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Interview of Adriaan Wesselink by David DeVorkin on 1977 September 23, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/4954-1
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Early life and family interests in Holland; study at the Rijksuniversiteit te Utrecht; courses in mathematics, physics and astronomy; move to Rijksuniversiteit te Leiden in 1929 and contact with Ejnar Hertzsprung; work for Hertzsprung on variable stars; Hertzsprung's career; Jan Oort's lectures on galactic rotation; recollections of Willem de Sitter; Leiden in the 1930s; Paul Ehrenfest's colloquium series; continued research with Hertzsprung during the 1930s; contact with Gerard Kuiper; research on Delta Cephei, dynamical parallaxes, and energy distributions in stellar spectra; Leiden Ph.D. thesis, 1938; stellar pulsations; Lodewijk Woltjer; year at Yerkes Observatory, 1938-1939; relations with Kuiper; recollections of staff research at Yerkes; South Africa, 1939; recollections of Marcel Minnaert. Short discussion of Jacobus C. Kapteyn, including plans and execution of 1936 eclipse expedition to Russia; war years in Holland; Dutch astronomy in World War II; living conditions; postwar move to South Africa and various positions there; move to Yale University in 1964. Also prominently mentioned are: Edwin Eugene Aldrin, Nikolai Pavlovich Barabashov, Bart Jan Bok, Dirk Brouwer, Subrahmanyan Chandrasekhar, Christie, Pierre Demarque, Arthur Stanley Eddington, Albert Einstein, Louis Henyey, Edwin Powell Hubble, Ivan Robert King, Hendrik Anthony Kramers, Bertil Lindblad, Edward Arthur Milne, A. Nijland, L. S. Ornstein, Henry Norris Russell, Karl Schwarzschild, Martin Schwarzschild, Otto Struve, Thackeray, Hendrik Christoffell van de Hulst; Bethany Observatory, Finsen Radiation Institute, Leiden Southern Station, and Radcliffe Observatory.
To start out I'd like to know when you were born and something about your early home life, who your parents were, and just a general description of the early influences that led you into astronomy.
I see. Well, I will do my utmost to give a faithful record of all the early days in my life, as soon as I start remembering something. Well, I was born in Hellevoetsluis on April 7, 1909. That means the sluice or the bottom of Hell. The Hell was a little river on the island on which I was born, and that's the origin of the name of that place where I was born.
What island was that?
That's one of the Dutch isles south of Rotterdam. Voorne and Putten. I am the youngest one of a family of four children. My father was a physician. That was a quiet little place — Hellevoetsluis — I think of about 5000 inhabitants — and there I went to school. I have had one brother and two sisters. I have still one living sister. My parents are, of course, dead for a long time.
Well, the early home life that you had then: your father was a physician during the entire period during which you were growing up?
Yes. My father started medicine as a practicing physician in Holland but not Hellevoetsluis. The two elder children were born there — that is, in Ruinen — in the northern part of Holland, very close to that part where people have trouble now with the Mallucans. But then he changed, only once, his residence and moved to Hellevoetsluis where my other sister was born and I was born two years later.
So then your recollections of early home life were of this one town and of let's say growing up with three older sisters and brothers.
Did your father practice medicine at home or did he have an office?
No, that was the beauty of it. He practiced at home, and therefore he had his office at home, and the pharmacy was also at home — it was his own. In those days you did have that. A doctor had his own pharmacy. For me that was a source of pleasure. Later when I was somewhat older I used the chemicals for my fireworks. They worked beautifully.
Well, I didn't shoot them off, but I started them and made beautiful colors — red or green or yellow. I could make anything. By adding a little strontium. I got beautiful red colors.
Did your father help you learn various chemical combinations that would do this?
No, he just let me have it. There was one little place in his pharmacy which he had the key of, because it contained too much poison, and he didn't let anybody in there. But otherwise he didn't mind if I took KCLO3 — This provides the oxygen for the burning process for fireworks.
What was your father's name?
My father's name was Jan Hendrik Wesselink.
What was your mother's background? What was she like?
My mother was a nurse in a hospital in Utrecht at the operation table. She was assisting a professor in gynecology at Utrecht. And that's where she met my father.
So your father went to the University of Utrecht.
My father went to the University of Utrecht, and that was probably the main reason why I started studying astronomy also at Utrecht, because he told me a lot about Utrecht, and that made me go there. He didn't know any other places in Holland to study astronomy.
Well, let's just stay in your early home life: your mother's name and her personality.
My mother's name was Adriane Marina Nicolete, and her family name was Stok.
What was your home life like, some of your earliest impressions, the activity that you and your brothers and sisters would get yourselves into or how your mother and father ran the house.
Well, it was quite a nice family life. Of course, everybody was always very busy. I thought my parents were very busy. My father was busy in his practice, and my mother was also socially involved in town. She was always president of the boards to care for the poor.
Did she continue to be a nurse?
No, she didn't continue to be a nurse. Her home works took all her time, but sometimes she presided over the welfare board. And she was very much loved by people, and every year there was a re-election for a new president, and when she came home I always told her: "You don't have to tell me — you have been re-elected." (laughs) And she was, yes.
What about reading and interests? Did your parents stimulate any particular types of interests in reading?
Well, yes, I think so. This mostly came from my father. He had very wide interests. He also had some interest in astronomy. He had some books on astronomy which I still have. He had some interest in botany and we went out every Sunday to the sea, to the dunes, looking for interesting flowers and plants. Sometimes we took them home and put them in the garden. He was a great source of knowledge on everything of nature, let us say.
That's very interesting. And his interest in astronomy — do you remember what kind of an interest? Was he interested in the names of the stars, the constellations, or was it something more than that?
He was mostly interested in the names of the stars. But he had a very general scientific background I would say, a general interest in science, in the purposes of science. And I must have inherited quite a bit, I suppose if I have any, from him on that score.
Your brother and your sisters — did they inherit any of these similar interests?
I don't think so. My brother definitely didn't. He came through school all right, but he had not such a strong bent for science or for any learning.
What did he eventually do?
He eventually became an engineer and director of a gas works in France.
So he was still interested in certainly a technical profession.
Yes, he was more of a technical man.
You said your father had some astronomy books and possibly we could get their names a little later, but what types were they? General popular works? And were they all in Dutch?
No, as a matter of fact, it was a book by Camille Flammarion — Astronomie Populaire. That is the name. I can show it to you in a moment if you want to.
I know that book.
It is written in French, and I must say I found it most difficult in my younger years to read. But maybe that was intended for me, to learn more French, at one stage in my life.
Did he present this book to you to read?
Yes, he showed it to me. I was most interested in the drawings and in the illustrations in the book. They fascinated me from a very young age.
About how old were you at that time? Approximately?
Well, I must have started in my interest at about ten I think.
That would make that 1919 already, yes. Was your family affected by World War I? Do you have any recollections?
Oh, yes, I have many recollections of that.
What was your schooling like, your early schooling primarily?
My early schooling: I think most of the time of the games we played in the schoolyard. They fascinated me. Running about with the others, you see, in the schoolyard — we had wonderful games, and I still know them all. And that I enjoyed tremendously. School itself: I did well at school, but I cannot say that I liked it.
In what way didn't you like it?
I felt a little bit in prison always. It was a kind of punishment to sit for long hours. I loved to read later, but I never enjoyed classes.
Could it have been the teachers or just the way the school was run?
No, I don't think it was the teachers. I think it was just the confinement, to sit there all the time. I always remember being bored very much for long periods of time. That made me long to go outside and run again. I ran usually in the street — I never walked. I always ran to school and ran home.
