STRöMGREN, BENGT GEORG DANIEL

(b. Göteborg, Sweden, 21 January 1908; d. Copenhagen, Denmark, 4 July 1987),

astronomy, astrophysics, stellar structure, interstellar gas, photoelectric photometry.

Strömgren has been highlighted as one of the most prominent astrophysicists of the twentieth century. Born and educated in Denmark, he spent more than sixteen years as director at Yerkes and McDonald observatories and as professor of astrophysics at the Institute of Advanced Study at Princeton. Strömgren’s scientific contributions can be separated into three main topics: problems of chemical composition of stellar structure and stellar interiors (1930–1940), the physics of interstellar gas (1938–1953), and photoelectric photometry of stellar spectra (from 1948). His contributions to the internationalization of astronomy were also considerable.

Origins The son of Svante Elis and dentist Hedvig, née Lidforss, Strömgren was brought up in Denmark into a life of science, as was his brother Erik (later professor of psychiatry at Aarhus University, Denmark). Elis Strömgren was professor of astronomy at Copenhagen University and director of the Copenhagen Observatory. He secured Bengt’s education at the best schools of Copenhagen and promoted his academic and practical training in astronomy. The son grew up surrounded by scientists: observers, assistants, and visiting international researchers in his father’s official observatory residence. Bengt’s scientific career began at a very early age: He published his first joint paper at the age of fourteen. After graduating from high school, he enrolled at Copenhagen University in 1925 to study astronomy and atomic physics.

In Germany, the commitment to advancing science led in 1863 to the founding of the international society Astronomische Gesellschaft (AG). Even before World War I, more than 400 members from mainly the Western world joined the AG, which was of significant importance to astronomical research, not least through the international astronomical periodical Astronomische Nachrichten, originally founded by H. C. Schumacher in 1821. German astronomers played important roles in the development of early twentieth century astrophysics. Being the national cradle of the observation of the photoelectric effect—and hence of practical photometry, i.e. the methods of studying stellar light—Germany fostered a series of important astrophysicists working with observational technology at a series of German observatories.

Already in 1921 and 1922, Strömgren’s father brought Bengt to visit the Bonn Observatory and Neubabelsberg Observatory in Berlin, directed by Frank Küstner and Paul Guthnick respectively. There he learned the theory and practice of photographic photometry. He joined his father at meetings of the AG and the International Astronomical Union and thus was introduced to international aspects of science at a very early age.

Bengt Strömgren was one of the first to use the photocell for astrometry, by which he managed to record accurate times of meridian transits (passages of a star through the observer’s meridian) by monitoring the image of a star while it crossed a grid of parallel wires in the focal plane of a telescope. His technological innovation of automated photocell-astrometry occurred when he was eighteen, and the national newspapers soon portrayed the young man as a genius destined to enter the world of science.

At Niels Bohr’s Institute of Theoretical Physics, the cradle of the quantum revolution, Strömgren was taught quantum physics by figures such as Bohr, Hendrik A. Kramers, Werner Heisenberg, and Oskar Klein. Thus in the right place at the right time, Strömgren graduated in 1927, already appointed assistant at his father’s observatory the year before—despite allegations of nepotism. In 1929, after working as observatory assistant and doctoral student for two years, Strömgren received his doctoral degree. He was just twenty-one years old.

In his dissertation, “Formulas and Tables for Determinations of Parabolic Orbits,” Strömgren presented extensive calculations using a so-called numerical Archimedes calculator. The advent of the electromechanical calculators in the 1920s made it possible for astronomers to carry out numerical calculations of the novel astrophysical equations. The handling of numerical calculations became a necessity in the 1930s, and the Copenhagen Observatory had to purchase electromechanical calculators for this purpose. On the basis of recent work by the astronomer Gerald Merton on modified Gaussian methods for determinations of orbits, Strömgren devised a new method for calculating comet trajectories.

The technological development of Danish astronomy, however, went slowly and was predominantly focused on classical celestial astronomy and numerical calculations. He was educated in classical astronomy, but owing to the spirit and stimulating environment at Bohr’s institute, Strömgren realized his professional future program: the application of the new quantum mechanics to astrophysics.

