Unsöld, Albrecht Otto Johannes
Unsöld, Albrecht Otto Johannes
UNSöLD, ALBRECHT OTTO JOHANNES
(b. Bolheim, Württemberg, Germany, 20 April 1905;
d. Kiel, Germany, 23 September 1995), astrophysics, stellar atmosphere physics.
German astrophysicist Unsöld, for the longest part of his career based at the University of Kiel, was among the first researchers applying the formalism of quantum mechanics to the physics of stellar atmospheres.
Early Life and Studies. Unsöld, son of a Protestant minister, attended primary and secondary school in Heidenheim, Württemberg. He developed an early interest in science because both his father and grandfather, a school teacher in Stuttgart, were botany enthusiasts and his uncle, who owned a notable physical cabinet, conducted experiments in wireless telegraphy and on dynamos. After reading Arnold Sommerfeld’s fairly technical textbook on quantum theory, Atombau und Spektrallinien (Atomic structure and spectral lines), which had then just appeared, the precocious fourteen-year old Albrecht wrote a letter to the prominent author, enclosing a manuscript of his own. (Transcription of these letters, also including Sommerfeld’s very friendly response, have been published by Weidemann.)
After passing his school finals (Abitur), Unsöld started studying mathematics and physics at nearby Tübingen University, choosing astronomy as his minor. He attended courses by Alfred Landé, Friedrich Paschen, and Ernst Back, all of whom specialized in spectroscopy, but after three terms Unsöld changed to the University of Munich to study with Sommerfeld.
His first two papers on spectroscopic issues appeared in the prestigious Zeitschrift für Physik in 1925 and 1926. At twenty-one years of age, Unsöld submitted his PhD thesis titled “Contributions to the Quantum Mechanics of Atoms” to the University of Munich, receiving his doctoral degree summa cum laude in February 1927. (According to the interview with Owen Gingerich, Unsöld had actually been assigned the task of calculating the molecular hydrogen ion, but this subject turned out to be too difficult because of two essential singularities, so Unsöld calculated other things, such as the spectrum of helium [with exchange terms], the polarization of the atomic nucleus by an electron in highly excited quantum states, and the quadratic Stark effect.) His mentor and thesis supervisor, Arnold Sommerfeld, was a most gifted teacher and author of famous textbooks. Many pupils of the so-called Sommerfeld school (Eckert 1993, pp. 132f., 144f.) were major contributors to the semiclassical quantum theory and the new quantum mechanics of 1925 and 1926; Werner Heisenberg and Wolfgang Pauli were among their number. Consultations with Gregor Wentzel, then Sommerfeld’s assistant, proved particularly helpful to the young Unsöld.
Unsöld’s thesis was the first dealing explicitly with Erwin Schrödinger’s version of quantum mechanics. It applied Schrödinger’s wave mechanics to various problems in quantum chemistry, especially heteropolar chemical bonds. Subsequently as Sommerfeld’s assistant, Unsöld continued to apply quantum mechanics to chemistry and crystal structure analysis.
Further Career. By 1927, Unsöld was convinced that theoretical astrophysics was the field most suited to him. “I thought, would it not be worthwhile just to try to understand how the Fraunhofer lines are produced?” he later told Owen Gingerich in an interview (1978, AIP transcript, p. 8f.). A colleague of Sommerfeld at Munich, Robert Emden, introduced him to the nascent field of theoretical astrophysics, in particular to the writings of Karl Schwarzschild. Emden, professor at the Munich Polytechnic, had pioneered the modeling of stars as hot gas balls. For the Sommerfeld Festschrift of 1928, Unsöld contributed a research paper on the influence of scattering on the Fraunhofer line profile, which followed up on techniques and ideas of Schwarzschild then still widely unknown to astronomers and physicists. But rather than conceiving this as a first step away from theoretical physics into astrophysics, Unsöld insisted on the continuity of his intellectual pursuits. When Gingerich asked how his colleagues had reacted to his shift in interest, he simply replied: “I have always remained a theoretical physicist.”