Was this a school that was supported by the city or private sources?
It was not a private school. It was a municipal school.
I know in Holland there were several types of schools. There was a Class A school or a Schedule A school. Maybe that was something that didn't go on in your area.
I cannot remember.
Okay. So this was a school that all your friends went to.
Yes. There was no distinction. There were two schools, but I don't think there was any reason to favor the one above the other.
Did you have any teachers from this period that you recall now as being influential upon you?
I still recall the first lady teacher I had. I don't remember how she could have influenced my later life. But I have a good recollection of almost all the teachers I had in my younger school.
Well, what was your first contact with science and mathematics in school?
Well, I still remember being asked a question and I couldn't answer it. It was a question in physics. It was rather fun because I still remember the teacher had a thermometer — a rather big one, which I could very well see — and he put it in hot water and everybody could see the level of the meniscus of the alcohol. I could see it very well. And what happened was that before going up rapidly, the meniscus went down a little bit. He asked us, "Did you see that?" I had seen it. So I may have been a good observer. He further asked: "Does anybody know why?" nobody knew why. It irritated me that I did not know why this happened.
Well, how did you go about finding the answer?
He gave the answer. He said, "It's simple." He said, "The glass expanded first before the alcohol was warm. And that's why the meniscus went down a little bit. But soon after, of course, the alcohol heated up too and the meniscus went up.
Did he usually do this sort of a thing? Or did he sometimes give you a chance to figure things out for yourself? Did you have laboratory experiments that you did yourself?
No, not at the first school I had, because this was — what do you call the equivalent — Grammar school.
Well, when you got to high school, this was still in your home town, I take it.
Yes, I went to high school. First I went for two years to what they called a French school in Holland in my home town.
Why did they call it the French school?
Because they taught French in that school.
But it was still a Dutch school.
It was a Dutch school. But for two years I had French there. And after that I went to high school, but there was no high school in my home town. So I went to another island. I had to go by boat, traveling half an hour by boat on one of the rivers in the Netherlands. I enjoyed that tremendously. I had to get up very early. My high school was in Middelharnis on the island of Goeree and Overflakkee.
What was the economic base for your home town? What did most people do?
The economic base of my home town was rather exceptional, as it depended on the Dutch navy. There were only two places in Holland where they had a wharf for repairing navy vessels. One of them was in DenHelder, in the north of Holland, and the other one was in Hellevoetsluis where I lived. In my days that navy yard was still active. There was a dry dock and navy ships came there for repairs. I had a lot of connections with that wharf, because most of my friends at school had fathers who worked there, and they had not difficulty of going into the dry dock. I was fascinated by dry docks in those years.
In the work itself?
Yes, the war was very close. I even heard guns shooting in Belgium near Antwerp. I still remember. And, of course, the food shortage in Holland was very bad.
Well, through your high school years, this would have been after World War I by this time, in the early '20s, you began to have laboratory experiences?
I'm interested that during that time you've indicated an interest in the ship building and in many of the things — there were other influences on you, botany and that sort of thing — when did you start thinking about career choices, what you were going to be doing in your life?
I thought about it quite a long time. I think at high school I started thinking about it, and I thought for a long time that I wanted to be a physician like my father. On the other hand, I was also very much interested in the sciences, and by the time I was 17 and I went to college, my father said, "Have you made up your mind what you want to study?" I said, "I haven't, but I would be glad to study astronomy."
How do you think you came about that? And I'd like to know what his reaction was to that.
And his reaction was favorable. He said to me: "That's all right. We have too many physicians and ministers in my family already." He said, "It's time for a change. It will be interesting what you make of it, be an astronomer."
So he was not the first physician either.
No, he had brothers who were physicians, and his father was a minister in the Dutch Reformed Church.
How do you think you came upon that decision then, to go into astronomy? When do you think it happened?
It happened very shortly before I went to Utrecht to study astronomy. I felt I had to make a decision, and I said to my father, "I want to know more about it, where to start. I'm really interested," I said. And he said, "Go ahead. I want to see what you do with it."
Were there any teachers in high school or courses that put you in that direction?
I do not think so, no. I cannot remember.
Had you ever attended any talks by astronomers or gone to any meetings? Did you have friends who were interested in astronomy also?
No, none whatsoever.
So this was purely a feeling that came from your reading and from your internal interests?
Very interesting. Well, how did you decide to go to Utrecht?
I had a sister studying at Utrecht.
Did you remain dependent upon your parents for funds or did you seek other aid?
I remained supported by my parents for four years I think. Later on at Leiden I got an assistantship very soon, which made me independent.
Okay. Well, let me turn the tape over, and then we'll talk about your experiences at Utrecht. Okay. We're supposed to be moving on to your years at Utrecht, but I would like to ask one more question about your family life, and that would be the religious background of your family and how religion was treated in your family.
I think that that will be a very short story. As a matter of fact, religion was not tolerated in my family at all. My father was very much opposed to it. My father had very very strong feelings about truth and about lying. I mean very early in my life I felt that lying was the worst crime a person could ever commit, and I think I still have that. I mean my upbringing was very tight on that score, and my father considered religion out of the question. He didn't believe, despite the fact that his father had been a minister. I think he must have had some sort of a conversion in his student times. And as a consequence he completely moved away from religion, and he didn't want the children to have anything to do with it.
So that was it basically. Did your mother have any feelings about it?
I've always suspected that my mother had more feelings about it than he had, but I think they were pretty much suppressed because of the whole atmosphere against religion.
Was this a common thing amongst families in your town?
No, I don't think that was very common. I think most people were religious all right. But I mean I would like to extend some particular aspect of this feeling for truth in my father with regard to fairy tales. I mean we were never taught any fairy tales. We were never told any. And I deplored that very much later in my life, because I think children derive a lot of enjoyment from listening to fairy tales, but they were not heard in my parents' house just because they were imagined. My father said, "They are not possible, and I don't want you to hear them."
No "Hans Christian Andersen?"
Nothing of this sort, although my mother saw to it that I visited somebody who told fairy tales, and when I came home I didn't speak about it because my father would be furious. I think it was my mother's doing somehow that I got to hear some. And then suddenly, very much to my surprise — there must have been something between my parents I guess — this stopped, and I didn't go anymore to that lady. He probably found out, and he didn't want me to hear them. Very odd, very strange.
Well, that's very strong. Could this have driven you more into a scientific sort of frame of mind, the search for truth.
I think so, yes. I think so.
That's interesting. Well, unless there's anything that we could pursue on your early home life, let's move on to Utrecht then. How far was Utrecht from your home?
My parents moved to the Hague at the time when I moved to Utrecht, and there he became a physician with a large practice.
What was the reason for that move?
Well, the move came because my home town really went down, because the government had closed the wharf in Hellevoetsluis. There were no more patients for my father to cure, and he still needed an income to sustain the family. I'm sure that this is why he went away to the Hague and took up some other practice there. I was in Utrecht, and Utrecht is about 50 miles from the Hague.
So you lived at the University.
No, I took rooms so to speak. I rented rooms in Utrecht.
What were the courses of instruction like in your first year at Utrecht?
I thought they were very good, specifically in mathematics. I thought the physics course and astronomy course were not at the same level, although interesting. But my mathematics training was very very good I thought — the early mathematics and geometry and analysis. Analysis was already at a very accurate level, a very good level.
Was this differential equations by that time?
No, no differential equations yet, but the theory of numbers and the principles of calculus. But I still remember one course in functions and series, convergence, principle of Cauchy, all these things, the convergence principle. We all had to study that in detail.