Chemical Composition of Stellar Interiors Strömgren was instrumental in transferring his inspiration and theoretical knowledge from Bohr’s institute to his father’s observatory. In 1930, Strömgren entered fully into the field of stellar models and chemical composition of stellar interiors. The year of his appointment as lecturer at Copenhagen University (1932), Strömgren published his landmark article, “The Opacity of Stellar Matter and the Hydrogen Content of the Stars,” in which he revised existing theory of the internal structure of stars, which had hitherto been formulated mainly by the British astronomer, Arthur S. Eddington.

In the paper Strömgren concluded that the main constituent of a star was hydrogen and not the heavier elements as was generally assumed in the late 1920s. The rapid development of physics in the mid-1920s had triggered Strömgren’s idea of large hydrogen abundances inside stars. As the first steps in determining the hydrogen content, the work of the Cambridge physicists John A. Gaunt and the Japanese physicist Yoshikatse Sugiura played an important qualitative role for astronomy because they set in motion the development of novel astrophysical theories. Detailed quantum mechanical calculations undertaken by Gaunt and Sugiura indicated very clearly to Strömgren that there were some fundamental complications in the basic theoretical assumptions about stellar interiors.

Strömgren’s hydrogen hypothesis was not quite novel in itself, but he made its basis more clear owing to improved calculation methods. The proposal of a large hydrogen abundance was advanced by Arthur Eddington at about the same time, although his reasoning was based on mathematical-analytical methods. Eddington’s classic The Internal Constitution of the Stars(ICS) (1926) is one of the great masterpieces of astronomical literature. Stellar interior theories had made great progress during the preceding years, and Eddington’s renowned monograph was long-awaited because of its all-embracing exposition of relevant subjects. ICS was also admired owing to its clear exposition of the theory of radiative equilibrium, and also because it emphasized two serious difficulties within the framework of Eddington’s “‘standard model.” The first of these was a persistence of an order-of-magnitude discrepancy between observed and deduced opacities of stellar matter. The other was the so-called stellar-energy problem of finding the source of energy-generation processes, which remained unsolved until specialists in nuclear physics entered the field in the late 1930s. In ICS, Eddington wrote that the order-of-magnitude opacity discrepancy could be removed by assuming a high hydrogen abundance in the stars, but he did not regard this a proper way out of the problem. Consequently he would wait for the discovery of either new absorption mechanisms, or some further development of Heisenberg’s quantum mechanics.

The methodological differences between the works of Eddington and Strömgren were manifest in this connection: Strömgren adopted the mixture of elements found earlier by the American astronomer Henry N. Russell in the solar atmosphere and calculated its so-called opacity (a property of matter that prevents light from passing through it). Eddington instead employed a general theory, which he claimed should apply to any likely mixture of chemical elements (hydrogen, helium, and heavier elements).

With Strömgren’s paper, the abundance of the light elements in stars—and hence in the entire universe—was closer to being determined. The hypothesis of hydrogen preponderance provided the missing link in the understanding of the values of stellar luminosities and radii. In addition, Strömgren’s paper furthered the knowledge of the temperatures inside stars—which had far-reaching consequences, as it meant a radical change of the prevailing views upon the physical conditions inside stars. Strömgren’s numerical style turned out to be essential for the new astrophysics owing to the empirical-inductive character and, contrary to hypothetical-deductive methods as undertaken by Eddington, James Jeans, and Edward A. Milne, it has remained so ever since. At the same time, Strömgren’s efforts secured Denmark a significant position on the international scene of theoretical astrophysics.

American Astronomy The shift in focus of Danish astronomy toward the United States became apparent in the 1930s. There were political-economic motives as well as techno-scientific reasons for the Western orientation of Scandinavian astronomy. One reason was the boycott of German astronomy after World War I, which hampered international contact between astronomers inside and outside Germany. Internal issues also characterized the problems of German astronomers: lack of resources, dismissals of Jewish scientists, and the shut-down of projects. At the same time, numerous ideas and astronomers from foreign countries, the Netherlands in particular, made their way to the United States between the wars and in so doing furthered the development of American astronomy and to laid the foundations of many new theories.