Unsöld realized that in order to make any further progress, more precise observations of select spectrum line profiles were needed. Quite in line with this, and funded by the Notgemeinschaft der Deutschen Wissenschaft, Unsöld arranged for a research stay at the Einstein Tower telescope in Potsdam (cf. Hentschel, 1997, p. 98f.), where he worked with Erwin Finlay Freundlich, Hermann A. Brück, and Walter Grotrian and familiarized himself with the experimental techniques they used. With a fellowship from the International Education Board, he then went to the Mount Wilson Solar Observatory as guest researcher in 1928 and 1929. With the help of Charles Edward St. John and Harold Babcock (who prepared hypersensitized infrared photographic plates for him), Unsöld recorded chromospheric emission lines with the 150-foot tower telescope and investigated spectroheliograms. In California, Unsöld also met the elite of contemporary astrophysics, most notably Henry Norris Russell and Cecilia Payne-Gaposchkin, both pathbreaking theoreticians (cf. Unsöld, 1979; Haramundanis, 1996) On his way back, Unsöld visited Harvard University, where the twenty-four-year-old gave a talk on stellar spectra, with Harlow Shapley, Edward Arthur Milne, and Arthur Eddington sitting in the audience. Unsöld later recalled in an interview (1984, transcript p. 8f.), that his Harvard talk managed to convert everyone present to his point of view.
Unsöld and his listeners realized that stellar spectra held the key to understanding the physical conditions and processes in the interior of stars. It had been known since Gustav Kirchhoff and Robert Bunsen’s discovery of qualitative spectrum analysis in 1859 that the presence (or absence) of characteristic lines in a stellar spectrum reliably indicates the presence (or absence) of specific chemical elements. But these qualitative features were not all that was encoded in spectrum lines. Quantitative information like the prevalence of an element in a star was correlated with the intensity of each line. Even finer characteristics of spectrum-line profiles could be explored to learn more about a star’s temperature, pressure, electron pressure, and variations of all these parameters at various depths of its atmosphere. But exactly how these inferences could be drawn was still under intense discussion at the time. Henry Norris Russell, for instance, declared that it was hardly to be doubted that the quantum theory, when properly applied, would clear up these questions. So the situation was perfect for Unsöld. As a student of Sommerfeld, he had obtained the best possible training in quantum theory. Even Russell, thus far the leading figure in applying astrophysics to quantum theory, wrote grudgingly to William Meggers at the Washington National Bureau of Standards: “I am rather inclined to leave the job for the present to Sommerfeld and his students, who are probably already deep into it” (quoted in DeVorkin and Kenat, p. 197). As a first step, Unsöld concentrated on sodium, which displays one of the most characteristic absorption lines in the solar spectrum, the Na D line. Combining Schwarzschild’s transport equation with microphotometric data on the Na D line profile, Unsöld arrived at a first quantitative determination of the percentage of sodium in the solar photosphere.
Shortly after returning to Germany, Unsöld submitted his habilitation thesis (for certification as a university teacher) on the Balmer series of hydrogen to the University of Munich in 1929 (it was published in 1930). For eighteen months, Unsöld worked in Hamburg as assistant to Wilhelm Lenz and at the Bergedorf Observatory with his colleague and close friend, Walter Baade. During the great economic depression he was fortunately appointed Ordinarius for theoretical physics at the University of Kiel in September 1932 (thus becoming the youngest full professor on record in Germany). Unsöld pursued nearly the entire remainder of his career at Kiel as director of the Institute for Theoretical Physics and Observatory—a typical Unsöldian combination, quite unique among German universities with their strong tendency toward specialization. This professional arrangement was only dissolved in the 1990s when an independent Institute for Astronomy and Astrophysics was established.
In 1932, Unsöld developed the weight function method, which allows interpretation of the quantitative determinations of weak Fraunhofer line profiles and the wings of strong lines, provided one has a model of how the absorption changes with depth. Soon he would also study the influence of radiation damping (which broadens spectrum line profiles), Doppler shifts, electric fields, and varying pressure on the broadening of Fraunhofer lines in the solar spectrum. Unsöld also showed that the Doppler shifts were linked to convective currents in the solar atmosphere, particularly in the ionized hydrogen convection zone right under the solar photosphere. Using these techniques to analyze many other stellar spectra, Unsöld was later able to tabulate physical atmospheric parameters of the Sun and red giant stars as a function of depth and even to estimate the overall abundance of the lighter elements throughout the universe.
