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Famous scholars from Kiel:

Albrecht Unsöld

Astrophysicist, set out the principles for determining the physical conditions in stellar atmospheres, Professor and Director of the Institute for Theoretical Physics in Kiel from 1932 to 1973.

Kiel University can indeed consider itself fortunate that when it appointed Albrecht Unsöld to the Chair of Theoretical Physics in 1932 it was appointing a young research scientist who was to bring world-wide fame to Kiel astrophysics. His standard work "Physik der Sternatmosphären" (Physics of Stellar Atmospheres) became the international "bible" of astrophysicists and, with the help of the post-war generation, he developed what became known across the whole world as "the Kiel school". After the end of the war in 1945, as first Dean of the Faculty of Philosophy and Natural Sciences, he breathed renewed life into the faculty and during his year of office as Rector in 1958/59 worked tirelessly for the reconstruction of the university, in recognition of which the university finally made him an honorary senator in 1983.

Albrecht Otto Johannes Unsöld was born in Bolheim (Württemberg) on 20th April 1905, the son of a Swabian protestant clergyman. When he was 14 he was already reading what was at that time the standard work of the new science of atomic physics "Atombau und Spektrallinien" (The Structure of Atoms and Spectral Lines) and was corresponding with its famous author Arnold Sommerfeld (1868-1951), moving to Munich to study under him as soon as he could after obtaining his university entrance qualification (Abitur) in Heidenheim and completing his initial university studies in Tübingen. He attended Sommerfeld's lectures, immediately took part in his seminars and was awarded his doctorate as early as 1927 (when he was 21) with his dissertation "Beiträge zur Quantenmechanik der Atome" (Contributions to Quantum Mechanics of Atoms). Sommerfeld's institute had just commenced work on Schrödinger's new wave mechanics and, at the same time, on Heisenberg's quantum mechanics; these were the foundation of Unsöld's doctoral work in this field. Sommerfeld endorsed his interest in the application of the new quantum theory to the interpretation of spectral lines in the sun and obtained a bursary for him from the "Notgemeinschaft der deutschen Wissenschaft" for observations from the Einstein tower telescope for solar research in Potsdam and later a bursary as a Fellow of the International Education Board (Rockefeller Foundation) for observations at Mount Wilson in Pasadena, California which contained what was then the world's largest reflector telescope (100") and was a Mecca for astronomers. They were so enthusiastic about his abilities there that they offered him a permanent position. Unsöld, however, refused the offer, preferring to return to Munich to obtain his "Habilitation" (qualification to teach at a university in Germany). On the way home he visited astronomical centres in Chicago (the Yerkes Observatory) and Harvard, Massachusetts where he lectured and became acquainted with leading American astrophysicists such as Harlow Shapley (1885-1972). He also stopped off in Cambridge (England) where he was invited both to tea and dinner by the famous Arthur Stanley Eddington (1882-1944). Unsöld used this esteem to write to Sommerfeld to submit proposals for the advancement of astrophysics in Germany. Amongst his proposals was a case for establishing a southern observatory and the foundation of an astrophysical institute - plans which only became reality 30 years later.

His dissertation for his Habilitation "Über die Balmer-Serie des Wasserstoffs im Sonnenspektrum" (On the Balmer Hydrogen Series in the Solar Spectrum) summarised the results of his observations and deliberations on the theory of line generation. In the years that followed he was to develop these ideas systematically with reference to radiative transfer and the quantum mechanics of line broadening caused by radiation loss, reflection loss and electrical fields (Stark effects). It was in this way that the ultimate objective, the quantitative analysis of solar spectra, and particularly the definition of the chemical composition and the incidence of elements in solar atmospheres was pursued. From May 1930 he worked in Hamburg as a lecturer and assistant in the Institute for Theoretical Physics where a friendship developed with Walter Baade (1893-1960), However, Baade left Hamburg in 1932 to take up a permanent position at Mount Wilson where he remained because of its outstanding conditions for astronomical observation even though he was offered the position of Director of the Hamburg Observatory in 1938 in succession to Richard Schorr (1867-1949).