One of the foreigners to visit the United States was Strömgren. His first visit lasted from 1936 to 1938 and was at the invitation of the director of Yerkes Observatory—and colleague through many years—Otto Struve (Strömgren left Denmark again to the benefit of the United States in 1951–1967). Work that was not possible to carry out in Denmark but possible in the States was multifarious: close cooperation between theoretical and observational astronomy; spectral photometry requiring comparisons between theory and practical astronomy; scientific confirmations and refinement of both theory and experiment; and practical work in geometrical optics. Had Strömgren stayed in Denmark, mainly pure theoretical astronomy of all sorts would have constituted the outcome of his research activities—predominantly due to lack of financial resources and modern observational technology.

In the United States, Strömgren followed up his work on hydrogen abundance. He wrote an important paper on the theory of stellar interiors and development (1937). In this paper, Strömgren sought to answer how the addition of helium would change his stellar model. The only observational basis to support a helium hypothesis was estimates of relative amounts of hydrogen and helium, but determinations were rather weak. The final result was a model comparatively close to the present relative chemical composition by weight, which Strömgren found to be 60 to 70 percent hydrogen, 26 to 36 percent helium, and 4 percent heavier elements.

As it was also a survey article, accumulating existing knowledge in one place, it opened the problem of stellar interiors and energy production for investigation not only by astrophysicists but also by theoretical physicists. Strömgren was accustomed to collaboration between astronomers and physicists already from the early 1930s and naturally he welcomed their views on the riddle of the time: What sources of energy powered the radiation output of stars?

Solar Energy Production Strömgren’s hydrogen hypothesis—later improved by his helium assertion—paved the way for Bethe’s and Weizsäcker’s 1938 theories of stellar energy loss through the conversion of hydrogen into helium in nuclear reactions. It was thus a central step in a fruitful line of reasoning in the understanding of nuclear physicists of stellar energy sources at the end of the 1930s.

Before leaving the United States in 1938, Strömgren was invited by George Gamow to participate in a Washington conference attended by a select group of theoretical physicists and astronomers. The theme of the 1938 Washington conference was the sources of stellar energy. Strömgren attended along with figures such as Edward Teller, Donald H. Menzel, Merle Tuve, George Gamow, and the Nobel laureates Hans Bethe and Strömgren’s lifelong friend, Subrahmanyan Chandrasekhar.

Strömgren opened the conference by outlining the present status of the problem of temperature and density distribution in the interior of stars. For the theory of internal structure of stars, he argued that two issues were of particular importance, namely, the calculation of opacities and the rate of energy production. In a subsequent publication by Chandrasekhar, Gamow, and Tuve, the results of the Washington conference were issued in Nature, in which Carl Friedrich von Weizsäcker’s so-called “Aufbauhypothese” was presented. The key point of the hypothesis was that stellar interiors continually build up heavier elements from hydrogen and that such processes liberate sufficient amounts of energy to account for the radiation of the stars. But Weizsäcker’s model schemes of transmutations of helium into lithium and back to another helium isotope, while emitting radiation and positrons, were contradicted by experimental evidence. It was uncertain which processes accounted for the stellar energy production.

Strömgren’s experience of the first part of the conference was that no solution was in sight. However, input from Hans Bethe gave hope, as the Washington conference occasioned Bethe’s interest in the nuclear aspects of the energy problem.

The ideas of Weizsäcker, Bethe, and Gamow about nuclear energy production in stars were later also discussed with Strömgren at the 1938 International Astronomical Union’s General Assembly in Stockholm. Bethe managed to calculate the energy production in the interior of the Sun by the proton-proton chain in which part of the matter involved is converted into electromagnetic energy by hydrogen fusion. Moreover, Strömgren believed that Bethe used better empirical data than Weizsäcker. From the new knowledge about stellar temperatures, densities, and chemical composition, Bethe looked systematically for the possibilities of reactions under those conditions. Until 1950, it was thought that the so-called carbon cycle was the main process of stellar energy production (in stars with central temperatures greater than 15 million Kelvin, the carbon cycle takes over the dominant role of energy production from hydrogen fusion). In 1950, however, it became clear that the proton-proton process was the primary process for the Sun.