His textbook on the physics of stellar atmospheres, first published in 1938, quickly became a classic. Even though it was never translated, it was nevertheless widely used in foreign countries, as no other comparable summary of quantitative spectroscopy and its astrophysical applications existed. Further expanded editions appeared in 1955 and 1968 with a new subtitle, indicating special attention to the Sun. It is still used as a reliable reference work. Many of its sections open with historical introductions and the author‘s effort at lucidity and well-balanced acknowledgment of all contributors to the field is noticeable throughout the work. His own contributions are alluded to in the third person and are given rather less than their due.
Prior to World War II, Unsöld obtained a guest professorship at the Yerkes and McDonald Observatories, where he worked for six months in 1939 with Otto Struve on a coudé-spectrograph analysis of Tau Scorpii, a hot star of spectral type B0. The outcome of his quantitative study, the first detailed analysis of an early-type star other than the Sun, was an abundance of hydrogen (85%) and a surprisingly high percentage of the inert gas helium (nearly 15%), so rare on this planet. Furthermore, he was able to determine that this star had the very high surface temperature of 32,730 K. Unsöld could also specify its internal pressure, density and other physical characteristics. This study of Tau Scorpii became paradigmatic, setting the standard for similar work on other stars by Unsöld himself and by his pupils, who continued to refine and improve his techniques. Between about 1950 and 1970, many doctoral theses at the University of Kiel under Unsöld’s guidance used his sophisticated model atmosphere techniques (Feinanalyse) to determine the chemical composition and physical conditions of various stars.
Shortly before the beginning of the war, Unsöld returned to the University of Kiel, where he built up a flourishing school of theoretical astrophysics, thus disseminating his extraordinary skill in mathematical physics combined with a sound intuition in interpreting experimental data. Many of his pupils (e.g., Bodo Baschek, Karl-Heinz Böhm, Erika Böhm-Vitense, Hartmut Holweger, Kurt Hunger, Dieter Reimers, Gerhard Traving, and Volker Weidemann) themselves became professors of astronomy or physics. Their teacher also established close ties with experimental physics, most notably with Walter Lochte-Holtgreven, who became Ordinarius for experimental physics after the war and specialized in plasma physics. Other of Unsöld’s contacts included the nuclear physicists Heinrich Rausch von Traubenberg (who was fired by the Nazis and committed suicide in 1944); August Eckardt and Rudolf Gebauer; and the applied physicist Werner Kroebel and his assistants Wolfgang Priester and Franz Dröge, who screened the Northern Hemisphere with a radio telescope he had built himself (see below). The collaboration on the reaction Li7 + H1 = 2He4 was related to Unsöld’s interest in the mechanism of nuclear energy production in stars (see his letter to Karl Hufbauer, 1978, and see Schönbeck, 2006, particularly on Rausch von Traubenberg’s work with Unsöld).
Ties with the many centers for astrophysics outside Germany were also fostered. Particularly intense ones were established with the observatories in Utrecht and Edinburgh, where Unsöld’s close friends Marcel Gilles Jozef Minnaert and Brück were directors. Unsöld and his pupils, also referred to as the “Kiel school,” were frequent visitors at several observatories in the United States as well.
Journals, Editorships, Etc. The English-speaking world’s leading periodical in this field since its inception was the Astrophysical Journal (appearing since 1895). In order to make room for high-class German contributions to the literature, it was decided to found the Zeitschrift für Astrophysik in 1930, to which Unsöld was a regular contributor. He subsequently became coeditor (from 1947 on) and after the death of Walter Grotrian was promoted to editor-in-chief in 1955 until the journal was merged with other European periodicals into Astronomy and Astrophysics in 1968. Unsöld also edited the more popularly oriented Himmelswelt from 1947 to 1950. According to Seaton (1997, p. 38), Unsöld ruled the Zeitschrift für Astrophysik with an iron hand, rejecting about one-third of the submitted papers out-of-hand (“I don't need a second opinion to recognize shoddy work”), accepting another third without qualification, and only sending the rest to referees. In his interview of 1984, Unsöld assured Harwit that this was ten times as fast as in other journals without any loss in quality.
Nazi Period. The rise of National Socialism was a heavy blow to Unsöld’s internationally oriented research. Many members of the Sommerfeld school were either forced into exile or, like Hans Bethe, decided to leave Germany in order not to compromise with the new anti-Semitic rulers. Unsöld, however, decided to stay, partly because he felt that others needed the scarce jobs abroad more urgently, partly because he simply wanted to continue his own line of research and not give way to political pressure. When academic positions had to be refilled, he tried to uphold academic standards and not let party membership or other nonscientific criteria decide. But he deeply regretted the isolation and drastic deterioration of physics in Germany.
