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Interview of Albrecht Unsöld by Owen Gingerich on 1978 June 6, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/4924
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Family background; education at Tubigen Univ.; Schrodinger Theory thesis at Univ. of Munich (1927), influence of Sommerfeld, Heisenberg. Potsdam experiments inspired by K. Schwarzschild, Emden: Local thermodynamic equilibrium and curve of growth. Spent 1928 at Mt. Wilson. Return to Germany to teach physics during Depression; use of microphotometer, Coude spectrograph; analysis of Tau Scorpii. Pre-war professorship at Yerkes with Struve: line profile information, stellar composition. WWII return to Kiel, star temperature detection, solar spectrum analysis, elements/energy production. Isolation and destruction of German physics, anti-German attitude. Remarks on history of science, views on contemporaries and astrophysical (radio) research; fate of Unsold's correspondence.
It was at the Liege meeting. As president of the meeting, I had to give some sort of coordinated introduction. And I did it in the form of giving a 50 years retrospect, because it's just now... 50 years, since the modern development of astrophysics into a quantitative science began, based on quantum theory, quantum mechanics and so on ... I made a photocopy of my introduction for you. I did not present it exactly in this form, but I think I will have it printed in this form in the Liege reports.
All right. My suggestion for procedure would be that we do this in two sessions, with some sort of break in between that will make it more relaxing. There is a twofold purpose for my talking to you. One is that I would deposit the tapes with the American Institute of Physics at the Niels Bohr Library, where they have an archive of many of these tapes, and for that part particularly I would like to talk about your own rather personal view of doing astrophysics, and how you changed from physics into astrophysics; your education; the people you worked with; your teachers, and so on. In the second session, I would like to ask you your broader evaluation of what you think were the most important trends in astrophysics, in this part. So maybe I can read over the paper very quickly, in the recess between the sessions.
As I said, you can also just take it along. I am sure that it will be printed in the Report of the Liege meeting.
I am happy you have done it, because these are always very useful kinds of surveys.
It was, even for me, a little bit surprising to see how in these rather few years between 1926 and 1928, almost all the very important concepts of modern astrophysics, spectroscopy, — I should say — were almost completed. And the reasons of course were simple because just in the preceeding few years, quantum theory of atomic physics made a tremendous jump, beginning with Heisenberg's invention of matrix mechanics.
This was the same time when you were beginning to publish heavily.
Yes. Well, you see, I started with theoretical physics, and had written my doctor's thesis or rather received my doctor's degree in February, 1927. This was probably — as far as I know at least — the first doctor's thesis based on the concepts of the Schrödinger mechanics.
And you did this in Munich working with Sommerfeld.
Yes. But also Schödinger Was rather frequently there, and I became very early in these times acquainted with Heisenberg, so we talked about these things. Then I continued first for some time working on problems of theoretical chemistry. You will laugh, perhaps —
No. It ties in remarkably well. We have a list of citations to your papers beginning with 1961, and one of the most heavily cited papers is the one in the ZEITSCHRIFT FUR PHYSIK in 1927, which is not astrophysics, it's chemical physics.
The main problem was the quantum mechanics of the heteropolar chemical bindings, of substances like NaC1, sodium fluoride and so on. Well, some of the quantum mechanical methods which I used are perhaps still in use in chemistry.
Apparently, because it's more heavily cited in the seventies than in the sixties.
Well, a very funny thing happened to me many years later. When I visited McCrea in the University of Sussex, we went to lunch in one of the college cafeterias, and just by chance, McCrea met one of his chemical colleagues. We sat together for lunch. Well, we talked first about various matters. Then he asked, "Well, may I just ask, was that work done by your father or by you?" I explained to him that I had just "changed the subject a little bit." My father had been a theologian, a minister. Which is somewhat different from science.
Was that a Lutheran?
Yes. And a similar thing happened at one of the IAU meetings. There was an excursion, and I went along with my wife. And Dr. Vinti — you know him probably?
...joined us, and when we had become acquainted a little bit, he said, "Well, may I just ask: Are you the father, or the son Unsöld?" I looked a little bit at my wife and said, "I am the father of several Unsölds." He of course didn't quite understand. We laughed. Then of course I explained to him how things were really!
May I ask about the paper in '27? That was essentially your doctoral thesis?
In "Annalen der Physik"?
Well, I have published some more papers in '27. In a paper in ZEITSCRIFT FUR PHYSIK I attempted to develop the quantum theory of the hydrogen molecular ion H2+. Then there came in NATURWISSENSCHAFTEN a paper on,the Born-Lande repulsive forces in heteropolar molecules and crystals. You see, I had studied with Lande earlier in Tübingen before I came to Munich. (That's the personal connection with Lande.)
I see. So this is related to but not actually your doctoral work.
It's just a continuation, you might say. The general idea was to calculate the forces which come out if the smaller ion penetrates into the function cloud, of the other one. That gives a very steep repulsive potential gradient. I have not taken into account exchange forces, so my theory was of course not complete.
All right. I'd like to back up somewhat. Since you've told me that your father was a preacher, how was it that you became interested in science and went in this direction?
Well, that is also somewhat of a funny tradition. My father was very much interested in natural science also, and especially my grandfather Unsöld. He was a school teacher in Stuttgart, south Germany. He was greatly interested in botany and such things, but also generally in science, and then, I had also an uncle, a brother of my grandfather's wife who, if family tradition is right, had invented the self-exciting dynamo machine, only a few weeks after Siemens independently. But he came too late with his patent. I still knew him, as a schoolboy, and he was quite a clever man but a little bit queer also. He had quite an interesting physical cabinet of his own, with an electrostatic machine, Leyden jars, Geissler Tubes and such things. He was an electrical engineer and he just experimented as a hobby and published a little bit strange papers, well, among other things, about the connection between wireless telegraphy which was quite new at that time, and the Kingdom of Heaven. He was a very religious man. For instance, he speculated also about the physical theory of the rainbow in connection with its meaning, in Genesis, as the sign of peace between God and man. You can probably understand this milieu only if you know South Germany pretty well. Perhaps Robert Mayer was also somewhat a man of that sort, also Kepler.
Yes. Yes. Very much so.
Such ideas were a little bit in the air — rather different from North Germany.
But in a certain sense, this rather recent paper you've written is in that tradition, too. The one that you sent to me.
