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This year, Utrecht Professor Emeritus of Astrophysics Kees de Jager is celebrating his 100^th orbit around the sun. Over the course of his career, De Jager earned international acclaim with his solar research, and he laid the foundations for space research in the Netherlands. Sonnenborgh Observatory played a major role in his life: he studied there, hid there during the war, and later became the director of the observatory and lived there with his family.

As a young man, Kees de Jager heard Professor Minnaert speak in 1939 and was immediately hooked on astronomy. “I was so fascinated by what he said, that I decided right then and there to study Astronomy. I wrote to tell my parents in the Dutch East Indies, but they weren’t very happy about my choice. ‘There’s no money in it’, they said. So I talked to Minnaert about my doubts: ‘But De Jager, there’s more to life than money’ he laconically replied”, recalls Kees de Jager, laughing over the telephone from the island of Texel. De Jager persisted, however, and eventually became a world-famous astrophysicist. He was a pioneer in the field of solar research, and he put the Netherlands’ space research on the map. His career is not only exceptional due to its length, but also the large number of highly diverse positions that Kees de Jager has filled over the years, such as his long term as Director of Sonnenborgh Observatory in Utrecht.

Sonnenborgh Observatory

The observatory in Utrecht played a key role in his career, as well as in his personal life. While De Jager was a young student of Astronomy with a minor in Physics and Mathematics, the Germans invaded the Netherlands and closed all of the universities. De Jager chose to hide in the observatory. “Together with my fellow student Hans Hubernet, we hid in a little room at Sonnenborgh, with a centimetre of dust on the floor. We sat silently studying at a table all day long”, sighs De Jager. “After 6 o’clock, when the staff when home, we’d leave the room to cook and stretch our legs. At night we sat in the observatory library and I studied the moons of Jupiter.”

Shall we start on your dissertation, then?

De Jager made good use of his time, and continued studying throughout the war. “Minnaert would sneak in and administer exams to us. I kept all of those exam papers, so when the university re-opened after the war I was able to graduate right away. That’s when Minnaert suggested: ‘Shall we start on your dissertation, then?’ So that’s what I did.” 

That was also when Kees reconnected with his future wife in the Netherlands. “My father would bring us food from time to time”, he explains. “One day he told me that Doetie was going to visit – a girl I’d known at school on Java. We used to play sports and dance together. She’d returned to the Netherlands with her family, so that night I took a chance and cycled home. That’s when the spark hit, and we were married in April 1948.”

In 1952, De Jager earned his PhD cum laude under Professor Minnaert, on research into the hydrogen spectrum of the sun. The research allowed him to gain insight into the life cycle of the sun by looking up to 800 km deep under its surface, which had until then been considered impossible.  His later research into the brightest stars and their life cycles could be considered as an extension of this early research.

After earning his PhD, De Jager was busier than ever. “I was able to start as one of Minnaert’s assistants, which I felt was a great honour, even though it was an unpaid position. For my day job, I worked for another professor, Rosenfeld, for 100 guilders per month. In addition, I’d go to the observatory at 6 o’clock, where I’d work until midnight. Night-time is of course the best time to look at stars. I kept that up for a year, but then I told Minnaert: there’s more to life than money, but I still need to put food on the table. That’s when I was finally able to get a position as a paid assistant.”

Pioneer in space research

In 1961, De Jager founded the Space Research Laboratory in Utrecht, now the SRON, and spent the next 20 years guiding its lines of research as Director. He still gets excited talking about the instruments developed there: “Our fine mechanics lab developed and built instruments for conducting experiments on sounding rockets, and later aboard satellites. For years, the Space Research Laboratory in Utrecht was the only place that could make transmission and reflection gratings for X-ray spectral research. They allowed us to filter the light from X-ray radiation. We also developed a spectrograph that could measure the ultraviolet radiation from stars. And instruments for studying solar ejections in X-ray radiation.”

At the same time, De Jager took over from Minnaert as the Director of Sonnenborgh Observatory in Utrecht. Now, De Jager’s family could call the observatory home, as he once had: “Doetie and I lived there together for 40 years. Our children grew up there. It was a fantastic time, and I’ll never forget it.”

From the sun to the climate via hyper-giants

Before founding the Space Research Laboratory in Utrecht, De Jager studied the structure and movement within the atmosphere of the sun, which is used to produce models for understanding processes in and around the photosphere; the visible surface of the sun. De Jager also came up with new hypotheses on how the sun’s thin outer atmosphere, the corona, can reach temperatures of millions of degrees, even though the visible surface is ‘only’ around 5,000 degrees.

As space research possibilities expanded, De Jager’s attention shifted to solar flares and their relationship to magnetic fields and electric currents on the surface of the sun. Today’s satellites also allow scientists to view stars in the ultraviolet and X-ray spectrums, which De Jager uses to observe super- and hyper-giant stars with extreme luminosities. De Jager has written a scientific monograph, The Brightest Stars, and more than 20 other publications on hyper-giants to understand the evolution of these extreme stars. Hyper-giants are important in the field of Astronomy, as at the end of their relatively short lives they cause the most high-energy explosions in the universe; gamma bursts.

After his retirement, De Jager delved into the relationship between climate developments on earth and the periodic changes in the sun.

You need to put yourself in your audience’s position. Every student should learn how to do that during their studies.

Human scale

In addition to his unbridled interest in astrophysics, Kees de Jager also pays attention to the more human-scale aspects of life. Despite the advances in automation over the years, he always adds a hand-written greeting and signature to his business correspondence. Kees explains: “At one point in my career, I was writing such an enormous number of signatures that I just had a stamp made. That saved me a lot of time, and I preferred to use that time to add a personal salutation to my letters. It’s important to show that you know whom you’re writing to, and that you pay personal attention to one another.”

De Jager also reveals his attention to the human side of science through the energy he invests in making it accessible to a wider audience. “Society makes it possible for us to conduct scientific research, so I believe that every researcher should be able to explain their research to an interested layman audience. To do that, you need to put yourself in your audience’s position. Every student should learn how to do that during their studies.” For many years, De Jager has written about his personal and scientific experiences in the popular science magazine Zenit; his contributions have since been bundled in the anthology ‘Terugblik’ (‘A Look Back’).

Back to Sonnenborgh

De Jager served as the last Scientific Director of Sonnenborgh Observatory. But Sonnenborgh continued to play a role in De Jager’s life, even after he returned to his birthplace on the island of Texel. After his wife Doetie died some years ago, he ran into a woman named Margriet, whom he’d first met in the 1970s. She is the granddaughter of Minnaert’s predecessor Nijland, and was born at Sonnenborgh. Her son wanted to peer at the stars through his great-grandfather’s old telescope, so she contacted De Jager. “We’ve been together ever since then”, De Jager recalls cheerfully. “The stars brought us together.”