It is somewhat striking, but not atypical, that it was theoretical physicists, and not astrophysicists, who eventually solved the problem of nuclear reactions in stellar interiors. Even though Strömgren and the community of astronomers were very familiar with nuclear reactions, Strömgren found it “so difficult to sort out what was going on in nuclear physics at the time that it took nuclear physicists who had devoted all their time to the field to sort it out” (quoted in Rebsdorf, 2005, p. 301).

Interstellar Gas Bethe’s nuclear energy results had important consequences for the astronomers’ estimates of stellar age. The lifetime of solar-type stars was anticipated to be about 10¹⁰ years, but at the same time it was evident that massive main-sequence stars had much shorter lifetimes, of about 10⁷ years in extreme cases. This led the astronomical community to conclude that star formation must occur under conditions such as those observed in the earth’s neighborhood of the galaxy at the present epoch. A compelling question naturally arose from this conclusion: Is there a sufficient amount of matter in interstellar space to allow for such processes?

It was well known that particles existed in interstellar space and that they caused the absorption and reddening of starlight travelling through space, but the amounts observed were rather modest. Strömgren investigated theoretically the photoionization of interstellar gas and discovered the great importance of the radiation from the relatively rare, hot O stars in fixing the physical conditions in space. As an important result he published “The Physical State of Interstellar Hydrogen” (1939). In the paper he showed that homogeneously distributed interstellar atomic hydrogen was ionized by ultraviolet light from hot O and B stars in particular, out to certain sharply determined radial distances from these stars. These shells of ionized hydrogen, such as the diffuse Orion Nebula, later became known as Strömgren spheres.

The paper was based on relatively simple methods, considerations, and calculations. Strömgren’s theoretical calculations also turned out to match the intensities of hydrogen emission lines in extended areas of the Milky Way, observed by Otto Struve at the McDonald Observatory on Mount Locke in Texas. The match was evident when the hydrogen density in interstellar space was of the order of magnitude of one atom per cubic centimeter. The crucial conclusion reached by Strömgren was that the galaxy contains very large amounts of hydrogen outside of the hydrogen observed in ionised regions.

Strömgren’s interest in the interstellar medium and the problem of the formation of these so-called H II regions around stars was triggered by Struve’s development of a new technique for observing very faint stars. In fact, in 1936 Struve had made the important discovery of Balmer line glow in extensive regions of the galaxy, and this triggered Strömgren’s interest in questions of the interstellar medium. Until that time it was known that a number of diffuse nebulae emitted an emission spectrum of strong Balmer lines, arising from n = 2 transitions of the hydrogen atom. What Struve found, in addition to these well-known areas, was that even large regions of hundreds of square degrees on the sky had a faint glow. At the McDonald Observatory Struve developed what was called the Centipede: a device pointed toward the celestial Pole, which could record faint spectra of extended areas and had a focal length of one hundred meters. By use of this instrument, Struve and the American astrophysicist Christian T. Elvey were able to measure this glow. They found emission lines in eight out of fifteen fields, immediately challenging Strömgren to develop a theoretical model to match the observations.

Strömgren constructed a stellar model of a luminous star with high temperature embedded in a uniform medium of hydrogen. The salient result of his investigation was a sharp transition. Despite numerous efforts at Mount Wilson Observatory to identify more absorption lines, the general assumption had been that the density of interstellar gas was very low. It was not possible to conclude that the amount of mass was distinctly high or that there was a large amount of hydrogen. In his 1939 paper, Strömgren attributed to Struve and Elvey the discovery of extended areas in the Milky Way in which the Balmer lines were observed.