Yes. That is perhaps not so surprising. We don't like narrow-minded specialists. For instance, in geology, which is of course very interesting and important in the mountains of the Schw übische Alb, the largest and most important collection of ammonites has been made by a local person, just in a small village, as my father was. Besides being a very good and recognized preacher to his people, he made, in the course of a very long life, probably the largest collection of ammonites in the world. Just as a quite private hobby!
There's a very long tradition, even into the 16th century, of the educated preacher in this part of the world.
Who can be quite a specialist in something scientific.
Yes. Yes. For instance — you know certainly — in the Black Forest, there is a large industry of watches and clocks. This was established literally by a preacher, Hahn. There's a monument to him in Stuttgart on one of the streets. At first he used to build clocks, globes and all sorts of instruments as a hobby, besides having his quite regular office as a preacher. And then, he thought, what he could do for these, at that time, extremely poor folks in the Black Forest. This is a country which is not very fertile, there grows very little. And its people were extremely poor. And so he got the bright idea, couldn't they just learn to make clocks and sell them? He taught these country folks just how to make these — you know — cuckoo clocks?
Yes, I've seen them.
Unsöld: They built them in their homes, and some went away to sell them. That way, many of these extremely poor people were quite a bit better off. Later, this art developed into a real industry, as it is nowadays.
Now, you were growing up some place in this southern region of Germany.
Well, I went to school — that's perhaps the easiest way to tell — in Heidenheim.
Where is it? Near —?
This is near the Bavarian border. And by the way, the school director was the father of the famous General Rommel.
I see. Then you went from there to Tüllbingen? Unsöld Then, having passed the "Abitus" examination ... — as we say— I went first to Tübingen.
And by then you were already specializing in science?
Yes, in science. Well, in physics, there were Paschen (spectroscopy), Lande and Back (you know the Paschen-Back effect.) But later, I think it was after one and a half years, I decided to go to Munich to join Sommerfeld.
Why did you make that decision? Advice from someone?
Well, no, I had seen Sommerfeld's book and became interested in it. Also, I had some courses, of Lande's, on the very beginning of quantum physics.
Were you going into it more from spectroscopy, or —?
Not from practical spectroscopy, but from the theory of spectra, the theory of atoms. That was one aspect. But moreover Paschen left Tübingen, in order to accept the directorship of the Physicalisch-Technische Reichanstalt in Berlin. And it was not clear who would succeed him, and so I felt it might be better to move. So I went to see Sommerfeld in Munich. He had a system of education, which is very different from what you are accustomed to in America, and also in many other universities in Germany, and which (I think) is still the most effective one, at least for training young people who want to become scientists. It would be not good certainly for training the large numbers of school teachers, engineers and so on. I went to ask Sommerfeld what I should do and so on. He just said, "Come to our seminar." I had not really had any of the ordinary courses in theoretical physics, as you are accustomed to nowadays. He said, "There is an interesting paper by Born on quantum mechanical perturbation theory, (of course on the basis of the Bohr Theory still) and you might report it in the seminar. Just take Poincare's METHODES NOUVELLES DE LA MECANIQUE CELESTE for the general background of it." I never had a course in mechanics or anything like that. Well, I went to work, and I talked with Wentzel, who was Sommerfeld's assistant at that time, and things went on. Sommerfeld was just using the well-known method to learn swimming: just throw the fellow into the water; either he will swim or he will get drowned.
So you swam.
That was the beginning. Later on, I asked Sommerfeld, "Could you give me a subject for a doctor's thesis?" And he said that was when the Schrödinger theory was already published — "You might treat the hydrogen molecular ion, by the Schrödinger theory." That is a most ugly differential equation, which due to the two nuclei, has two essential singularities, as one says in theory of functions. There is nothing like a general theory of such differential equations known in the literature. Well, I looked into the matter, and after having worked on that problem for about a month, I saw that it was not the proper thing for me. During the vacation, I simply began to calculate all sorts of things which just interested me, some particular problems in spectroscopy, the spectrum of helium, i.e., exchange terms in the two electron problem, the polarization of the atomic core by an electron in highly excited quantum states. Then I dealt with the quantum theory of quadratic stark effect. I just wrote these things together into a paper, and went back to Sommerfeld. "Well, I have not calculated the hydrogen molecular ion, but I have done some other things." First he was not very enthusiastic about my work, but after having looked into the matter, he became more friendly, and finally quite enthusiastic. I got "summa cum laude" for my thesis.
Who were the other students working with Sommerfeld at that time?
Well, there were quite a lot of them. Hörrl was still there. He was a little bit older than I. You know him?
I know the name.
He did the well-known work on multiplet intensities. Then there was Heitler, who is still living in Zürich. There were Bethe, Peierls, Frühlich. Then there were several Americans. Pauling was there for a long time. Then Houston, Margenau, and Eckart, working on the electron theory of metals.
He had discovered the Schrodinger equation independently only about four weeks after Schrodinger! ... Going "the other way," he had started with the Heisenberg matrix theory, and then used this general concepts of Hilbert's theory of eigenvalue problems to derive from the matrix presentation a differential equation.
He didn't get much credit for it, being four weeks late.
Yes. Indeed, Eckart's paper is almost unknown nowadays. You see also, this is just only one case. How frequently a discovery is made, almost at the same time, by several people!
It happens very frequently. For instance, if you take the conservation of energy, Robert Mayer, Joule, Helmholtz and perhaps others.
But each approaching it in a somewhat different way.
Yes. Well, and also, in quantum mechanics, Heisenberg and Dirac; in relativity, Einstein and Poincare.
That happens I think quite often. Many people are nowadays inclined to over-estimate the importance of the individual, while what is important is actually what C.G. Jung, famous in tiefenpsychologie, would call the "collective unconscious."
I have mixed feelings about this, of course. There are many problems that are ripe at a particular time, that are sort of in the air. And it isn't necessarily a matter of asking a new question, but just to understand the context of the work that's going on. On the other hand, there are some more major things that happen, by people who, because of the sort of totality of what they do, that you feel really have changed the course of things, or that the science wouldn't be quite that way if they hadn't done that.
Yes, but even that is to a certain extent, I would say, a kind of collective process. For instance, I think it is a little bit unjust how, in most historical books, Hooke is treated" in comparison with Newton. Truly Hooke must have been personally an extremely ugly person. No one liked him very much. But that is no reason, I would say, not to acknowledge his scientific work, which was to a considerable degree ahead of Newton.
Well, this is true. And of course Newton, as a man who later came into the ascendancy, did everything he could to lower Hooke's reputation, so that there is today no known portrait of Hooke.