In 1932, at the age of only 26, Unsöld was delighted to accept the Chair of Theoretical Physics at Kiel University in succession to Professor Walther Kossel (1888-1956). During the initial years of the Nazi era that followed he lectured intensively on theoretical physics and devoted much time to writing his book "Physik der Sternatmosphären (mit besonderer Berücksichtigung der Sonne)" (The Physics of Stellar Atmospheres (with special Reference to the Sun)". Dr Liselotte Kühnert, whom he married in 1934, typed the whole work of 500 pages for him and it was published by the Springer Verlag in 1938. In the foreword to the book Unsöld acknowledged the critical and collaborative work of Dr Walter Lochte-Holtgreven (1903 - 1987), who was later appointed to the Chair of Experimental Physics, and the assistance of Dr Gerd Burkhardt (1911-1969) who became his assistant after the war.

Shortly before the outbreak of war in 1939, Unsöld accepted an invitation to be a visiting professor in Chicago. While in Chicago he was invited by Otto Struve (1897-1963) to accompany him to the inauguration of the McDonald telescope in Texas. Struve allowed him to take some high resolution photographs which Unsöld took back to Germany on which he worked in the years that followed.

This was, in fact, how he was able to undertake a thorough analysis of a hot star (B0 Tau Scorpii) and publish the findings in four parts in the journal "Zeitschrift für Astrophysik" (Journal for Astrophysics) while at the same time meeting his war service obligations as a meteorologist. When bombs destroyed the Kiel Observatory, which had been closed since 1938, Unsöld nevertheless succeeded in moving to Kappeln (using a requisitioned lorry) the valuable astronomical library which went back to Hans Christian Schumacher's (1780 - 1850) Altona Observatory (ca. 1800). After the destruction of both his home and his institute he moved in with Dr Werner Kroebel (1904-2001) at Bredeneek Castle which later housed the department of Applied Physics, while his family lived in Schönberg.

In the summer of 1945 the British appointed the politically untainted Unsöld as Dean of the Faculty of Arts and Humanities at the same time as he was preparing for the resumption of his lecturing work for the 1945/46 winter semester (some of the lectures were held on ships in the harbour) and, in a barracks in the Hospitalstrasse, also helping those who were returning from the war. This was before the university found a new home with ELAC (the electro-acoustic equipment manufacturer) in the Westring. Unsöld was able to obtain a whole floor there where he established the "Institute for Theoretical Physics and Astronomical Observation", complete with the old astronomical library which he brought back to Kiel.

It was this address that made Kiel Astrophysics famous across the world. Unsöld kept this address until his retirement although Theoretical Physics moved into its own Institute in the 1960s and Astrophysics and Astronomy obtained a second chair. Years of fruitful collaboration with students, most of them returning to Germany after the war or displaced from lost German territories in Eastern Europe (Annemarie Rosa, Erika Vitense, Dietrich Labs, Hans-Heinrich Voigt, Karl-Heinz Böhm, Kurt Hunger, Max Reichel, Gerhard Traving, Volker Weidemann) and also his own work made a revised new edition of "The Physics of Stellar Atmospheres" (1955) both necessary and possible. Other students (Ludwig Oster, Hartmut Holweger, Bodo Baschek, Wilhelm H. Kegel, Dieter Reimers, Thomas Gehren) followed in the next decade and, due to the time that most of them spent on research work in the USA and the chairs they later held, they spread the fame of the "Kiel School". The liber amicorum prepared to mark Unsöld's 70th birthday "Problems in Stellar Atmospheres and Envelopes" is ample testimony of this.

In the meantime Unsöld had refused offers of chairs at Tübingen and Munich - not least because of his close collaboration with Walter Lochte-Holtgreven and his plasma physicists in Kiel - but he accepted a number of tributes (the Gold Medal of the Royal Astronomical Society (1956), the Bruce Gold Medal of the Astronomical Society of the Pacific (1957) as well as honorary doctorates from Utrecht (1961), Edinburgh (1970) and Munich (1972)).