During the war, Unsöld was drafted as a meteorologist together with his friend Lochte-Holtgreven. He was stationed first at Kiel-Holtenau airport, then at nearby Travemünde, which meant he could continue some of his research on Tau Scorpii and other esoteric subjects under the cloak of “secret war research.” Kiel observatory had been closed down in 1938 and most of its buildings were destroyed by the bombing that reduced about 85 percent of the city to rubble, but Unsöld managed to rescue the valuable old Schumacher library (Heinrich Christian Friedrich Schumacher’s observatory in Altona had been moved to Kiel in 1873) by preemptively arranging for its removal into storage in a small town fifty kilometers north of Kiel.
Postwar Functions. After the fall of the Nazi regime, people such as Unsöld who had not compromised themselves by becoming members of the party, the university lecturers’ league, the SA, the SS, or other affiliated Nazi organizations, were in high demand. Unsöld became the first dean of the philosophical faculty when the British Military Command allowed the University of Kiel to reopen in late November 1945. Conditions were extremely difficult then, as most buildings of the university had been destroyed, the faculty had shrunk drastically, and there was a dire scarcity of food, clothing, and housing. During this period under Allied occupation, Unsöld also served as a member of the local denazification committee, whose difficult task was to cleanse the personnel of former Nazis. By declining various calls to other universities, most notably to Munich, Unsöld managed to strengthen the local conditions for astronomical and astrophysical research in Kiel. A radio-astronomical observatory—initially with a dipole apparatus (built in 1952), later with a 7.5-meter dish—was operated on campus until 1975. In the following academic year Unsöld served as university president (Rektor), and in 1982 celebrated his fifty-year golden jubilee as university professor. The University of Kiel conferred upon him the distinction of honorary senator.
From 1949 until its dissolution in late 1951, Unsöld served as a member of the West German federal research council (Forschungsrat), initiated and presided over by Werner Heisenberg. Unsöld was also member of the senate of the German national research foundation (Deutsche Forschungsgemeinschaft) from 1954 to 1957. After his retirement in 1973, Unsöld remained active in research for another fifteen years. That Unsöld was also asked to serve as president of the Astronomische Gesellschaft in the immediate postwar period of 1947 to 1948 is also indicative of the unbroken trust in him by the international community. In 1988, the society elected him as its first honorary member. Unsöld was also a frequent speaker at conferences organized by the International Astronomical Union, but he recalled being shunned by French astronomers after 1945 (Gingerich interview, p. 23) and did not hold any offices there, although he took part in its structural reform in 1957 (see Blaauw, p. 209). He remained active long after his retirement at the age of sixty-seven.
Popularization and History of Science. Roughly a hundred years after the publication of Alexander von Humboldt’s grand compendium Kosmos, Unsöld decided it was time to attempt another synthesis of the knowledge acquired in cosmology, astronomy, (astro)physics, chemistry, and allied sciences. He wrote Der neue Kosmos with the set purpose of making known to nonspecialists the major changes, revisions, and additions to the basic concepts of astrophysics. It went through six revised and expanded editions (the last four coedited by his pupil Bodo Baschek) and was also translated into English, Spanish, Italian, and Japanese.
As an aside, Unsöld’s interest in the history of science in his later years should also be mentioned. In 1957, he wrote a short booklet about Max Planck; later he published a note interpreting Heinrich Hertz’s principles of mechanics as foreshadowing elements of Einstein’s thinking (see Unsöld, 1970, especially on geometrization and on the elimination of forces by constraining motion onto geodesics).
Unsöld’s general view was that prior, much less known developments had prepared the way for revolutionary breakthroughs in the sciences, often occurring in more than one place at a time, Nicholas Cusa and the Copernican Revolution being other cases in point. An anthology of some of his semipopular papers and portraits of people he had met appeared in 1972. In his last book from 1981, Unsöld reflected on the evolution of cosmic, biological, and mental structures and sought a grand unifying view, thus building bridges across to other domains of knowledge.