Strömgren’s investigations revealed that high-temperature stars and clusters of such stars in particular were found to be capable of ionizing interstellar hydrogen in regions large enough to be of importance in problems of interstellar space. Strömgren’s sought to arrive at a picture of the actual physical state of interstellar hydrogen, and he calculated that the Balmer-line emission should be limited to certain rather sharply bounded regions in space surrounding O-type stars or clusters. Moreover, Strömgren found relations between the gas density, the luminosity of the star, and the size of the sphere of ionized hydrogen around it. He found that such regions should have diameters of about 200 parsecs, or c. 650 light-years, which was found to be in general agreement with Struve and Elvey’s observations. Strömgren calculated the density of hydrogen in these areas, later to be denoted H II regions, or Strömgren spheres, namely 3 per cubic centimeter.

Strömgren’s paper ushered in a new line of research for him, one that he would follow until about 1953, when his photoelectric photometry research on spectra took over. In fact, Strömgren’s last paper on interstellar emission was published in 1951.

Photoelectric Photometry In 1952, the year after he succeeded Struve as director of Yerkes and McDonald Observatories, Strömgren pioneered a new photometric system to succeed the supplement of an existing photometric system, the so-called UVBGRI system invented in 1943 by Joel Stebbins and Albert Whitford at the Washburn Observatory, Wisconsin. Strömgren’s novel narrow-band system made use of a photomultiplier (converting light into an electric signal), the so-called abcdef system, which was a six-color system like the UVBGRI. The abcdef system was not very practical for photometric work when dealing with very faint stars, though.

Then Strömgren investigated the possibilities of spectral classification (the classification of stars according to their spectra; each major spectral classification is given a letter, with additional numbers providing further subdivisions) through photoelectric photometry with interference filters. The first observations were made in 1950 at the McDonald Observatory, and by 1951, he had published a paper on his ideas of the system. In collaboration with Danish astronomer Kjeld Gyldenkerne, Strömgren made photoelectric measures with the 32-inch reflector of the Observatoire Haute Provence in France, using a set of twenty-six filters covering the wavelength region from 3350 to 5500 Å. The American astronomer William W. Morgan helped determine spectral and luminosity classes

of a series of standard stars. From the analysis of this material, Strömgren concluded that accurate two-dimensional classification was possible for certain types of stars. After Strömgren left his directorship at Yerkes in 1957 for a professorship at Institute of Advanced Study in Princeton, he began developing the widely used intermediate-band uvby system, which was based on the early abcdef system, but improved it by employing wider passbands (that is wider than twenty nm).

This system became closely connected to the β system of H β line photoelectric photometry, which was invented by David Crawford when he wrote his PhD dissertation under the tutelage of Strömgren in the late 1950s. A complete discussion of the properties of uvbyβ photometry and its use for stellar classification was given in Strömgren’s most cited article (Strömgren 1966; Olsen 1994). The uvbyβ system became the most widely used intermediate-band system of astrophysics. Photoelectric photometry was also widely utilized at the Danish Brorfelde Observatory, the building of which was set in motion by Strömgren during the World War II, and followed-up by the successor of Strömgren’s Danish professorship in 1958, Anders Reiz.

International Science After researching at and directing the Yerkes and McDonald Observatories from 1951 to 1957, Strömgren formally left Danish astronomy in 1957 and moved with his family from Yerkes Observatory in Williams Bay, Wisconsin, to the Institute of Advanced Study (IAS) at Princeton, New Jersey. This was a year of changes, as the satellite Sputnik ushered in the space age and occasioned an explosive post-Sputnik growth of space science, astronomy, and national space programs in the 1960s. Strömgren’s appointment as professor of astrophysics in Princeton that year was at a tremendously interesting time for space and astronomy research. Strömgren took over the office formerly occupied by Albert Einstein, who died in April 1955, and he stayed with his wife Sigrid Strömgren in Princeton for ten years until their return to Denmark in 1967.

Strömgren had earned much international recognition. In 1954, he was elected a member of the Advisory Panel of the National Astronomical Observatory that was supported by the U.S. National Science Foundation. In April that year, it was settled that the panel should consist of the chairman, Robert R. McMath, Ira S. Bowen, Whit-ford from the Washburn Observatory, Struve, and Strömgren. The following year, Strömgren was elected a member of the Academy of Arts and Sciences.