Can you tell me how you got interested in astrophysics? Unsöld Yes, I had been interested in astronomy and astrophysics already as a schoolboy, to some extent. As a student I had attended some lectures, just as a sideline — you might say, as a hobby — in Tübingen, where Rosenberg had a small private observatory at that time. Then in Munich, I had also considered the matter from a practical side: one had to have one additional sideline for the doctor's examination, and I decided to take astronomy. That was a rather funny sort of astronomy: It consisted only and quite exclusively of the theory of minor planets and special perturbations!
Have you ever done anything further with this?
Did that study in any unexpected way, provide insights later on?
No, not at all. It was extremely dull.
Who was the professor for that?
A. Wilkens. interesting thing: He was a rather strange type of man. But an one of my fellow students was Bröck. He had also studied physics to a certain extent, but more astronomy, and so one day we talked, "What shall we do now?" He was not quite sure what he should do. And I told him, I was an assistant in Sommerfeld's institute "well, just come to Sommerfeld's institute and work also on chemical bindings and crystal structures and so on, from the quantum mechanical viewpoint." Bröck actually did that, and began also to work on the quantum theory of the forces in heteropolar bindings. Later he also changed with me from physics into astronomy. You know that he later became Astronomer Royal for Scotland. He's now retired. Well, the astronomy courses were extremely dull, and the only thing that came out was that I had fulfilled a certain requirement for my doctor's degree. But then, suddenly I became interested in the solar spectrum, in connection — I think it was — with the lecture of a colleague on the relativistic red shift, which at that time was a very obscure matter. I had that impression too. And I thought: would it not be worthwhile just to try to understand how the Fraunhofer lines are produced? Well, I looked into books. The most modern book was Russell, Dugan and Stewart, of course. We had it in the physics library, and I found that even Russell had no clear idea how the lines originated. So, I began to think about that problem. I had seen that Minkowski, who was an experimental physicist in Hamburg at that time, had investigated (I think it was at Pauli's suggestion) the radiative damping in the sodium D lines. And in that paper, one could see also these broad wings, — as one used to say — of the lines, which Russell had emphasized so much, but not quite understood. So I began to think about the solar Frauhofer lines. Now at that time, one thought that the pressure in the solar atmosphere was much lower than one thinks nowadays, about 10-4 atmospheres only. (Well, now it is almost 1/10 atmosphere.) And in these circumstances, radiative damping might be a good mechanism. Well, then I looked further into the matter and soon became acquainted with Emden, who was professor at the technical high school in Munich. Emden was a good friend of Sommerfeld's, and when I talked with Sommerfeld about it, he said, "Well, I'll make you acquainted with Emden, he knows about it and he also knows about Karl Schwarzschild's work, because he was a cousin-in-law of Schwarzschild's," and so he had a complete collection of Schwarzschild's papers', which at the time no one knew. In all my astronomical studies, I had never heard a word about the work of Karl Schwarzschild.
But the papers had been published?
They had been published, but in very awkward periodicals. The early ones were in the Gr0ckttingen Academy, and later ones in the Prussian Academy in Berlin and these publications were mostly not available in the Institute libraries. So a student couldn't see them. They were accessible officially, but only in the big libraries.
This was before the days of xerox machines.
Yes, of course. But you see, even in Potsdam, where Schwarzschild had lived in his last years, no one of the astronomers, except Freundlich, had the slightest idea about his work. I'm fully convinced there were at that time only two people who had read and understood his papers. One was Eddington, and the other one was Emden — that was really, in the strict sense of the word, all. You may believe it or not.
I'm very much surprised, but it's interesting, that you say that.
Well, I came later to Potsdam. I tried to talk with one or the other about this. The older astronomers, who had lived with Schwarzschild still there, said, "Oh well, he has done various strange things, but that has all been very inaccurate." That was the only comment. Emden was very nice indeed; he made it easy to talk with him. And he made no difference between a professor and a student. He borrowed me for reprints of Schwarzschild's papers. The other access of course was through Eddington's book, which had appeared just before; it was also accessible in the physics library. There also the important papers of Schwarzschild's were quoted. Evidently Eddington was the man who really has understood Schwarzschild. So I saw immediately (you would say, that essential idea), that in order to understand the Fraunhofer lines, one had first to deal with the exchange of radiation, the transfer" of radiation as we say now. At that time, one used for everything the term Strahlungs_gleich_gewicht in a somewhat sloppy way. The other problem was the broadening of spectral lines. That problem had remained rather obscure, of course, in the time of the Bohr theory. But now, when quantum mechanics came along, one could get a clearer judgment which of the semi-classical theories made sense and which not. And one could begin at least to deal with these matters. Only later on, one began to think about the importance of "models." What it meant, what was the importance of the pressure… temperature stratification and of the kind of exchange of radiation. You see, Schwarzschild in his really very careful paper of 1914, in the Berlin Academy, had already put forward the concept which he called "scattering and absorption." "Scattering" was later called by Milne monochromatic radiative transfer, and what Schwarzschild called "absorption" was termed local thermodynamic equilibrium. And Schwarzschild also had used two different models for the solar atmosphere, one that followed the classic Kirchloff scheme, a low photosphere producing only continuous spectrum, and above it a thin layer producing only lines, and another model where both had the same dependence of depths. But he used the layer model only with scattering and the other model only with absorption. And so he got somewhat mixed up, and he made some wrong generalizations, because he had not exchanged both, the structure and the mechanisms.
This is something you noticed rather soon when you started looking at this?
That was not the first thing. At first, I just followed Schwarzschild's scheme on scattering because one knew that the absorption and reemission of sodium vapor with radiative damping had the character of scattering. That was clear from Wood's experiments and also from theory, the electron jumping just up and down. So everything seemed to be quite all right. But I saw at the same time that it would be necessary to get more and well planned observations. The only thing that was known was the photometry of the Hand K lines in the solar spectrum by Schwarzschild. And by the way, he had already asked his student Karl Böttlinger, with whom I became acquainted later, to measure line profiles in stellar spectra, already in 1914 or '16, but no one read that paper or did anything with it, because simply no one understood what it might possibly mean. And so I decided, that I should make measurements. I had heard that von Klüber has begun at the Einstein Tower at Potsdam to measure solar line profiles, but without any theoretical background. So I thought, I should first measure the profiles of resonance lines, because they could be explained theoretically. And so, I talked with Sommerfeld about my project. He favored it and I got a fellowship of the Deutsche Forshungsgemeinschaft. Sommerfeld arranged with Freundlich that I could work at the Einstein Tower, and so, I started experimenting! I had no training in experimental physics at all and some colleagues felt a bit skeptical what I might do with the telescope. But I was not very bashful!