International Functions and Honors. For his pathbreaking contributions to the theory of stellar atmospheres and quantitative spectral analysis, Unsöld obtained many honors, most notably the Bruce Medal of the Astronomical Society of the Pacific in 1956 and the Gold Medal of the London-based Royal Astronomical Society in 1957. The Royal Astronomical Society of Canada named him honorary member in 1956.
Unsöld’s broad interest in more general issues of science is also reflected in his engagement for the Gesellschaft Deutscher Naturforscher und Ärzte and in his election as member of the Deutsche Akademie der Naturforscher Leopoldina in 1962, to whom it awarded its prestigious Cothenius Gold Medal for outstanding life work in 1973. Unsöld was also a member of the Göttingen and Bavarian academies of science, and of the International Academy of Astronautics. Honorary doctorates were conferred on him by the universities of Utrecht (1961), Edinburgh (1970), and Munich (1972). The University of Königsberg awarded him their Copernicus Prize as early as 1943.
He was quite averse to hagiography and undue deference, though—his harsh words about the idolization of genius on the occasion of the centenary of Einstein’s birth in 1979 caused quite a stir in the scientific community and beyond (see Unsöld, 1980, and various letters to the editor in 1981; Unsöld’s disgust about self-advertisement of scientists and the article from 1980 is also mentioned in the interview with Harwit [1984, transcript pp. 11f., 23f.]).
A Personal Note. Being a child of the province of Württemberg, Unsöld always retained a slight Swabian accent, which added a charming touch to his lectures. In 1934 he married the biologist Dr. Liselotte Kühnert (d. 1990), with whom he had four children. He was fond of music, played the first violin in the Kiel university orchestra as well as at private concerts, and also enjoyed watercolor painting. As one of his pupils, Volker Weidemann, said in his obituary, “Unsöld was a towering personality, a giant not only in physical stature.”
There are papers at the Unsöld Archive at the Institute for Astronomy & Astrophysics in Kiel. Unsöld’s correspondence with Otto Struve is microfilmed and available at the American Institute of Physics (AIP) under call no. MI 78. The archive of the Deutsche Akademie der Naturforscher Leopoldina in Halle has a membership file on Unsöld. The Royal Astronomical Society in London and the Astronomical Society of the Pacific have files related to his prize awards. A select bibliography of some two dozen scanned articles authored by Unsöld and more useful historical articles are available from http://www.physastro.sonoma.edu/BruceMedalists/Unsold/UnsoldRefs.html.
WORKS BY UNSÖLD
“Beiträge zur Quantenmechanik der Atome.” Annalen der Physik, 4th ser., 82 (1927): 355–393. PhD diss., University of Munich.
“Ueber die Balmerserie des Wasserstoffs im Sonnenspektrum.” Zeitschrift für Physik 59 (1930): 353–377. Habilitation thesis from 1929.
Physik der Sternatmosphären[Physics of stellar atmospheres]. Berlin: Springer, 1938. 2nd rev. ed., 1955; 3rd ed., 1968.
“Die kosmische Häufigkeit der leichten Elemente.” Physikalische Zeitschrift 41 (1940): 549–552.
“Quantitative Spektralanalyse des B0-Sternes T Scorpii.” Zeitschrift für Astrophysik 21 (1941): 1–21, 22–84, 229–248; 23 (1944): 75–97. Partly translated and commented on in A Source Book in Astronomy and Astrophysics, 1900–1975, edited by Kenneth R. Lang and Owen Gingerich. Cambridge, MA: Harvard University Press, 1979.
Der neue Kosmos. Berlin: Springer, 1967. 2nd ed. 1974, 3rd ed. 1981, 4th ed. 1988, 5th ed. 1991, 6th ed. 1999, 7th ed. 2002. Revised and expanded, since the 3rd edition together with B. Baschek). Also translated into English as The New Cosmos. New York: Springer, 1969. 2nd ed. 1977, 3rd ed. 1983, 4th ed. 1991, 5th ed. 2001.
“Heinrich Hertz. Prinzipien der Mechanik—Versuch einer historischen Erklärung.” Physikalische Blätter 26 (1970): 337–342.
Sterne und Menschen. Aufsätze und Vorträge. Berlin: Springer, 1972.
Interview with Owen Gingerich. 1978. 2 cassettes, 27 pp. transcript. AIP. Call no. OH 500.
Autobiographical letter to Karl Hufbauer. 15 March 1978. AIP. Call no. MB31477.