Although there were no observational possibilities in Princeton, Strömgren soon got himself involved in the large scale project of the establishment of the Kitt Peak National Observatory (KPNO). Numerous astronomers petitioned the federal government for funds to build a research center available to the entire American community of astronomers, a national facility. By 1957, the advisory panel had decided to organize an Association of Universities for Research in Astronomy (AURA) to manage the observatory and in May 1958 the National Science Foundation secured the mountaintop, Kitt Peak in Arizona, as the site for a national observatory. Strömgren was on the planning committee of the observatory, having useful and relevant experience from setting up the new Brorfelde Observatory in Denmark. The KPNO became the most frequently used observatory for Strömgren’s observations for ten years, where his close coworkers were David Crawford and Charles Perry.

With the establishment of the National Aeronautics and Space Administration (NASA) in October 1958, Strömgren became a member of a committee to help plan an astronomy program for the space agency. In 1960, he was chosen as a consultant research associate for the Goddard Space Flight Center’s theoretical division. The following year he was appointed as consultant for the Institute of Space Studies in New York. In the following years, Strömgren became increasingly occupied with political and organizational matters of international astrophysics and big science and large observatory projects.

He also won many awards. One of the most important was the Catherine Wolfe Bruce Gold Medal, which he received from the Astronomical Society of the Pacific (ASP) in 1959 for being “one of the leading astrophysicists of our generation” (Mayall, 1959). The highest honor awarded by the Royal Astronomical Society was given to Strömgren in 1962. He received the Gold Medal for his contributions to stellar and interstellar astrophysics. According to the admiring presidential address by William McCrea, Strömgren’s work was “characterized by penetrating scientific insight, a comprehensive knowledge of the subject in hand and by skill and patience in the use of the required techniques whether of mathematics, of computation or of observation.”

In his later years, Strömgren got numerous honors, honorary doctorates, awards, and directorships. Medals, prizes, and honorary lectures were: the Augustinus Prize (1950), Halley Lecture (1958), Ole Rømer Medal (1962), George Darwin Lecture (1962), the Rosenkjær Prize (1963), H. C. Ørsted Medal (1965), H. N. Russell Lectureship (1965), Juel Janssen Gold Medal (1967), and the Karl Schwarzschild Medal (1969). He was general secretary of International Astronomical Union (IAU) (1948–1952). He held the directorships of the Copenhagen Observatory (1940–1954), Yerkes and McDonald Observatories (1951–1957), and NORDITA (1971– 1975). He served as president of the American Astronomical Society (1966–1967), Royal Danish Academy of Sciences and Letters (1969–1975), IAU (1970–1973), ESO’s Scientific Policy Committee (1971–1974), and the ESO Council (1975–1977).

In 1965, Strömgren was offered the opportunity to move into the Carlsberg Mansion of Honor, formerly inhabited by Niels Bohr. He accepted in 1967; interestingly, 1967 was also the year of formal Danish membership of the European Southern Observatory (ESO), thus going back to Denmark did not mean the loss of professional observational research anymore. The Carlsberg Mansion of Honor was originally intended to be a stamping ground for large groups of learned scientists, artists, and politicians. Living in the Carlsberg Mansion committed the inhabitant to live a relatively public life and to function as a hospitable host of prominent foreign visitors. This also applied for Strömgren, who moved in with the iconic memories of a series of important Danish academics, in particular Bohr. Strömgren was buried in Copenhagen in the summer of 1987 and the memorial service was held in the Carlsberg Mansion of Honor.

BIBLIOGRAPHY

WORKS BY STRÖMGREN

“The Opacity of Stellar Matter and the Hydrogen Content of the Stars.” Zeitschrift für Astrophysik 4 (1932): 118–152.

With Elis Strömgren. Lehrbuch der Astronomie. Berlin: Springer, 1933.

“Die Theorie des Sterninnern und die Entwicklung der Sterne.” Ergebnisse der exakten Naturwissenschaften 16 (1937): 465–534.