You were working basically with the strong lines?
Yes, and with resonance point was reached where we could to be made still — and theory. Schwarzschild's papers with the was backed up experimentally by already. lines, because I thought that a connect observations — which have That might be achieved by combining theory of resonance scattering, which Minkowski's work, which I mentioned That was a start at least. Well, then things went on. In Munich, I could not get a position for a longer time, and Sommerfeld thought it would be good if I would learn really astronomy, and so he arranged that I should go for a year to Mt. Wilson Observatory. He took up contact, as he had done for many other young physicists before, with the Rockefeller Foundation, and I got a very well paid fellowship for one year, for working chiefly at Mt. Wilson Observatory, but also for going, as it might turn out, to other American observatories later on.
What year was this? Was that around '29?
It was from the fall of '28 to the fall of '29. At Mt. Wilson then I took up contact and was coordinated, with St. John. He was the main man in solar physics and through him I became soon a friend of Theodore Dunham's.
Oh yes, I still see him very often. He has an office at Smithsonian Observatory.
Oh, if you see him please give him my regards. He was and he is an extremely nice man, and also Mrs. Miriam Dunham.
I don't know her.
When at Mt. Wilson,I first took up work on the solar chromosphere, in connection with this strange theory of Milne's. Perhaps you have read about it in the older literature? I felt that this work of Milne's was purely theoretical or rather pure speculation. Why not begin experimenting? And I took spectra of the H and K lines with the l50-foot tower, as well as one could do, in different heights above the solar limb (of course with quite poor resolution). And of course also the Hand K lines on the solar disk. So I had spectra of the same object, seen from two virtually perpendicular directions. From these spectra I tried to find out the stratification of the ionized calcium vapor and to look into the line-broadening mechanisms. I found that the line profiles could not be explained purely by radiative damping and thermal Doppler effect, but one had to assume in addition broadening by random motions in the chromosphere. That was the origin of the concept of "turbulence"; in this particular case only, of course.
Let's see. There were two interesting kinds of things that you were getting onto at about exactly this same time. One was the great abundance of hydrogen in the universe.
Oh, that was much later.
That was quite a bit later.
Your earliest papers on this must be '28 or '29. Well, maybe I shouldn't have said in the universe, I should have just said, in the sun.
No, that was really later. I can't tell you at the moment, how things at Mt. Wilson developed… See, this the work on the chromosphere, was one object. It soon became clear, what I liked most was this business with the turbulence. That was quite new.
— well, another thing you were closely working with was what might be called the curve of growth.
— did that relate to the turbulence, or not?
That was — well, you see, it was not called curve of growth officially. But of course, I looked at the calcium in the optically thin layer, up and down, and in the thick layer potentially, to the surface, so the idea was actually, that of using different optical depths. But it was not yet — I did not yet have the general theory of the curve of growth. That came later.
And when you were talking about the concept of turbulence, at this point it had no relationship to the curve of growth interpretation?
Well, only for this very particular case. You see, one didn't yet see the general importance of it. That came later. It was just this, rather particular problem of the chromosphere. And well, the generalization came later, for very different and somewhat complicated reasons. Then to investigate at Mt. Wilson the question of LTE (Local Thermodynamic Equilibrium) by comparing the H and K lines and the infrared calcium lines. For this program Harold Babcock hyper sensitized for me some of the infra-red plates which had become available at that time. And do you know, how long the exposures were with the solar tower for getting the infra-red calcium lines?
Is it measured in hours?
Remarkable. One would have supposed the plates would have lost their special sensitization by the end of five hours.
When that investigation was completed, I became interested in the hydrogen lines. At that time one could read in the (old) Rowland tables, that H was the last "clear-cut" hydrogen line, while at the wave lengths of the higher members of the Balmer series there could be seen just some faint haze. No one knew exactly what that meant. So I decided to make photometric measurements. On the microphotometer bracings one could see in fact the profiles of the higher series members very well. But one could not see these lines with the eye on the plates well because they were subdivided by so many deep and sharp metallic lines. But interpolating on the microphotometer tracings I could measure the profiles of the Balmer lines up to the main quantum number n=16 And the impressive thing was that all the lines from H a(n=3) to about H 11 (upper quantum number n=ll) had almost the same profiles, although their f values were so very different. These f values had been calculated by Schrödinger just a few years earlier (about 1926). And so there arose now the quite definite quantitative question: How could the lines be so similar, when their f values are so different? And I thought this could be, because of the widths of the Stark effect patterns increased so tremendously with the upper quantum numbers. I calculated that these two effects, decrease of the f values and increase in broadening, just about compensated each other as long as the lines were absorbed in large optical depths. Th,en, for n 11, the lines became weaker and flat, up to n 16. For layer n they practically disappeared. Then I could also evaluate the number of hydrogen atoms in the second quantum state, using the lines which were absorbed in optically thin layer. But, as to the abundance of hydrogen, things were extremely unclear still, because earlier work by Russell, Adams and Moore seemed to indicate that there was something very peculiar about the excitation of the hydrogen atoms. All the astrophysicists were quite confused; was it a matter of abundance, a matter of line broadening, or strange excitation conditions? So, the problem of the hydrogen abundance remained quite unclear. Well, and it was only then considerably later that I resumed the problem, in connection also with the abundance of helium. But in between there came the problem of the curve of growth. When I worked at Mt. Wilson Observatory, I soon became acquainted with Russell. He, together with Admzas and Moore, had just developed his calibration of the Rowland intensities "as a method for the quantitative" evaluation of the Fraunhofer lines. Russell at once became very interested in the work I had done in Potsdam, attempting to begin with a theory of the Fraunhofer lines. We soon realized that we had tried to get to the mountain from two different sides. But at times we had been not quite sure to be on the same mountain, without having seen it! Minnaert and his group, Mulders, Slob and others — taking Russell's earlier "multiplet calibration" as a starting point — began by establishing empirically for many multiplets the relation between the equivalent widths and the f values of the lines. These curves were then shifted together to form the "curve of growth."
I gather basically that the discovery of the curve of growth was very much an empirical observationally based thing, on Minnaert and Mulders part, and that you and perhaps to some extent simultaneously Menzel were working out the theoretical side of it.