What about physics and astronomy? You mentioned that astronomy wasn't as interesting. But did you have laboratory courses in physics and chemistry?
I had laboratory courses in physics which were good. The laboratory courses attracted me, maybe because they were practical. In those days I liked to use my hands on instruments. There was a course on instrument making, too, and I made a galvanometer myself.
You hadn't had the experience, like any kind of shop experience, in high school?
No, never. This was the first time I made something.
And it appealed to you.
Yes, it appealed to me.
Do you recall some of your teachers in physics and astronomy at that time?
Yes, my physics teacher was L.S. Ornstein.
And what kind of a lab did he run?
He ran a laboratory where they studied light and spectra. And he was primarily interested in the measurement of the intensities of spectra lines. The whole laboratory was devoted to that problem.
This was his own research?
His research and his collaborators of course. He was very good at that and known world-wide.
Did he ever discuss or did you see any astronomical applications of this kind of work at the time — calibrating the solar spectrum or calibrating stellar spectra or anything like that?
No, as a matter of fact, I was still an undergraduate.
Yes, this was just your first year.
Yes, and I didn't have much contact with him about his own work.
Yes. What astronomy did you take?
I took astronomy with Professor A. Nijland.
Was this a general introduction to astronomy?
A general introduction to astronomy, spherical astronomy, and there was another course which I also followed, with Nijland, on what he called popular astronomy. That was on comets and was less mathematical.
And you took both of these courses.
I took them both.
Okay. What did you think of astronomy as you were going through these courses? Did you have any personal contact with him?
Yes, I had quite a lot of personal contact with him. I had quite an appreciation for Nijland. He was an observer and when it was clear in Holland, which is not all that frequently, he was observing variable stars visually with a 10-inch refractor. I had not a very great liking for that type of work. I thought even then that this was an old-fashioned way of doing it. Of course, photoelectric photometry didn't exist yet. But even so I thought that photographic methods would be far more efficient and more accurate than visual observations of variable stars.
So there was no instrumentation really that you had seen at Utrecht.
I was not allowed to come near to the instruments at my undergraduate level. I saw the instruments, but there was no possibility to do any work with them. But, of course, I became involved more and more in astronomy and also more and more involved with what was going on in the world in astronomy and in the Netherlands.
Did you start reading journals at that time as an undergraduate?
I did read some journals but very barely.
Which ones were they? What was available to you really?
What was available to me was the B.A.N. and the Monthly Notices of the Royal Astronomical Society and the ASTRONOMISCHE NACHRICHTEN in German.
How were your languages at that time?
My languages were fair. I had no problem of reading English or French or German. I could read it all.
The B.A.N. was in English.
As you say the B.A.N. was published in English.
Did you ever wonder about that, why a Dutch publication would be in English?
No, I didn't wonder for very long in any case. I mean there are so few Dutch astronomers. They realized very soon that they had to publish in English to be understood by all the Americans.
Was it the Americans that they were pointing their interests at, or was it the Germans?
Well, I would say they were pointing their interests at the whole western world, so to speak. I mean Holland scientifically felt that it was part of the western world. I mean England and the United States, and of course South Africa, Australia, all the English-speaking people, and the French, of course — the French scientists, most of them can read English. And French is more difficult for the Dutch then is English, so I must also have contributed to the publishing of the magazine in English.
That's quite interesting. Did you have any contacts with astronomers other than Nijland?
I do not think so, not in those days. There was a colloquium once every three months in the Netherlands, and I think that is still so. There I met astronomers from all parts of Holland.
This is while you were still an undergraduate.
Where was this colloquium held?
This colloquium was held either at Utrecht, Groningen, Amsterdam or at Leiden. It traveled around, and I was a junior member.
Do you recall any particular instances of meeting people that impressed you at these colloquia or were they too few?
I did meet E. Hertzsprung and J. Oort both from Leiden. Oort was very young at that time. And both impressed me very much indeed, so much so that I began thinking of completing my studies at Leiden rather than at Utrecht.
Did you talk to anybody about this? Did you talk to Nijland about it?
I talked to Nijland about it. Nijland was human enough to be somewhat disappointed. He said because I had passed my third-year exam for the Candidats and I still remember visiting him and thanking him. I wanted to thank him for all he had done. Furthermore, I was going to tell him that I was going to leave. And I think he was a little bit shocked.
Did they have a graduate program at Utrecht?
Yes, but there were a few graduate students, maybe two or so, and I think they did some observing, visual observing.
And that was it.
That was it. That didn't appeal to me.
Were there any other students at Utrecht as you went through who later became astronomers?
You mean graduate students?
Well, graduate students, undergraduates, anyone you would have had contact with, who became, let's say, reasonably well known people that we would recognize.
Yes, V.D. Kamp, Houtgast, de Jager etc.
P. van de Kamp was at Utrecht.
He was at Utrecht.
And he went to Groningen.
He went to Groningen. And Klein Wassink, but he's not very well known. Van de Kamp is quite well known, of course. He became quite prominent in astrometry, later in the United States.
Well, then, how did you apply to Leiden? Who did you write to and how did the procedure develop for your moving to Leiden? Wesselink: Well, I think it happened on summer vacation of 1929. I went to Leiden from the Hague, from my parents' house, and talked things over with Hertzsprung. And he said, "Why don't you start now?" because he was very keen. He couldn't stop. And he wanted me. I said, "I will come at the beginning of September." He wanted me to start at once looking at photographic plates with an eyepiece, to study variable stars. And as soon as I came — I started doing that, examining a huge plate collection with an eyepiece.
Well, what kind of mechanical things did you have to go through as far as applying to Leiden, any administrative problems?
I could go there without any problems. I was taken on regardless.
Just if Hertzsprung wanted you, that was it. That would have been the same with any faculty member at Leiden?
Yes. I suppose so.
What were your first impressions of Hertzsprung? You've already given me some of them. You knew he was a very hard worker.
He was a tremendously hard worker, and I became more and more familiar with his work, too, reading his research work and research papers. And I got to like him for his scientific spirit because he was indeed very very careful. I was struck by the fact that everything he discovered was usually very well backed by evidence, more so than I had ever seen before. And I found it most attractive, because I developed a feeling that there was too much speculation in astronomy going on — beautiful ideas, many ideas people had, which they could not sustain later on in life and they were rejected very soon. And I never thought that was a good way of doing it. I thought if you had a good idea, get the evidence for it so firm that the likelihood that it will be ever shaken is small. And I think that was part of Hertzsprung's training, that he was a very skeptical man. Anything you said he was skeptical about, which was frightening to a lot of students. Most didn't like that. They didn't understand the spirit behind it. However the spirit was the real love for truth, I think. He really wanted things to be that certain. And that appealed to me. I thought this was the way I wanted to do astronomy.
This seems to be a similar kind of thing in some ways to your father's search for truth or your father's feelings for truth.
Yes, I think that feeling was inherited partly. It was a little bit embarrassing sometimes. I mean if you had done something for Hertzsprung — e.g. you had examined 200 plates; and you would think this ample he said, "There are 500 more. You still have to do them." You see? He was not soft. As a matter of fact, he could be quite hard with you and make you work very hard.
Did he train you in the process of determining magnitudes on plates, or did he just give you an eyepiece and say, "Here, go do it."
He first said, "Here's an eyepiece and go ahead." Arid you were embarrassed with it. You didn't know what to do. But after an hour or so he'd come over and look over your shoulder: "What did you do?" he would say. If you said, "I didn't do much because I didn't know what to do," then he would start explaining. He said, "I did it on purpose." He said, "I wanted you to think about it before I started explaining to you what to do. Now you learn much better from me what to do than if I had started out telling you right away."