“On the Helium and Hydrogen Content of the Interior of the Stars.” Astrophysical Journal 87 (1938): 520–534.

“The Physical State of Interstellar Hydrogen.” Astrophysical Journal 89 (1939): 526–547.

“On the Density Distribution and Chemical Composition of the Interstellar Gas.” Astrophysical Journal 108 (1948): 242–275.

“Spectral Classification through Photoelectric Photometry with Interference Filters.” Astronomical Journal 56 (1951): 142–143.

With William W. Morgan and M. Hugh. “A Description of Certain Galactic Nebulosities.” Astrophysical Journal 121 (1955): 611–615.

“Problems of Internal Constitution and Kinematics of Main Sequence Stars.” Quarterly Journal of the Royal Astronomical Society4 (1963): 8–36. George Darwin Lecture.

“Spectral Classification through Photo-electric Narrow-Band Photometry.” Annual Review of Astronomy and Astrophysics 4 (1966): 433–472.

With B. Grønbech and Erik H. Olsen.” Standard Stars uvby Photoelectric Photometry South of Declination +10°” Astronomy and Astrophysics Supplement Series 26 (1976): 155–176.

With Erik Heyn Olsen and Bengt Gustafsson. “Evidence of Helium Abundance Differences between the Hyades Stars and Field Stars, and between Hyades Stars and Coma Cluster Stars.” Proceedings of the Astronomical Society of the Pacific 94 (1982): 5–15.

“Scientists I Have Known and Some Astronomical Problems I Have Met.” Annual Review of Astronomy and Astrophysics 21 (1983): 1-11. Valuable yet unassuming autobiographical article.

OTHER SOURCES

Kulsrud, Russell M. “Bengt Strömgren.” American Philosophical Society Yearbook (1987): 216–222. Obituary.

Mayall, Nicholas Ulrich. “Award of the Bruce Gold Medal to Dr. Bengt Strömgren.” Publications of the Astronomical Society of the Pacific 71 (1959): 79–82.

McCrea, William H. “Presidential Address: On the Award of the Gold Medal to Professor Bengt Strömgren.” Quarterly Journal of the Royal Astronomical Society 3 (1962): 81–83.

Nissen, Poul Erik, and Bengt Gustafsson. Astrophysics : Recent Progress and Future Possibilities: Invited Reviews at a Symposium in Honour of Bengt Strömgren (1908–1987) held in Copenhagen May 25–26, 1988. Copenhagen: Royal Danish Academy of Sciences and Letters, 1990. Gives a selected bibliography for Strömgren.

Olsen, Erik Heyn. “On the Complementarity between MK Classification and Strömgren Photometry.” In The MK Process at 50 years: A Powerful Tool for Astrophysical Insight: A Workshop of the Vatican Observatory, Tucson, Arizona, U.S.A., September 1993, edited by Christopher J. Corbally, Richard O. Gray, Robert F. Garrison. Astronomical Society of the Pacific Conference Series, vol. 60. San Francisco: Astronomical Society of the Pacific, 1994. Gives a comprehensive bibliography of Strömgren’s publications in this field.

Rebsdorf, Simon Olling. “Bengt Strömgren: Growing up with Astronomy, 1908–1932.” Journal for the History of Astronomy34, part 2 (May 2003): 171–199. Biographical paper on Strömgren’s early years

———. “The Father, the Son, and the Stars: Bengt Strömgren and the History of Twentieth Century Astronomy in Denmark and in the USA.” PhD diss., University of Aarhus, Steno Institute, 2005. Systematic biographical dissertation on Bengt and Elis Strömgren. Gives a complete bibliography of Strömgren and lists relevant archives and oral interviews.

———. “Bengt Strömgren: Interstellar Glow, Helium Content, and Solar Life Supply, 1932–1940.” Centaurus 49 (2007): 56–79. Biographical paper on Strömgren’s prewar years.

Woltjer, Lodewyk. “Bengt Strömgren (1908–1987).” Messenger 49 (1987): 1. Obituary.

Simon Olling Rebsdorf