Well, indeed Minnaert and Mulders certainly came from the empirical side, and then, learned, in connection with my work at the Einstein Tower, began to develop also the theory of the curve of growth. At exactly the same time, I had developed the curve of growth for interstellar lines! When I returned from Mt. Wilson, I visited various American observatories, also Yerkes Observatory, where I became acquainted with Struve and Elvey. They had just done a lot of work on interstellar lines, and found ratios of K:H 1 to 1.2. I saw immediately this was a question of optical thickness, i.e. of the curve of growth and developed the theory for that case. By the way the theory of the curve of growth was also developed at about the same time by W. Schütz in Munich and by von der-Held in Ornstein's laboratory in Utrecht for laboratory absorption lines. And the superposition of Doppler effect and damping had been dealt with by W. Voigt in Gürckttingen, already in 1912, however, in a paper which was published in the Bavarian Academy, and which no one ever has read. Moreover it was rather bad to read! So, the concept of curve of growth came from many different sides almost at the same time.
My preference would be now, to stop and take a brief recess, because we have gone for one hour.
Oh yes. [Recess]
Let me put some identification on the tape. This is an interview with Professor Albrecht Unsöld, with Owen Gingerich as the interviewer. We're in Bad Gastein Austria, and it is June 6, 1978. Now, we had been talking about your work, first of all in Munich, then in Potsdam, then at Mt. Wils0n Observatory, and some of the foundation things in physics being applied to the interpretation of spectra, stellar spectra and solar spectra. How did your colleagues in physics feel about your — moving more and more into spectroscopy and astrophysics?
Well, officially I have always remained a theoretical physicist. When I returned to Munich in the fall of 1929, Sommerfeld asked me to become a privatdocent, to make my "habitation." He said, "Just present any paper you have on hand. That will be all right.” Then, I think it was about four weeks before the beginning of the winter term, "you might give a course in special and general relativity." I had no idea about it. I had never heard a lecture on relativity. But that was the way one used to do things in Sommerfeld's Institute. And I had quite a famous audience sometimes: Pauling, Margenau, who else? And besides I continued my astrophysical work. But already in the spring, 1930, I was asked to take a similar position in Hamburg. In Munich it was simply not possible to get a reasonably paid position. These were extremely poor times in Germany. The Depression had just been in America, so its shadow was quite dark also in Germany. So, I went to Hamburg. I had to lecture chiefly on theoretical physics. I could lecture on astrophysics also, or rather, I was permitted to do that. But, for instance, in Munich I had been privatdocent for theoretical physics and astrophysics. When I wanted to have the same position in Hamburg, the director of the observatory, Professor Schorr, said in the faculty, (as was reported to my by friends), "Well, that is very nice, and he also can do what he wants at the observatory, but he cannot become privatdocent for astronomy at the same time. That might be a damage to the observatory." And so I was privatdocent for theoretical physics only. But of course, sometimes I also lectured on astrophysics. What I began to investigate was a problem which had been mixed up since the time of Schwarzschild, namely: What is the difference, really, between lines produced by scattering or absorption? In modern terminology the first would be called an extreme NLTE model, the other an extreme LTE model. What difference does it make how the "stratification" is, the dependence of the ratio of line absorption to continuous absorption upon depth? What difference does it make how the newly introduced weight function varies with depth? And so on. These were rather complicated mathematical problems. And of course we didn't yet have any computers. One had to invent analytical methods which could be handled numerically just with ordinary desk computers. I think that it became fairly clear in course of time — and it was interesting to see — what was important and what was not important. And it turned out that applying the new method of weight functions, which I had established, one could hope to analyze the composition of the sun or stars, within error factors of about 2 to 3. That was at that time also roughly accuracy of the observations anyhow, so that was not so bad. You see; to calculate models as one does nowadays was completely impossible, because the computing would have taken a lifetime.
When you moved up to Hamburg you were, for example, much closer to Copenhagen. Did you have contacts with the Copenhagen people at all?
Because some of the weighting function things had also been developed in Copenhagen.
No, that was not so. And I had no personal connections. But I had very close and interesting connections with Walter Baade, of course. He was still at the Hamburg-Bergedorf Observatory at that time. We met quite frequently and that was always extremely interesting, because he had a very broad outlook and was an extremely brilliant and clever man, no doubt. We regretted of course very much when he went to America (while I was still in Hamburg).
Yes. How long did you stay in Hamburg?
Not very long. Only until spring, 1932. Two years only.
And then you went to Kiel?
Then I got a professorship for theoretical physics in Kiel, which of course was much better, a permanent position, and it was quite clear that I had to take it. In Kiel, you see, there was at that time still an observatory of the rather old fashioned type. And. I had to do — single handed — all the lecturing in theoretical physics. I had to give the complete course in theoretical physics for all the physics and mathematics students, which was quite a job. Not much time was left for astronomy.
Did you have any students during that time who have become known as astrophysicists?
No. I had mostly students who worked on physics. Well, a few also in astrophysics, but no graduate students.
So the Kiel astrophysics school didn't yet begin really to develop —
That began only after the war. But I ought to mention one man who became known in a different way in astronomy, namely, Dr. Hans Wolter. He was my first student working for a doctor's degree and, as you know, he was the one who first constructed an optical system which could be used for an X-ray telescope. Hans Wolter wrote his doctor's thesis on optics of thin films of metals, evaporated on quartz. Then be became a school teacher, because there was no position available. During the war, he worked on radar and such things in the Navy. After the war, he returned to Kiel, and since he had lost his position in the school, I said to him, "Well, stay here at the university," and we got a position for him in the experimental institute. There he became interested in the problem of a X-ray microscope for biological purposes, and the principle of this optical system was later used without any change, only in larger scale, by Giaconni for solar X-ray photographs.
Now, just before the war, you went to McDonald Observatory and to Yerkes.
Yes. You see, then in Kiel, I began soon to write the first edition of the STELLAR ATMOSPHERES, which of course was quite some job. Then I felt, it was in time really to try all these things which I had figured out and considered theoretically. I took up contact with Otto Struve and he made it possible that I, which was very difficult at that time, due to the political situation with the Nazis, got a guest professorship at the Yerkes Observatory, and could take part at the dedication of the McDonald Observatory, and then together with him, begin to observe there with the brand new 82-inch telescope. That worked perfectly well. The first plates which were taken at all with the new Coude spectrograph (at that time of course a prism spectrograph) were those of Tau Scorpii.