Because you'd been staring at it for an hour.
Yes, because you had been staring at it for an hour. I liked that. I thought that was a good way of teaching. So he explained to me how to do it, and well, of course, then I got the experience later on.
So this was your primary work in the beginning — variable star work.
Did he have any formal lectures that he gave or did any of the others?
He gave one lecture in the week, which he didn't like giving. But it was excellent. It was very well prepared, and it contained a lot of valuable information. I still have these lectures.
You have the lecture notes?
I have the lecture notes. They're in Dutch.
That still is very very valuable — a complete lecture series, let's say, that you would have had is an extremely important document for the state of mind of the lecturer and the person taking the notes at the time, and this is something that you should preserve.
Yes, I agree.
I'm only saying that to remind you. Historians are very well aware of it, but astronomers aren't.
Did you start having contact with the other students?
Yes, very much so. At Leiden there were a good number of graduate students. I think there were about seven or eight, and I had good contact with them.
How many were working for Hertzsprung?
Most of them were working for Hertzsprung. There were other teachers than Hertzsprung. Maybe you want me to talk about them, too?
Certainly. First, though, I'd like to know what Hertzsprung lectured on. What were his primary interests?
His primary interests were Practical Astronomy. And that was photometry and astrometry also and the treatment of observations. I enjoyed that part of astronomy.
Okay. I do want to ask you a few other questions about Hertzsprung, but certainly we'll be asking them all the way through your period of time at Leiden.
Well, let me add something. I wanted to talk to you about what struck me most about Hertzsprung's teaching. I mean from him I learned I think that if you have a set of observations, you should always try your utmost to get all the information which is in there. I mean you are very careful to make sure that you had all the information, that there wasn't any more information available in those observations. I think that's very important in astronomy and something which is not always recognized, even today —
Did he ever talk about his own career with you? Did you establish a personal social relationship?
Yes, he did tell me about his own career. You want me to talk about that?
I would like that very much. Wesselink: His personal career: Hertzsprung's father was an astronomer. Being an astronomer, he felt, didn't give you enough to live on, and he therefore told his son: "You should never become an astronomer." In those days a son was told by his father what to do. In his case his father told him to become a chemical engineer. Hertzsprung studied chemistry and he became a chemical engineer who became interested in the process of making photographic plates. I have never met an astronomer who knew more about photographic plates — about the structure of it, about the chemistry and their possibilities — than Hertzsprung. And then he became an amateur astronomer, who started using photographic plates, and he did so with more success than most professionals knowing so much about plates. Most of his lectures were also concerned with that. I mean we learnt much about emulsions from him. Of course, astronomy is more than just knowing about photographic plates. It's the interaction between the telescope and the photographic plate — what light is doing, what kind of focus, what the focus is doing to photometry. All these things I learned from Hertzsprung in great detail.
Did he ever talk to you about how he decided to work on the strange spectral peculiarities that he found in the Harvard Classification System — his famous work on Miss Maury's system of 1897?
He never told me how he came to do that. But he still must have been an amateur astronomer when he did that, and he corresponded a lot with Karl Schwarzschild, who very fortunately for astronomy, saw how important that work was. So Schwarzschild asked him to go to Gottingen where he became a professional astronomer. And when Schwarzschild became the director at Potsdam, Hertzsprung moved with him to Potsdam as a professor.
During these years Hertzsprung actually did produce a diagram, a Hertzsprung-Russell diagram. I'm wondering: did he ever talk about what prompted him to compare the colors or the spectrum and the absolute brightness of the star?
No. I don't think he ever told me, what prompted him to do that.
The idea I have in mind is that it seems that it was a purely empirical type of search that he had made. He was looking for what correlations there would be. And I'm just trying to see if this is something that he would instill in his students, too. Did he ever talk about the process of investigation in the analysis of data?
Yes, I think you are right. I mean everything he did was empirical almost. I think that was partly because he never received a formal training in astronomy. That's also the source of his originality. I mean the combination of a very critical mind on his own work specifically and a lack of training made him original because he didn't get it from anyone. There was something else I wanted to say on that. Yes, I still remember that he wanted to talk to me I said, "Excuse me, Professor but I have a course with one of the other professors —” I said, "I would rather like to go now. Otherwise I miss that." Why," he would say, "do you want a course? I never had a course in astronomy." I said, "Some people need it — some don't."
Interesting. Did he talk about Schwarzschild and his association with Schwarzschild?
He talked quite a lot about Schwarzschild and his association with Schwarzschild. Schwarzschild was one of the few theoreticians he appreciated. Hertzsprung generally, in my feeling had too little appreciation for theoretical work and theoreticians. He considered them too speculative, and considered most theoretical work not worth knowing.
Yet in a paper that you showed me many years ago, the 1906 Hertzsprung paper on using black body radiation to predict the angular diameters of stars he certainly was using laboratory work and theoretical work — Planck curves — at that time. Did he ever talk about why he did that?
Yes, I think so. I think about the time, it was only a few years after Planck had discovered his curve and obviously Hertzsprung was interested in the amount of radiation coming from a hot body like a star. And the Planck curve definitely inspired him. He saw the possibility of using it for his work on the stars.
But he didn't seem to continue that after that paper.
I think he did. I mean he always returned to the color of the stars. Even in 1920, much later, he wrote a rather fat volume in which he collected all the available colors at that time on stars. You remember that?
I don't know it in particular, but the important thing is that he was definitely working from the observational side?
Very much from the observational point of view. All that work is now obsolete, but it was very much the best at that time. Of course, now we have photoelectric colors which are superior to anything published then. But even so, at that time it was the best.
He published that paper before he went to Potsdam with Schwarzschild — this was probably just at the time when he and Schwarzschild were getting together in 1906? Is that about right?
Yes, that's true.
And in Potsdam, of course, Wilsing and Scheiner and all the others who were doing this color work, it seemed.
Eberhard was also in Potsdam. And quite a photographic expert. Eberhard, of course, was the discoverer of the well-known Eberhard effect.
And I'm wondering if he ever talked about the feeling he had going to Potsdam, the first really big observational center and technical center.
No, he never told me much about it.
Did he ever speak more about his own personal career, about the many techniques that he derived, certainly with the HR diagram and then with the hypothetical parallaxes and his work on binaries?
Yes, he told me about that. I mean Hertzsprung did quite a bit of statistics, stellar statistics, and the work was generally ingenious. I think that statistics is one of the most interesting and difficult subjects in astronomy but very very important, much more important than most astronomers realize. Hertzsprung was very good at that. He was well aware of selection effects — I mean the problem of biasing your results by selecting the stars according in a particular way and its effect on your conclusions. I've never seen anybody taking more pains in making sure that no selection effects got in the results. I learned a lot from that. I've always tried to be aware of that problem in Hertzsprung's way. But apart from his stellar statistics, I have always appreciated his way of handling Practical Astronomy — I mean not so much handling the accidental errors, because accidental errors are not difficult to handle in principle. I mean in the first place, of course, you make them as small as possible. But if they are not that small, by making a lot of observations, and taking means, good results are obtained. But I think Hertzsprung made me more aware than anybody of the importance of systematic error. He was more ingenious than most astronomers to eliminate them. (Systematic errors). I have known in my life lots of astronomers who love to apply corrections to the results — corrections for this, corrections for that, corrections for temperature of the measuring machine — and Hertzsprung always disliked that. He arranged observations in such a way that he systematically and automatically eliminated systematic errors. For instance, he would measure a double star plate, photographic double star measures: he would measure a plate twice once measuring on the film, and then after reversal the plate measure through the glass. In that way the obliqueness of the measuring wire in the microscope is eliminated in the mean. Other astronomers would treat that differently I think. They would determine the tilt of the measuring wire and apply a correction to the result, accounting for it. His methods would be otherwise, as I say. He rather liked to measure the plate twice — once through the glass — and once on the film, and knowing the mean result of the two sides: the effect of the tilt of the wire would be automatically eliminated. I've always appreciated that. I asked, "Why don't you, apply a correction in a much shorter time?" "Oh," he said, "it's very simple. I might apply the correction with the wrong sign."
