he was appointed professor and in 1960 professor at Utrecht University. In 1963 he succeeded Minnaert as director of the Utrecht Observatory. 

In the meantime, he had seen the tremendous new possibilities that spacecraft research beyond our atmosphere offers for astronomy. In 1961 he therefore founded the Utrecht Laboratory for Space Research (now part of the NWO Institute SRON), which quickly grew into one of the most important laboratories in this field in Europe, with more than 150 employees. He headed this institute until 1983. In the meantime, he also participated in the founding of the European space research organization ESRO (now ESA). In his laboratory, instruments for satellites were built for the measurement of ultraviolet and X-rays from celestial objects, such as the Sun and stars, including for the first Dutch satellite ANS, launched in 1974. 

One of his lab’s great successes was also an instrument built for NASA’s Solar Maximum Mission satellite. With this it was discovered in 1980, by his colleagues Max Kuperus and Peter Hoyng, 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, as a result of a kind of short circuit of electrical currents of trillions of amperes generated by the magnetic fields of sunspots.   

Building on the experience gained with this instrument, De Jager built, at the invitation of the Soviet Union, a successful instrument for the Russian space station MIR, for measuring X-rays from stars. This instrument made the world press because, after something went wrong with it, two Russian astronauts were able to fix it thanks to a spacewalk of many hours outside the space station.  

Thanks to this pioneering work, the Utrecht laboratory has become the world leader in building X-ray spectroscopy instruments in space, used for NASA’s Chandra X-ray observatory and ESA’s XMM-Newton observatory.

Besides being a great scientist and organizer, De Jager was also an exceptionally inspiring teacher and popularizer. The lectures on the construction and evolution of stars that he gave in the 1960s in Utrecht and Brussels (where he was part-time professor from 1960-1970) laid the foundations for the thriving research groups in this field at the University of Amsterdam and the Radboud University in Nijmegen, as well as at the Free University in Brussels.

Around 1980, De Jager decided to also focus on the research of stars other than the Sun, the heaviest and brightest among them. He wrote the book “The Brightest Stars” about this and has conducted important research in this area for decades with a whole group of researchers. 

In addition to his scientific work, he devoted a lot of attention to the popularization of astronomy throughout his life. For example, he organized eclipse expeditions with groups of astronomy amateurs, gave numerous lectures to laymen, wrote articles and some popularizing books. After his retirement, thanks to his efforts, the former Utrecht Observatory was transformed into the current “Museum Sterrenwacht“ Sonnenborgh ”, where visitors can get acquainted with what can be experienced in the universe.   

In addition to his scientific and organizational talents, De Jager also turned out to be an excellent diplomat. From 1970-1973 he was Secretary General of the International Astronomical Union, and with the agreement of both the United States and the Soviet Union he was twice elected President of the world space research organization COSPAR (1972-1978 and 1982-1986) . In the ten years he led this world organization, he managed to find a satisfactory solution to a number of delicate international problems in space research. He himself attributed his diplomatic gifts to the contact in his youth with Indonesian culture, with his refined manners.

Kees was also the co-founder and chairman of the Skepsis foundation, which is committed to combating pseudoscience and quackery, to which he devoted a lot of attention after his retirement.

After his retirement he also turned out to be a fascinating writer about what he had experienced in his life. For many years he wrote the column “Retrospect” about this in the popular scientific monthly magazine Zenit. These columns range from pieces about his childhood in Indonesia to the solution of diplomatic problems between China and Taiwan. His last column appeared this month. Part of these columns are bundled in two books entitled “Retrospective” which show what a great and humorous storyteller Kees was. These books can be seen as his autobiography.

In addition to all this, Kees was very interested in athletics from his youth in Java, especially long distance running. At the age of 75 he ran the New York marathon all the way.

De Jager has received numerous national and international awards for his scientific work, including two honorary degrees and the Gold Medal from the Royal Astronomical Society, widely regarded as the highest international award in astronomy. 

With the death of Kees de Jager, an exceptionally amiable, enthusiastic and inspiring world-class scientist passed away. It is a great privilege to have had him as a teacher.

Featured image: Kees de Jager during one of his last public appearances at the Gala of Astronomy in December 2019. Photo credit: Bob Bronshoff / New Scientist


Provided by NOVA

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