This was then really the first serious application of the line profile information, beyond the sun. Unsöld Yes. We could use the Coude, and moreover Struve had built an excellent ultraviolet Cassegrain spectrograph, with two big quartz prisms, which was far above everything that was available elsewhere at that time. These spectrograms had very high resolving power, because we could use the new "'Eastman process" plates, similar to what one calls lantern slide plates. And we took mainly quite a bunch of plates for Tau Scorpii, and also other B and A type stars. Now, why Tau Scorpii, you will ask. Well, I had considered at that point quite carefully, having seen a paper by Adams and Dunham on Tau Scorpii, showing reproductions of Mt. Wilson Coude plates, with identification for many lines at least. And, in that paper, one could see moreover, that Tau Scorpii was a B star with quite sharp lines. That is important, because most B stars, as you know, have very wide, rotationally broadened lines, so that faint lines cannot be measured at all. Then I saw from Adams and Dunham's paper that many elements were represented in several stages of ionization, and with hydrogen-like quantum states. So one could, on the one hand, calculate from quantum theory many f values, and one could determine the temperature and the pressure by using quite a number of different ionization equilibria. This star is in this way singular — of course it had also to be bright enough to be taken with the Coude spectrograph. Later, for instance, Pannekok told me, he had also had the idea of analyzing B stars, and had taken many plates at Victoria Observatory, which also had good spectrographs. But he couldn't achieve anything because all his stars had such broad lines that the faint lines couldn't be seen and the program was a complete failure.
Do you think Tau Scorpio is pole on, or?
I don't know. One cannot find out. I would not know how.
I was interested. Of course, as you know, I've looked at the Tau Scorpii paper particularly carefully, since I translated part of it. I noticed there you right away pick up on some of this work of von Weizsäker on the nucleogenesis, the nuclear reactions. I would be interested in your general remarks about how that kind of material impinged on your consciousness.
Well, that was certainly not at the beginning. At the beginning, the essential point was to have a star which one could hope to analyze, using the means which we had available at that time. And only during the working out of the plates and films. I should say (with Weizsäcker's ideas) I saw the connection.
Von Weizsäcker has specified or mentioned in his interview that people thought it was an extremely speculative paper and were not necessarily willing to take it very seriously. But obviously you took it quite seriously, in order to make the observational test.
Well, at least I saw the connection that in knowing the composition of a star, one could perhaps think with good reason about the origin of the elements. The ideas were of course rather general at that time. I, in that way a completely empirically minded person, thought, well, first let's see how things are, and then what we can speculate about them.
So the Tau Scorpii work was one of your most empirically related things, in the sense that you went out and got lots of observations there.
Yes, but the observation was, perhaps more than you might think of, a blend with the idea of being evaluated by means of the theory. At that time, it would have been quite senseless just to take some spectra and try to analyze them.
But I think because of your book on the physics of the stellar atmospheres, most people think of you basically as a theoretician. And it's interesting to me to see you going and getting the spectra.
I looked very closely into these matters, and...
But apart from Tau Scorpii, I suppose that what followed was partly a matter of the political situation that then developed. You didn't have much opportunity to do that kind of thing again.
Well, when I left Yerkes Observatory; Struve and I decided that he would keep most of the ultraviolet plates, taken with the quartz spectrograph, of many A and B stars, and work on the wavelengths and identifications of the lines for P Cyani and what else. I made the microphotometer records of all the Tau Scorpii plates and films at Yerkes Observatory, because they had a very good microphotometer, which, I think, Morgan had built. I had worked with it day and night during the short time that I had left. And I took all the Tau Scorpii materials with me to Germany. Well, that seemed to be all right, but when I was in Germany for two weeks, I was in the Air Force, without being asked much about anything else!
I see. So were you actually away from Kiel or were you assigned back to Kiel?
Well, I had managed, that I could remain in Kiel as a meteorologist in the Air Force, and so, I had enough time left to do a lot of astrophysics and other things. I always left the military people in the dark, what I did and how it was important and so on, in military matters a secret is always good. So I had still quite some time left for my work on Tau Scorpii. The students were largely away anyhow, so the university work was much reduced. So in Kiel actually I first measured all these microphotometer tracings, rolls and rolls, which of course was quite a job. Then I carried on identifications of lines in the ultraviolet. For instance, the great intensities of the neon II spectrum was still quite unexpected at that time. And…well, then I began to work on the whole pr0blem, and first determined the composition of the star, together with the temperature and pressure in its atmosphere. Well, it was possible to publish that in the ZEITSCHRIFT FUR ASTROPHYSIK and a few copies or reprints still came even to America, and at least an abstract was published by Kuiper in the ASTROPHYSICAL JOURNAL. The next question was, what is the constitution of the atmosphere of Tau Scorpii? I looked into the matter first from the viewpoint which was the only one available, using a grey atmosphere, of course, in hydrostat equilibrium. It turned out that such a model gave a completely wrong thickness of the whole atmosphere. So I thought for many weeks that there might be some additional mechanism of continuous absorption, besides the well known hydrogen and helium absorption, which I had calculated, and which was well-suited as a quantum mechanical problem at that time. That didn't work. Finally, I got the idea, in connection with previous solar work, that the non-grayness of the atmosphere might be the important point. And so I calculated what I think was really the first non-grey model atmosphere, by using in a suitably modified way, Chandrasekhar's previous calculations on the picket-fence model, applied it to Tau Scorpii and it turned out, that was quite good. I could calculate then from the average temperature of the atmosphere its effective temperature, which of course was very interesting, also in connection with the absolute magnitude and such things.