(laughs) I can see why he didn't want to apply it. Did he ever talk about the detection of possibly his most famous systematic error, which turned out to be the existence of giants in his samples of work, in his early work?
No, he never told me so.
Did anybody ever ask him about this in your company?
I can't remember that anybody ever asked him about it, but my own feeling is that it is his memory, and furthermore his almost personal interest in stars. I mean he knew data of such a great number of stars by heart. I mean Betelgeuse, for instance: he could recite all the quantities that were known of it. And I think that faculty of knowing so much about the individual stars contributed quite a lot to finding things. He would remember that that star was peculiar in some way, and then he would remember that that same star had the same color as some other star. Then he would still remember in what respect they were different, and then he would start wondering why. And that I think led him to discoveries like the difference in luminosity, you see between Betelgeuse and 61Cygni.
Well, he would remember in what respect it differed from a giant of the same color.
How about the old historical question as to who first named them giants and dwarfs? Did he ever talk about that? Whether he did or whether Russell did or somebody else?
He never gave the name giants and dwarfs. He called them "Whales among the fishes," implying at the same time their frequency. I mean whales are far less frequent than herrings, for instance. And that was implied in his remark "whales among the fishes." I mean the supergiants and the giants, of course, are very rare objects in the whole volume space. And by saying "whales among the fishes" he implied that also to be true among stars.
You know, it seems that it was right about at the time that you were working with him — this is 1929, 1930, when you went to Leiden — that Bok or someone else had decided that the Russell diagram should now be called the Hertzsprung-Russell diagram. Am I correct in that?
I don't remember that.
Was there any pressure that you remember?
It's possible. I don't remember that, because Bok wasn't in Holland. Bok had already left Holland, I think in 1930, and I never met him. Bok went to Harvard, and he came back later in 1932 for his PhD.
Did you meet him then?
No, because he got his PhD with van Rhijn in Groningen. Bok didn't work with Hertzsprung but had quite a good appreciation of him, though.
Well, did Hertzsprung ever include the use of the diagram in his lectures, in his weekly lectures?
What did he call it? What did he call the diagram? Wesselink: The color-magnitude diagram. Furthermore, he embarrassed all his listeners by having it 90 rotated. I mean he always had luminosity as abscissa and the color as ordinate. I asked why he did that, while being out of step with the rest of the world?" "Well, after all," he said, "I made the first one and it was this way." "It was only Russell who turned it around." (laughs)
Did he ever say anything else about Russell?
Yes, Russell and Hertzsprung were very good friends.
Did Hertzsprung ever say why he would not turn it around?
Yes, I know it. Because he felt that abscissa should be the more accurate coordinate. In most diagrams the abscissa is the independent variable which has a small error. In most observational diagrams it's customary that the accurate coordinate is the abscissa, and the luminosity was more accurately known in those days than the color.
The luminosity was more accurately known?
I think so, because, you see, in clusters he used the apparent magnitude rather than absolute magnitude. And It's definitely true that the apparent magnitudes were more accurate than the colors, you see?
Yes, I do see that.
It is true that absolute magnitude may not be. The apparent magnitude in the clusters were far more accurate than the colors. Of course one should consider the total range also.
That's the fascination, because Hertzsprung continued to use the apparent magnitude, which made sense. But Russell's first diagrams were absolute magnitude.
Maybe I am somewhat mistaken there. I don't think he ever published one with absolute magnitude.
That's very interesting.
And as soon as he started publishing them, he used apparent magnitude as an abscissa. That was Hertzsprung's philosophy.
That was Hertzsprung's philosophy and this is something that you had derived directly from talking to him.
Okay. But Russell did not have this philosophy.
No. Well, his spectral types were of course in his case the abscissa. They may have been better known than the absolute magnitudes. And Russell did use absolute magnitudes, but he may have had the same philosophy but with other data. And for those data that's true. But the world accepted his position on the Hertzsprung-Russell diagram — using colors: (b - v), as it is now, as abscissa.
Why do you think the world did accept his use of the diagram in that direction as opposed to Hertzsprung's direction? Russell's presentation?
I do not know. I do not know why the world did that. Maybe Russell's results were better publicized.
But Hertzsprung never seemed to be too concerned about it?
No. I still remember Oort lecturing (at Leiden) and using color magnitude arrays, and he said, "It's very difficult for me at Leiden to know how to put this diagram," he said. "I rather like to do it the same as the rest of the world does it, but Hertzsprung being at Leiden, I feel some hesitation of not doing it his way." (laughs)
How did Oort do it?
I think he did it like the world did it. I could tell you some more about that, because Hertzsprung used a different absolute magnitude than we are using now. Hertzsprung used m + 5 log. I mean he didn't have that extra 5: m + 5log of nowadays. But the modern definition is based on a different parallax — of course, a standard parallax of .1 second of arc. That was Kapteyn's idea. And I know that Hertzsprung didn't follow suit on that.
He used 1 second of arc.
1 second of arc. He said, "That's the unit — why not use it?" But Kapteyn would say too many stars will have negative absolute magnitudes on Hertzsprung's scale which they have anyway now on Kapteyn's definition also.
Right. But that's an interesting thing. He wanted one system where all the numbers would be positive, and it's justifiable from the standpoint of simple arithmetic. That's very interesting. Yes, so his absolute magnitudes were a little bit different. Did you attend Oort's lectures too?
What did he usually lecture on?
Well, I was fascinated with Oort's lectures because he lectured on galactic rotation. And I became acquainted with a lot of the astronomical literature through him. And, of course, Oort's lectures were more modern than Hertzsprung's. I mean he was really "in the stream" of contemporary astronomy on the galaxy. Oort himself was researching in the galactic system, — stellar motions, magnitudes, colors.
This was a very very important period of time. Of course, it was just after Hubble's work, and his own on rotation.
Yes, he spoke a lot about Hubble's work. I still remember when Hubble's counts — nebular counts — came out. It was only 1935 I think. And I already had lectures from Oort from 1930 onwards. Well, Oort talked quite a bit about the theoretical implications of galactic rotation as first proposed by Lindblad.
Was there any contact with Lindblad?
I didn't have any, not much later. I had contact with Lindblad in World War II.
Yes, certainly. But at this point in time did Oort ever talk about the legacy that he had had from Kapteyn interpreting the star streams in terms of rotation?
Yes, Oort talked quite a bit about Kapteyn, the great effect that Kapteyn has had on his own development and his work. But Oort also felt very much obliged to Hertzsprung for his statistical work. He found that most interesting and fascinating, and he told me he was very strongly inspired by Hertzsprung's work. But primarily he talked about Kapteyn's work. He once gave a lecture — a special seminar I think — about Shapley's book, Star Clusters. Oort found that book very important and lectured about it at great length.
This was in the '30s?
Well, the implications of Shapley's book, of course, dealt with the size of the galactic system.
And, of course, it had an immediate bearing on his own work, on galactic rotation.
