- Donald E. Osterbrock
Strömgren, Bengt (21 January 1908–04 July 1987), astronomer and astrophysicist in Denmark and the United States, was born Bengt Georg Daniel Strömgren in Göteborg, Sweden, the son of Svante Elis Strömgren and his wife, Hedvig Lidforss Strömgren. Elis Strömgren, an old-school classical astronomer, had recently become director of Copenhagen Observatory, Denmark, and it was there that Bengt grew up, was educated, and worked for many years.
He was a brilliant boy, and his father delighted in teaching him calculus and numerical computing methods before he was twelve. Bengt began a regular astronomical observing program at age thirteen and published his first paper at fourteen. He had the run of Niels Bohr's Institute for Theoretical Physics, and he began helping in the reduction of laboratory spectroscopic data at sixteen. Strömgren graduated from high school at seventeen, from Copenhagen University at nineteen, and earned his Ph.D. at twenty-one with a thesis on the determination of near-parabolic cometary orbits.
Throughout his life Strömgren made important contributions to solving several of the most important astrophysical problems of his time, a number of which he had recognized himself. In all his research he drew on his deep knowledge of physics, the then-new quantum mechanics as well as classical theory, his solid experience with the older astronomy of stellar positions, dynamics, and kinematics, and his skill in numerical computing. At Copenhagen, analyzing the internal structure of stars using his own quantum mechanical calculations of the opacity of ions at high temperature, he demonstrated that hydrogen is an extremely abundant element. In 1935 he worked out the detailed theory of the optical aberrations of the recently invented Schmidt telescope, a mathematical project he could do alone in his office with his “computer,” a glorified electromechanical adding machine.
Strömgren had married Sigrid Caja Hartz, a Dane, in 1931; they had two daughters and a son.
Then in 1936 Otto Struve brought Strömgren to the University of Chicago as one of the group of brilliant young astrophysicists who helped him rebuild its Yerkes Observatory in Williams Bay, Wisconsin, as an outstanding research center; the others were S. Chandrasekhar, G. P. Kuiper, and W. W. Morgan. There Strömgren analyzed and solved the nature of the large, irregular volumes of interstellar matter in the Milky Way that faintly radiate emission-line spectra. He showed that these “H II regions,” often idealized later as “Strömgen spheres,” are ionized by high-temperature, high-luminosity stars within them.
Strömgren had written Struve that he would accept the Chicago post only on a temporary basis, and in 1938 he returned to Denmark as a full professor, though the Yerkes director tried desperately to persuade him to stay. Strömgren could see that World War II was near and wanted to be in Denmark with his family, his father and mother, and his countrymen. There, unable to travel or to communicate with astronomers in England or America after the Germans invaded Denmark, he worked on stellar atmospheres, calculating “models” free of the simplifications used in earlier treatments, to match the observed spectra of stars. This involved varying systematically the assumed abundances of the elements, effective temperature, and gravity in the star's atmosphere. An important element in these calculations was the opacity of H-, the negative hydrogen ion, which Rupert Wildt had suggested would be important in the sun and stars like it. Strömgren and his group's calculations confirmed that it was. When Elis Strömgren retired in 1940, his son Bengt succeeded him as director of Copenhagen Observatory.
Soon after the end of World War II, Bengt Strömgren returned to Yerkes Observatory as a visiting professor for the academic year 1946–1947. He continued and extended his research on interstellar matter, analyzing and interpreting both the emission regions and interstellar absorption lines, which provided further quantitative information on its physical nature. After a few years back in Denmark, Strömgren returned to Yerkes Observatory as its director, succeeding Struve (who resigned to move on to Berkeley) on 1 January 1951. The University of Chicago operated McDonald Observatory, located in the mountains of west Texas, on behalf of the University of Texas, and Strömgren directed it, as Struve had before him. By then Strömgren was developing his plan of using narrow-band (“interference”) filters and the methods of photoelectric photometry to do accurate, quantitative classification of stellar spectra. With these filters it was possible to measure parameters equivalent to effective temperature, surface gravity, heavy-element abundances (“metallicity”), and age, (the last depending on further theory). He and his students did much of the observational work at McDonald Observatory. In 1957 he left Chicago and Yerkes Observatory to become the first professor of astrophysics at the Institute for Advanced Study in Princeton, “the man who got Einstein's office.” There Strömgren continued the narrow-band filter or “stellar-population” research on the ages and elemental abundances in stars. Part of the goal was to trace back a star's orbit in the Galaxy (the Milky Way), from its current position and velocity, using its age, to its place of formation or “birth.” Scientists at the NASA Institute of Space Sciences in New York collaborated with him in this work, using their powerful, advanced electronic numerical computers.
In 1967 Strömgren was elected extraordinary professor of astronomy by the Royal Danish Academy of Sciences and Letters, perhaps the highest honor a Danish scientist or scholar may attain. As part of this appointment, he and his family lived for the rest of his life in the Carlsberg Foundation's “Mansion of Honor,” dedicated to the Danish people. Bohr had been the first person to hold this appointment, and Strömgren was the first scientist after him. During and after World War II, Strömgren had led the planning for a new astronomical observatory outside the lights, smoke, and exhaust fumes of Copenhagen, and by 1967 the new station was in operation at Brorfelde, in the Danish countryside. Students, postdoctoral fellows, and faculty members collaborated with him in pushing forward his research there as well as at a newer Danish telescope at La Palma in the Canary Islands and at the European Southern Observatory in Chile.
Throughout his career Strömgren took the widest possible view of science, kept up to date on research done everywhere, and was famous for his masterly reviews, published and oral, of all the fields in which he had worked, including stellar interiors and atmospheres, nebulae, interstellar matter, and their respective evolutionary histories. He was a true scientific leader. As a citizen of a small, traditionally neutral country poised near the border between East and West, he was also a superb diplomat. Trusted by nearly all, he was elected secretary-general of the International Astronomical Union in 1948 and its president in 1970, and he was chosen president of the American Astronomical Society in 1966 and of the Royal Danish Academy in 1968. He retired from his professorship in NORDITA, the Nordic countries' Institute of Astrophysics, located in Copenhagen, in 1978 and died there nine years later.
There are many letters to, from, and concerning Strömgren in the Yerkes Observatory Archives, Williams Bay, Wisc. He published a short autobiographical account of his astronomical career, “Scientists I Have Known, and Some Astronomical Problems I Have Met,” in Annual Review of Astronomy and Astrophysics 21 (1983): 1–11. The best published memorial biography of him in English is Russell Kulsrud, “Bengt Strömgen 1908–1987,” in American Philosophical Society Yearbook (1987), pp. 216–22. Another good but shorter one, which emphasizes his Danish years, is by Mogens Rudkjùbing , “Bengt Georg Daniel Strömgren,” Quarterly Journal of the Royal Astronomical Society 29 (1987): 282–84. See also Adriaan Blaauw, ESO's Early History: The European Southern Observatory from Concept to Reality (1991), and Donald E. Osterbrock, Yerkes Observatory 1892–1950: The Birth, Near Death, and Resurrection of a Scientific Research Institution (1997).