And that no doubt worked out very well. I've since seen that much more elaborate model calculations for the determination of the effective temperature of Tau Scorpii, by many astrophysicists, have led to almost the same value which I had found with this rather primitive attempt. Now, since I didn't have the observational material on the other stars, the plates of course were in America, I began to think about the helium lines. I had previously (1931) published a paper on the abundances of hydrogen and helium, treating the helium lines similar to the hydrogen lines, and found a ratio of about helium to hydrogen 1:100. Because I thought that these are the two most important elements, in various ways, in astrophysics, I decided to determine their abundances more accurately. Moreover it had not yet been possible, in my first paper, to determine the ratio of hydrogen and helium to the metals with any reasonable accuracy. In the Tau Scorpii at least that was possible; I got for the first time with some accuracy the ratio of hydrogen to helium and I found that neon, which is a "rare" gas on the earth, is quite an abundant element cosmically. Its abundance came out a little bit too high by modern standards, but anyhow…then I could determine for the first time reliably the abundances of nitrogen, carbon, oxygen. These light elements were interesting in connection with the energy production in the hotter stars and related things. And I developed, which· was unknown so far, the theory for the broadening of the helium lines which has static broadening in the wings and collision broadening close in the core. The helium lines present a very complicated transition case between linear and quadratic stark effect. A big paper on that subject had been published in the PROCEEDINGS of the Royal Society in London, by F. S. Foster from the Copenhagen Institute. And so one could now determine the helium abundances from different lines and also compare to a certain extent the profiles of the helium lines, with observation, which seemed to me to be fairly satisfactory for that time. Then, I did some more work on helium and hydrogen abundances in other stars, using observations from various sources. But that may be perhaps not so important.
I'd like to inquire about how it came about that the Kiel astrophysics group really got started. These were students that just happened to come because they were interested in such things and knew of your book, or ?
Well, you see, during the war there were no students, for obvious reasons. And I continued my work during the war years quite alone, along with a lot of other duties.
I gather that when you first went to Kiel you were in comparative isolation, compared to the time you had spent in Munich or in Hamburg.
Yes. In Kiel, when the analysis of Tau Scorpii was reasonably complete, I began a complete analysis of the solar spectrum. I had got during the first time of the war still, a copy of the Utrecht Atlas of the Solar Spectrum, and so, began to measure solar lines. I also used Allen's previous tables of the equivalent widths of solar lines. In the work on Tau Scorpii, I had learned how to manage the calculation of values of many, many lines. That was not so obvious as you might think nowadays. I collected all what one knew about the quantum mechanics of the problem. The chief point was to find out, for which lines one could at all hope to calculate f values, and which method would be most advantageous. So, I made this rather extensive analysis of the sun, and it was very nice to see that the compositions of the sun and of Tau Scorpii agreed with each other, as far as one could at all compare them. That was still more interesting because in the meantime one had noticed that Tau Scorpii was a young star and the sun a much older one. Somewhere, in between I wrote. a paper on cosmogony and related problems, the age of the world, the age of the sun and the abundances of the elements, using a kind of thermal, equilibrium theory of nuclear transformations. Such ideas had been started by Farkas and Harteck and later on by Chandrasekhar.
By then some students must have re-arrived. This is at what time?
No, work with students could begin only after the war. My official position during most of the war-time was that of a meteorologist in the Air Force mostly in the Kiel airport, for some-time also in the neighboring airport of Travemünde. And then, before the war was to end but at a time when it was clear, at least to intelligent people, what the end might be, I had managed to join a group which was headed by Kiepenheuer, and which should have made important discoveries in high frequency techniques and such things. Following very nebulous ideas of some military people such work should enable Hitler to win the war, when in reality it was completely lost! You see, Hitler, when the French campaign was at ·an end, completely forbade to do any research work on high frequency, on centimeter waves, because — he said now the war was practically over and we needed no new discoveries. Soon the submarine war was practically lost due to the British radar detection.
Then, Hitler ordered furiously that all available people should work on high frequency techniques, and overtake the British. It was perfectly clear, that Hitler understood about physics and electronics as much as a fly.
But I used this in a somewhat better way, to get out of the air force, and into a position where I could work for myself, and I did "very secret things," "very important things for winning the war," but in reality I worked on the solar spectrum and Tau Scorpii and such things. Anyhow no one would have understood what that was!
Now, towards the end of the war Kiel, where I had lived so far, was destroyed to about 85 percent. My house had burned down. The Institute had burned down. The observatory was evacuated. It was a very complicated kind of life.
And I went to a friend, Professor Kroebel, now, who had an institute working for the air force in a nearby castle, a little schloss, in the country. The place was fairly safe from air raids and there was a neighboring farm yard where we could get also, making friends with these very nice people, something to eat. That was also very useful. And I had one not too big room. And that was my living room, my kitchen, my laboratory if you may say so, and when the war was over and I became dean of the faculty, that was also the office of the dean and everything else. Very neat and near together! Then, when the war was over, I was in this castle, a very funny type of existence, and there carne the first students. There was no university yet, but in the course of some weeks, I became — as I said already — dean of the faculty (which did not exist for the most part) because I was one of the very few people who had not been members of the Nazi party. I had always managed somehow to get along without that. Well, one of the first students· who arrived, was Miss Vitense, now Mrs. BöhmVitense. And I had an assistant who had belonged originally to the experimental institute, but that didn't exist anymore, and so I simply took her over. That was Miss Rosa, a young lady who had got her doctor's degree in experimental physics, but who had no idea about astrophysics. So I showed her simply how to do stellar-atmosphere-calculations, and she did very useful work as a computer and assistant. Later, she went into electron microscopy and worked for the anatomic institute. Well, so things went on, and some more young people joined the institute. I had taken up work in connection with the convection layer in the sun and the thermodynamical properties of ionized gas in general. You know, in my Hamburg times I had first established the theory of the hydrogen convection zone from the thermodynamical side and now, I thought we should continue that work and also take into account the hydro dynamical aspects more closely, where Biermann and Siedentopf had done some very useful work. I got several of my students to help with these fairly long numerical calculations. In course of time, we went back to Kiel, and got some kind of institute. This was a part of a former factory for radars, which had survived in reasonable shape. But the first thing, when I moved into the new institute, was to get an institute for insect combating, because the bedbugs walked along in long black hordes. And mice came down through the cable tubes, and so on. You can't quite imagine, what kind of institute life that was. It was in any case amusing!
May I back up and ask another question concerning the wartime situation. To what extent did you get things like the ASTROPHYSICAL JOURNAL or publications from abroad?
During the war, not at all. After the war, a few copies were in Germany and one could get photostat copies of individual papers.
So during the war you must have been really in very intense isolation, as far as scientific work was concerned?
Well, I could just continue my work.
Did you have occasions to see other astronomers or other physicists? Unsöld: There were very few meetings. There was almost no one working on these lines, so it wouldn't have been· of that much value anyhow. But after the war, Otto Struve especially, wrote as soon as it was possible and sent reprints and papers, and food in between which was also quite important in that situation. Well, we came in touch with other people and little by little general connections were re-established rather soon, compared with what had happened after the First World War, e.g., when I came to Kiel, Rosenberg was still there in the observatory. And he told me when he attended the IAU meeting in Paris, I think it was 1931 or so.