Did he ever talk about the systematic errors that very possibly were in Shapley's work?
Yes, he went through Shapley's book rather critically I must say — appreciatively but also very critically.
And what was his conclusion?
Well, he said, "There's room for a tremendous amount of work and improvement, but even so it is a very interesting and beautiful blueprint, so to speak, to work from for other astronomers."
Did Hertzsprung have this opinion?
Yes. Also Hertzsprung had a high opinion of Shapley.
What about the possibility of absorption changing the scale of things? Did people consider that at Leiden at that time in your experience?
Yes, people were already considering that. I think Oort was very well aware of that. Already in 1932 I think he was. You can see that (in) Oort's papers on the motion of the stars in the direction perpendicular to the Milky Way. Do you know that paper? On the Z force, on the force of acceleration of gravity in the direction perpendicular to the plane of the Milky Way. (That) was studied by Oort in 1932, using stars at very high galactic latitude, and he did that purposely to avoid the absorption. And if you are avoiding absorption, that means that you are aware of it, see? And I think he was aware of absorption very much. But it was only six years later, Oort came out with a big paper on the density distribution in the Galactic System, in which he tackled the absorption, using all he knew about absorption in those days, from Hubble's counts, which were available then, and Stebbins and Whitford's colors of B stars were known. And they were all incorporated in Oort's 1938 paper.
Let's go back and talk about another aspect of all this kind of work and another person that you were in contact with there. I understand that at this time you had contact with de Sitter.
I'd be interested in the nature of that contact that you had.
Well, W. de Sitter, of course, was a very senior astronomer, and we were very much impressed with his personality I still remember. But, of course, he was the director of the observatory, and we knew of his important work on the Jupiter satellites. I must say almost none of us knew in detail what he was doing in that field. De Sitter lectured on that once in a while, — but I can't remember much of it. It didn't influence me much, but I don't want to imply that it wasn't important.
What about his cosmological work?
His cosmological work was, of course, also in those days, and he talked about the expanding universe and the recession of the nebulae. He didn't really lecture much about expanding universe from a theoretical point of view. Of course, he also contributed to that quite a bit. But he thought it was above the understanding of the students.
Did he ever have any reservations interpreting Hubble's observations in terms of an expanding universe?
No, I don't think he had. He was a great friend of Eddington, and was very much influenced by Eddington in his whole relativistic outlook. Of course, it was the other way around when relativity started. I think de Sitter's first papers on the implications of general relativity for astronomy aroused the interest of Eddington.
I think that's true. During all this time, especially after you went to Leiden, which was a major world center for astronomy with all these really active people, did you ever wonder or did you ever get into discussions, especially now 1929-1930 with the Depression striking the world, about the practical aspects of astronomy and why astronomy was still being supported? Did you ever wonder about it yourself?
I don't think I wondered much about it.
Now about the others — Hertzsprung or the others: Did it ever come out in lecture?
No, I don't think it came out in any lecture. The job market for astronomers was, of course, the only economical aspect I can remember. I mean it was of concern for the teachers how to get rid of all the students in astronomy later on in life. They all could become teachers in high schools in mathematics. I mean our training was such that we had to acquire enough credits in mathematics so that we could become teachers.
So as you were preparing for your Doctorandus, you were also preparing for a teaching credential?
Well, I have credentials for physics and astronomy.
I see. How did you go about satisfying the requirements for the credential? Did you get these at Leiden or at Utrecht?
I got them at Leiden.
What were the requirements?
Well, I mean you had to pass an oral examination with Oort in astronomy.
Is this for the Doctorandus again?
Yes. And I had it in physics with Ehrenfest and with Kramers. Kramers was present at my defense of my thesis.
How was the Doctorandus different from the Ph.D.?
Oh, for the Ph.D. you had to write a thesis.
And not for the Doctorandus?
Not for the Doctorandus.
I see. The Doctorandus; was the credential then to teach? And the necessary grade before the Ph.D.
I see. I wasn't sure about that. Okay. What kind of questions did Ehrenfest and Kramers ask you? Do you recall? Did you have contact with them other than at this exam?
Yes, I admired Ehrenfest tremendously. He allowed me to sit in at the physics colloquium and I enjoyed that. He was unrelenting with speakers. I don't know whether I express myself correctly. In saying unrelenting I mean that every speaker on Ehrenfest's colloquium had a difficult time. He would interrupt the speaker almost every minute, and remarkably enough he was able to do that without hurting the feelings of the speaker. "Most listeners, are more stupid than I am — I tell you," he said to the speakers, "and I don't understand it. I will say so from the beginning to end and you'll have to live with it." And all the speakers, de Sitter included, were harassed by Ehrenfest, and the Ehrenfest colloquium was the most famous physics colloquium in The Netherlands. And so all these great men — Heisenberg, Fermi, Einstein, Bohr — all came to the Leiden colloquium, and in his peculiar way, Ehrenfest was able to extract every drop out of what these people knew.
Are there any particular colloquia that you remember particularly — vividly — as being truly momentous?
Yes, I remember one colloquium with Pauli who was a good friend of Ehrenfest.
What about the times that Einstein or Fermi were there? Do you recall anything that happened then?
Yes. Einstein was there and he gave just a class, just a normal lecture in class. Ehrenfest introduced Einstein to us. "Children," he would say, "I have a surprise for you today. Einstein is here and he's going to lecture to you. I will step down," he said. "On such an occasion I don't have to say anything."
Did he treat Einstein the same way he treated others about asking questions?
No. Einstein was the only person he revered so much that he behaved somewhat differently; very interesting thing Einstein gave a very very good lecture. I still remember what he said.
What was it on?
It was on the Ergodic Theory in statistical mechanics.
So this was in a physics course.
We had a course from Ehrenfest on Kinetic Theory of Gases, you see, and about the statistical implications of that and Brownian Motion. Einstein had contributed much in that field. Einstein was touching on Brownian Motion also in this particular lecture. I still remember how clear it all was.
Did you take notes at these lectures?
Oh, yes, but I don't have them anymore. I don't know why.
You've only saved your astronomy notes then.
And some mathematics notes I have saved. I have some beautiful mathematical notes from Utrecht, on geometry.
Well, your contact with all of these physicists must have made you aware of the tremendous revolution going on in physics at that time.
Your own personal work — published work at least — was beginning by 1930, and you were working on binary stars, primarily binaries I imagine, wasn't it?
Variable stars. All of this was with Hertzsprung.
Yes, I didn't do much astronomical work with the other teachers.
Did you ever talk with Hertzsprung or talk with the other students who you were associated with about the relative importance of the work that astronomers were doing — Hertzsprung and all this tremendous revolution in physics? Did you ever have a feeling that maybe you would want to get in on that too? So many things were happening. Or did you see this as something else?
Well, my own feeling was that I was very very busy doing the observational work. As a matter of fact, I think that I have devoted too much to observing. I became a good observer under Hertzsprung, but I think I should have devoted more of my time to studying with Oort and with Woltjier. Hertzsprung's wished that I should devote all my time to his department, and it was very difficult not to. He was a very forceful person. But I deplore that somewhat for my own education. Later on in life I made up, what I lost then. But I think I still was too much tied to a telescope. I mean students in this country, for instance, were not nearly ever tied so firmly to a telescope as I was.
Not to jump forward, though, but how do you feel about that in terms of the balance? Do you think it's gone too far and students are not aware enough of —?
Yes, I think it's gone too far, and I think they have not enough training if they go to a major observatory. And therefore they perhaps do not value enough the importance of good, practical training.