“Introduction—A Fifty Years Retrospect.” In Les éléments et leur isotopes dans l’univers. Université de Liège: Institut d’Astrophysique, 1979. Page 7 reviews the “explosive development of quantitative spectroscopy,” starting with the contributions of Russell and Payne-Gaposchkin.
Evolution kosmischer, biologischer und geistiger Strukturen. Stuttgart: Wissenschaftliche Verlagsgesellschaft, 1981.
“Unsöld on Einstein.” Nature 291 (1981): 374. Letters by Unsöld and by Kal Kromphardt (editor of Physikalische Blätter), in response to “German Physicists in Row about Einstein.” Nature 290 (1981): 535.
Letter to the Editor. Physikalische Blätter 37 (1981): 65, 229–230.
Interview with Martin Harwit. 24 February 1984. 2 cassettes and 30 pages transcript. AIP. Call no. OH 501.
Baschek, Bodo. “Nachruf: Albrecht Unsöld.” Mitteilungen der Astronomischen Gesellschaft 79 (1996): 11–15. Abbreviated in Physikalische Blätter 52 (1996): 890–891.
Blaauw, Adriaan. History of the IAU: The Birth and First Half-Century of the International Astronomical Union. Dordrecht, Netherlands: Kluwer Academic Publishers, 1994.
Böhm, Karl-Heinz. “Albrecht Unsöld 60 Jahre.” Sterne und Weltraum 4 (1965): 89.
DeVorkin, David, and R. Kenat. “Quantum Physics and the Stars.” Journal for the History of Astronomy 14 (1983): 102–132, 180–222.
Eckert, Michael. Die Atomphysiker: Eine Geschichte der theoretischen Physik am Beispiel der Sommerfeldschule. Braunschweig: Vieweg, 1993.
Hentschel, Klaus. The Einstein Tower: An Intertexture of Dynamic Construction, Relativity Theory, and Astronomy. Stanford, CA: Stanford University Press, 1997.
Haramundanis, Katherine, ed. Cecilia Payne-Gaposchkin: An Autobiography and Other Recollections. 2nd ed. Cambridge, U.K.: Cambridge University Press, 1996. Page 179 gives an account of Unsöld’s first meeting with Milne, with whom he had conducted a “spirited controversy on the printed page, no holds barred.”
Jeffreys, Harold. “The President’s Address on the Award of the Gold Medal.” Monthly Notices of the Royal Astronomical Society 117 (1957): 344–346.
Kippenhahn, Rudolf. “Laudatio für Herrn Prof. Dr. Albrecht Unsöld anläblich der Verleihung der Cothenius-Medaille.” Nova Acta Leopoldina 42 (1974): 61–64.
Seaton, Mike. Obituary. Astronomy and Astrophysics 38 (1997): 37–38.
Schlüter, Arnulf. Obituary. Jahrbuch der Bayerischen Akademie der Wissenschaften 51 (1995): 285–287.
Schmidt-Schönbeck, Charlotte. Dreihundert Jahre Physik und Astronomie an der Kieler Universitat. Kiel: Hirt, 1965.
Sommerfeld, Arnold. Atombau und Spektrallinien. Braunschweig: Vieweg, 1919.
———. Wissenschaftlicher Briefwechsel, edited by Michael Eckert und Karl Märker. Berlin: Deutsches Museum, GNT-Verlag, 2004. Volume 2 publishes part of Unsöld’s correspondence with Sommerfeld.
Voigt, Hans-Heinrich. “Zum 70. Geburtstag von Albrecht Unsöld.” Sterne und Weltraum 14 (1970): 112–113.
Weidemann, Volker. “Albrecht Unsöld (1905–1995).” Publications of the Astronomical Society of the Pacific 108 (1996): 553–555.
Wilson, Olin C. “The Award of the Bruce Gold Medal to Dr. Albrecht Unsöld.” Publications of the Astronomical Society of the Pacific 68 (1956): 89–91.
Wolfschmidt, Gudrun, ed. Entwicklung der Astrophysik. Nuncius Hamburgensis, 4. Norderstedt: books on demand, 2007. A series of papers honoring Unsöld’s hundredth birthday in 2005. See contributions by Volker Weidemann, Bodo Baschek, and Charlotte Schönbeck. Unsöld’s correspondence with Sommerfeld and with Lochte-Holtgreven is partly published in Weidemann’s contribution.