Must have been '34.
No, not so late.
Let's see, '32 was in Cambridge, Massachusetts, and so '35 I think was in Paris, as far as the official meeting was concerned.
Tell, I don't know the year exactly, but anyhow, when he was in Paris, he tried to introduce himself to a French colleague. He turned around and ignored him.
I know that Germany was not a member of the Union.
Yes, but, you see, Rosenberg was certainly not a nationalist...
— no, no, I understand. I know that in the First World War, Hale and I think Adams and a number of others were very anti-German.
And were thus very influential in keeping Germany out of the Union.
I don't think there was really such a residue of this kind of thing after the Second World War.
Yes, well, I have visited once in 1928 or 1929, Hale, when he was already in very poor physical condition, and it was not possible to talk long with him, because he had a very strong nervous trouble, but he was extremely nice with me.
Probably by that time he was —
Yes. With Adams, there was nothing at all.
You mentioned earlier that you had become acquainted with Heisenberg. That was in Munich?
That was in 1926. No, he was at home in Munich. His father was a professor in Munich University. Modern Greek and such languages. A very nice old gentleman, whom I knew also.
I see. But Heisenberg's own interests and work tended to be rather different from yours.
At that time, you see, we worked on the same subject actually. On the helium spectrum.
But I mean, after that time, did you keep in touch particularly?
Not very particularly. We always had very nice personal relations, and Heisenberg was quite interested in astrophysics, but not in an active way. But, for instance, in between, it was after the war — I was quite interested in the problem of the origin of cosmic rays, and also radio astronomy and their possible mutual relations. Heisenberg of course was greatly interested in these matters. So at that point we came again in closer contact.
This is now a related kind of question, to the interviewing: have you preserved your correspondence and your working notes at all, from these times?
This was something you just didn’t save, or was it destroyed, or —?
First it was of course a question of the space and how to keep it in order and so. You see, as to the institute: first I was at the old Physics Institute and that was bombed out twice. Then, I worked for some time at the old observatory. Then I was in Schloss-Brendencek. After the war we moved into this institute, a former factory in Kiel. Then last year we have moved again, to a new and better institute. I could not carryon all that old stuff, car-loads of paper.
You saved some of it surely?
Almost nothing. What would be the use of it? After all, you see, someone who even would take the time to read it, would mostly probably not have the connection, what it meant really. And I think the really important points are published anyhow.
I know many people feel that way. But particularly, as you've mentioned here, there have been cases of almost simultaneous discovery.
Where people, because they were second, haven't published it. And there are also interesting things one does which turn out to be wrong, which you find out are wrong before you publish them, but which are nevertheless very interesting to someone who is working on the history of science, because in reconstructing a given period, it's necessary to know all the kinds of things people were working on, not only those that happened to be right.
Yes. But I would say, in most cases, these points also lose interest. There is so much published nowadays, that probably gives already a fairly good record. What's the use of later saying perhaps, "Oh well, I had that idea in mind two weeks earlier than this so and so —"
No, people are not necessarily interested in priority, but they're interested in reconstructing a general milieu out of which various problems emerged as being important. And it's interesting to find out who talks to whom, and what kind of correspondence was going on. If you had letters from 1926-30, it would be very interesting to see what kind of technical points may have come up in the correspondence.
Well, I think that would bring not very much. I have kept the post-war-correspondence with Struve, and some correspondence with Minnaert, which is very nice. He wrote very nice letters, also personal ones. And I kept some correspondence with Bohm's, since they were in America.
I hope that such letters are not destroyed but that you have an archive at the university or someplace where this kind of scientific correspondence can be preserved.
I would say, many interesting parts in history of science anyhow are located in the more or less general unconscious, in the sense of Jung.
Yes, but Jung and Pauli particularly have found a great number of interesting things in examining Kepler, which he can try to penetrate into his pattern of thinking, and in that, it turns out to be extremely useful to have Kepler's correspondence, as well as what .he published in books. This is why Kepler is so fascinating compared to Copernicus. We have no letters at all from Copernicus.
Yes, but I am not Kepler. (laughter)
I don't think that's sufficient excuse. I really hope you'll make some effort to have this part of the correspondence, which you still have and value, saved somewhere. I'm wondering if you're interested in making some further general observations about where the action was in astrophysics, in this period, apart from the things that you yourself were doing?
Well, of course, in solar and stellar spectroscopy and stellar atmospheres, one very important "turn" was the introduction of the big computers, so that one could carry through very elaborate numerical problems in reasonable time. I have myself never worked with them. My students have done that much better. That's a matter of the younger generation.
I was thinking particularly of events, let us say, between 1930 and 1950, before the era of the computers. For instance, here in continental Europe, what were the main kinds of astrophysical things?
I would say that in this epoch — of course quite generally the main weight of astrophysical research in America, because astronomy in Europe had decayed, one by one, since the First World War. You see, in Germany, astronomy at big institutes like Potsdam and so on has never recovered from the First World War, actually, and had gone to sleep, one might say. The important points in the postwar epoch, I would say, of course were, on the one hand, the beginning of nuclear astronomy, that's one aspect. Then of course radio astronomy, which began very vigorously in England and Australia, but could not even get started early in Germany because research on short waves was forbidden by the Control Commission altogether. Somewhat later I did myself some work in radio astronomy and had the idea of having in Kiel an Institute which at least could prepare suitable young men for later work with bigger instruments. Together with Professor Kroebel we established a very modest radio observatory. Soon, it became possible also to form a group working on theoretical plasma physics, headed by Eggen Richter, who is now professor at the Technical University in Braunschweig. But just at the point when we had begun some observational work, and when this theoretical group had done some work, Richter obtained the professorship in Braunschweig, and I had no possibility to offer him something equivalent in Kiel. And for this small radio observatory in Kiel, I could never get really first class collaborators, because the television institutes and industry, paid so much more, five times as high or something like it. So these enterprises never became a great success. Then, of course, the second very important point was the beginning of ideas on stellar evolution. One usually quotes chiefly the papers by Sandage, Arp and others, but the real motor behind the new aspects was Walter Baade, no doubt.
Yes, I'm sure, he set the stage for it, although he published so little — he was intensely interested.
Yes, and he did not like publishing and publicity at all! Also, the paper by Schwarzschild and Hoyle, of course, was largely stimulated by Baade. That and radio astronomy were the really important new points.