Well, we can get to that and certainly we will, but let's stay in the '30s and start talking about your research. Was all of your research dictated pretty much by Hertzsprung or did you choose the particular topic which you would work on?
I would say at least half, 50%, was dictated by him, and the rest he would be very critical of. But he never held it against you if you did it. If you showed initiative, he would not stop you. But he could be extremely honest about these things. For example I had a plan — I can tell you what it was. I wanted to observe Delta Cephei using gratings. I wanted to use two gratings on top of one another.
Objecting gratings, crossing them, and use the faint companion of Delta Cephei as a comparison star. Now, you see, to do this photographically is difficult because the companion is several magnitudes fainter than the variable, and photographically it is a problem therefore to bridge that gap with your reduction curve. So that's where the grating came in. I used crossed gratings. One grating was to bring images into the field which was a good deal fainter than the variable but of comparable brightness of the central image of the companion star. The other grating was a 1 magnitude grating, with the purpose of providing the magnitude scale. And he said, "I'm not in favor of that, but if you want to, go ahead. I think you won't be successful," he said. "But go ahead," "You don't listen to me," "I know you want to find out for yourself, and that's good," "You should be a scientist. I want you to develop you, and therefore nature should be your best master, not me." See? And so I was successful — it worked all right; I got beautiful results; the measurements were accurate, more accurate than anything known before, Hertzsprung said, "You should go ahead with it. I was wrong," and, "You were right."
Did that result in a publication?
It was published much later on. It was published after World War II for some reason I don't remember. It was published just before I started on "Wesselink's Method." So it was much, much later.
Do you have any idea when it was published at this time?
I think it was published perhaps in 1943 or so if publication was at all possible in the war. It must have been published.
You were working on Cepheid type variables very early in the '30s — 1930 - 1932. This was all pretty close in association with Hertzsprung's interests.
But in 1932 you indicated you had contact with Kuiper, who had been in discussion with you on some of the Gamma Gem type stars and on others. How did that contact come about?
Well, Kuiper was there as a student. He was senior to me and he was one of the better students I knew him very very well indeed, and he was studying double stars for his thesis then, doing statistical work, studies of double stars; and he was measuring the difference of magnitude between two components. This was not well known in those days. I was always interested very much in techniques of calculating things from observations. I have always been.
Was Hertzsprung doing any work in dynamical parallaxes any more at that time?
Yes, I think he was. I think he wrote another paper in those days on dynamical parallaxes.
Did you become interested in that at all?
I haven't done anything on it, but Hertzsprung published a new formula giving the orbital motion in a binary when the apparent magnitude and the proper motion were both known.
One reason I wonder about that along with the HR Diagram is that this was another instance when Hertzsprung and Russell published that same method at the same time independently. Hertzsprung I know in correspondence with Russell thought this was most amusing, but I wonder if he ever talked to you about it also.
He talked to me about it, and said, "It seems to me that Russell and I always seem to be interested in the same problems."
Did he say it with a twinkle in his eye?
Yes. I mean he had no bad feelings whatsoever about that.
I know you acknowledged his help, but did he also direct your attention in 1935 to working on energy distributions in the spectra of stars?*
No, he didn't do that. As a matter of fact, it was the first time he didn't like my paper, because he had not so long before found another formula. He had found a formula for the comparative distribution of energy in stellar spectra. You know, in black bodies the difference in magnitude is a linear function of 1/T for a black body, to a great approximation, which is not perfect, but it's a good start. Hertzsprung found from practical observations 1/T4 was much better. He published that, and it was based on observation. It was just an empirical formula. He tried very hard. I didn't like that formula because I found it difficult to work with because the exponent 4 there was an empirical exponent, and it had to be 3.6 or something rather to give an even better representation. But then you would have to use logs all the way to work with that formula. We didn't have calculators like you have now. So I didn't like that. I thought of another formula which was far simpler and it did just about as well as Hertzsprung's and perhaps even better. That was 1/(T-Const.). The Constant was .275. That's what I used. And I showed to Hertzsprung that it could represent the same data as he had used at least as well. I mean there was a slight difference. But my formula was far easier to adapt to calculations than his. That was an empirical formula, a comparative distribution of energy in spectrum stars: The difference in magnitude between stars, to put that in a formula. And that had the difference in magnitude as a function of wavelength, and I made it proportional to 1/(T-.275).
Let me ask you about your thesis and then we will talk about Woltjier, unless Woltjier had something to do with your thesis. How did you decide on your thesis?
I don't remember. I decided on it when I had quite a good number of observations. I had several thousand observations.
Was it the matter of picking a particular star?
Yes. Well, you see, it was a particular star, it was an eclipsing binary but it had a component of approximately the same brightness at maximum. Then the companion was very close so that any defect in the photographic plates were eliminated again. And even the sky was no problem even in Holland, I could observe that star through clouds with perfect results, because the comparison star was almost next to it and still enough separated so that you could measure it independently.
What star was it? Was this published?
Yes, SZ Camelopardalis. This is a component of the visual double Struve 485. And I made I think what was called the best photographic light that was ever published on any eclipsing variable star. That sounds somewhat boastful, but I don't think it has been improved. Even modern photoelectric curves are that good. There's a recent photoelectric curve on that star, which is not as good as mine.
I don't have the particular reference to your thesis. When did that come out approximately?
It came out in English in 1941. I got my thesis in 1938. Because of the war, I didn't have it published or translated, until '41. But I added quite a bit. Since I made my thesis and before I translated it, I added quite a theoretical part to it. I'll talk to you about it, about the effect of gravity, the gravity effect, on the ellipticity. And I'm very happy I did that, because when I applied that on my star, SZ Camelopardalis, I got perfect agreement between observation and theory. This is impossible without the gravity effect. Therefore, there is reason to believe that the gravity effect is really there in stars and to the amount predictable by theory, which is very interesting. But I didn't have that yet when I wrote my thesis.
Yes. Would Hertzsprung have gone that far? — I mean to account for all of these possible variables?
I don't think it was in his nature to do that.
So you were definitely going farther than he was in the analysis of binaries and the variables. Well, since you were talking about eclipsing variables, so there seems to be a different sort of an animal from a straight variable star, the interpretation being simply different. You also did a lot of work and you became very interested in the '30s in the pulsation hypothesis of Cepheids.
Not so much in the early '30s. I became interested — that's true — in a theory of variable stars, pulsations, about 1934 I would say. But it's a long time ago, and I still haven't made much headway with my thoughts about that. I studied it in the perspective of relaxation oscillations. I hit upon the idea that relaxation oscillations were really the most important oscillations to consider for stellar pulsations from a theoretical point of view rather than the Eddington approach that the oscillations were very small and around an equilibrium position. Relaxation oscillations do not have a stable point of equilibrium. I mean any pendulum, a normal pendulum, which you have in a clock has a stable equilibrium position. It can hang still without moving, and relaxation oscillation cannot do that. These have no stable equilibrium position. Therefore, these two oscillations are completely distinct. And I thought that a relaxation oscillator is far more relevant for the explanation of Cepheids than the normal pendulum idea that Eddington brought in and Woltjier perpetrated at Leiden in those days. But they didn't want to hear of it.
So this does bring us into discussing your contact with Woltjier, and that's very interesting because at about that same time Martin Schwarzschild and of course Rosseland were all working on pulsation theory, and this was a very important period for that.
Well, I spoke with Martin Schwarzschild about it in 1938. Then he had a running wave theory.
Yes, where did you see him?
At Harvard. It was in 1939. He took me off the railway station in Boston for a very short visit at Harvard. And then we did some talking about pulsations. But he didn't like my relaxation oscillations and I didn't like his running waves.