During the period 1945 - 57 he was a member of the Senate of the German Research Foundation. He immediately set to work to rebuild international links, particularly with the USA but also with the UK and the Netherlands and thereby brought to fruition the hope that Sommerfeld, his old teacher, had expressed in a card in 1947 when he wrote: "Dear Unsöld, only a short but heartfelt thank-you for your faithful remembrance and the loyalty you have consistently shown towards me. You are one of the pillars around which Germany's international prestige should be rebuilt..."

In Kiel he was instrumental in the erection of a memorial in front of the HSH Nordbank to commemorate the 100th anniversary of Planck's birth. In his speech at the memorial's unveiling ceremony on 23rd April 1958, Unsöld quoted Planck whom he had accompanied through the ruined city in 1944 to Planck's hotel after a lecture at the Kiel Institute for the World Economy: "How happy I am to see the Kiel I love once again" (printed in "Sterne und Menschen" (Stars and Men) along with Unsöld's speech on his inauguration as Rector for 1958/59, "Physik und Historie" (Physics and History)). This speech became famous for its epistemological assertion "Science is a science conducted by human beings for human beings and for nothing more."

In the years that followed, Unsöld attempted to understand the results of his research relating to the rather uniform frequency of elements across the cosmos arising from the development of stars and galaxies, grappling with the newer hypotheses on the subject. In many lectures, e.g. to the "Gesellschaft Deutscher Naturforscher und Ärzte" he perceived the need to make the fundamentals of astrophysics more easily understood by laymen and colleagues with an interest in the subject. He therefore wrote his second book "Der Neue Kosmos" (The New Cosmos) which was published for the first time in 1967. This work was distributed across the world in the form of translations into Spanish, English and Japanese and in the form of further editions (the ones that appeared after 1981 were prepared with the collaboration of Bodo Baschek in Heidelberg) and is still considered as a standard text-book. In 1973 Unsöld received the Cothenius Gold Medal of the German Academy of Natural Scientists Leopoldina in Halle for outstanding life-time achievement. The encouragement he received at the meetings of the Leopoldina and elsewhere prompted him to publish a final book summarising his work "Evolution kosmischer, biologischer und geistiger Strukturen" (The Evolution of Cosmic, Biological and Intellectual Structures) (1981) in which he also expressed his personal vision of the human being as part of a whole.

He devoted much time during his retirement to painting water colours. He died on 23rd September 1995 at the age of 90.

Unsöld demanded much of himself and his students but was also truly concerned about them and followed their careers with a genuine interest. As editor of the "Zeitschrift für Astrophysik" (Journal of Astrophysics) from 1947 to 1968 he ensured that papers published in the Journal were of a high standard and regained international esteem for the journal - also for the German language in which he encouraged his students to publish. The "Astronomische Gesellschaft" (Astronomical Association) marked the 100th anniversary of Unsöld's birth (their first honorary member) at their Autumn Meeting in Cologne in 1995.

Prof. Dr. Volker Weidemann
Institut für Theoretische Physik und Astrophysik

First published in: Christiana Albertina, Forschungen und Berichte aus der Christian-Albrechts-Universität zu Kiel, Ausgabe 62

Important works by Albrecht Unsöld

  • Physik der Sternatmosphren (mit besonderer Berücksichtigung der Sonne), 1st edition 1938, 500 pages, 2nd edition1955, 866 pages, Springer Verlag Berlin, Göttingen, Heidelberg.

  • Der Neue Kosmos. 356 pages, Springer Verlag, Berlin (Heidelberger Taschenbuch 16/17), further editions 1974, 438 pages, 1981, 1988 and 1991 (with B. Baschek), 1999 and 2003 (B.Baschek).

  • Sterne und Menschen, Aufsätze und Vorträge. 1972, 170 pages, Springer Verlag Berlin, Heidelberg, New York.

  • Evolution kosmischer, biologischer und geistiger Strukturen. 1981, 150 pages. Wissenschaftliche Verlagsgesellschaft, Stuttgart.

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