Well, let's talk about Woltjier now and your experiences with him because this will take us back into the early '30s, and relaxation oscillations.
As a student you cannot have ideas of your own. I think that's what was behind it concerning my work on the Cepheid theory of variability and the theoretical interpretation of oscillations, which was also Woltjier's topic. Although later on, of course, Christie with the use of electronic computers made tremendous headway, and I think he is right in the explanation of Cepheid variation. I think Christie is the man.
This is much more recent.
Yes, much more recent, after World War II, in the '60s. And what he's doing there are just relaxation oscillations.
He's going back to what basically you had.
Although he doesn't use the word relaxation oscillation once, it's just what he's been doing. It is a self-sustaining oscillation in the atmosphere. Now, a lot of things I had already found out myself on this business.
Now, when did you first get interested in the problem? Was it talking with Woltjier?
No, I got interested in it by going to some congress in the Netherlands on something altogether different. It was a medical congress.
A medical congress?
Yes, I was there with my father, and I sat in on the explanation of the heartbeat. And the curves that were produced on the blackboard about heart beat just looked like Cepheids light curves. That interested me.
Did you reproduce them? You have a number of curves in this paper in the A.P.J. in 1939. Did you reproduce those curves in here? You had a number of different types of curves I recall. Here — these on page 663.
I mean there are some, the lowest one on the right hand page.
Yes, there's one marked "I."
The subject of relaxation oscillations is very dear to me because I still believe it's very important. And what I had there I'm still rather fond of. I will tell you more about that. (looking at the paper) I'm reading the last line of it, and that came very true, you know.
Okay, would you like to read it?
"An investigation into the details of the relaxation mechanism in the stellar envelop which is probably related to the ionization of hydrogen and the other abundant elements are beyond the scope of this paper." In fact, Christie's work is based on the ionization of hydrogen and helium. And that's because the hydrogen in fact, without the ionization, there won't be any Cepheid variation in his picture.
You've quoted from page 668. That's quite a prophetic statement. You were able to predict in almost direct Christie to the right direction. You understood where the limitation was.
Yes. I felt that the seat of the Cepheid phenomenon was there. And I do some explaining in that paper. It's a rather a negative explanation because it couldn't be anywhere else, because that's the only place in which we didn't know so much. I mean the interior of stars are much more known — everything is ionized and nothing can happen there anymore. You can't do much. But it is the instability caused by the ionization that starts off the pulsation. The pulsation is started in the atmosphere and the body of the star just is in resonance with the atmosphere. You see, that was my idea. But because I was so green in these matters and I had no mathematical background and was not well enough trained in theoretical astrophysics and I was too much of an observer for my money, for what I got for it, I couldn't pursue it.
I appreciate that situation at that time. But yet you were encouraged by people. Didn't Milne encourage you?
Hertzsprung encouraged me. Oort did not. He said, "That is no good." And I knew all the time I was right. But when I went to the States in 1938 for a visit to Yerkes and I was there eight months, when I was there I got a letter from Hertzsprung and he said, "Are you still interested in your idea about relaxation oscillations? If you are," he said, "by all means go ahead with that. That's important."
This is when you wrote this paper then.
Then I started writing the paper because I was already cornered by so many people about it from Leiden before. "Unless, the idea is not yours," he said, "but perhaps Professor Kramers." Because I did on one instance talk with Kramers about it, but it was not Kramers' idea. I got the idea first. But I asked him for some mathematical help, which never came forth.
He never did help you.
And I think that is not surprising in the light we are living now, because, as you know yourself from Christie's work, it involved electronic computers to get somewhere, and it needed heavy computers. I mean at that time I would never have been able to do what Christie did later, but I had the basic idea I think. I tell you because you can trust it and you do not think I'm a fool if I say that. It's true.
Well, this has certainly happened a number of times.
Of course these things happen all the time.
Yes. But you say that Oort didn't think too much of it. At one time or other in your career I know that Milne did encourage your work. Did Milne encourage this particular work?
What contact did you have with Milne? Did you stop in England on your way to the United States?
No, I had never met Milne, but I had corresponded in '35 on this business.
Did you write to him first?
I wrote him a letter about it because I wanted to have someone see it. Eddington wasn't interested in it, I surmised, because he proposed another model in his book, and Milne came out with an interesting paper in the Quarterly Journal of Mathematics in Oxford about 1935.
Yes, I think you do reference that paper.
And I went with that to Woltjier, that manuscript, and I said, "I think this is most important." Woltjier said, "I don't think so. I think that's one of the worst papers Milne ever wrote. I don't want to hear any more," and I went out.
But you continued working on it.
I continued working on it. I tried to boost my very meager ego that I had on the subject. I mean when these big shots tell you off and tell you that's no good, it's very difficult to stand up to them morally. I don't know if you know that feeling.
Maybe in your contact with me, I have cut you down sometimes. I don't know. It's a very difficult thing — if you have an idea, to work it out and to believe in it is very important, whatever anybody else says. It comes always back. I still feel that now in my retirement — one of the projects I want to do now in retirement is to re-study Christie and see if I could frame it my way with relaxation oscillations, because I think he has it. But I think it can be explained simpler that way.
Let's talk about the mechanics of how you went about working on it, because I know that you built an electronic device to reproduce different types of oscillations in a neon tube as part of that experiment.
Well, I didn't build it, but I think I just read about it from electronics magazines. I mean all kinds of oscillations in the world are relaxation oscillations — you know that.
Yes. Did you want to actually be able to see these kinds of oscillations in a neon tube?
I think what I could do one day is make an electrical model of a variable star, an analogue computer which has all the typical things of the stellar atmosphere which make it oscillate and can produce light curves very similar to those of Cepheids. And it's not because it's just a fluke but because there's really something similar in these two things.
Were you trying to do that here too to a certain extent?
I was to a certain extent trying to do this.
Yes, you were creating electronic relaxation of oscillation, with a capacitor?
My principle at that stage was to introduce the astronomical public to a type of oscillation that they did not know. I mean I talked to Chandrasekhar.
Was he at Yerkes by that time?
He was at Yerkes. He was of course, pretty prominent, a great astrophysicist. And I thought he had the ability, the mathematical ability. Once he had this idea, he could probably work it out, I hoped. And he said, "Adriaan, I am interested in this. I will come back to it, I promise." He never did. Maybe he was just being polite — I don't know — but I don't think so. Chandra was not that type of person. But I think of the few people I convinced, I convinced Kuiper. He put it in proper English. I wasn't so good in writing papers at Yerkes. He helped me somewhat with the manuscript.
Was he one of the ones who brought you to Yerkes for those eight months? How did you get there? What was the process?
The process was that he wrote me a letter and said, "Would you like to come over to Yerkes?" It was with the intention that if I had been very good, they would have kept me. But I wasn't very good, for the simple reason I got in trouble with Kuiper on the observing part. He took all my observing time away.
I don't know whether I want to put that down.
Oh, certainly. You have control, editorial control, over anything you say, but I would appreciate hearing these kinds of things.
Well, if I have editorial control, I can pick that up, because my impression of Yerkes was very poor because I was too unbending.
Yes. You see, Struve had promised me observing time and Kuiper said, "That's nonsense. I don't want you to observe double stars photographically."
Well, what did he say to you in the letter for you to come over? Did he say what the reason was?
Well, that was the reason. He asked me what I wanted to do and I wrote him I wanted to do that. He said, "That's okay." When I got there, he started undercutting and upsetting all my plans. He wanted to mold me to his wishes. And I didn't mind being his student for a while or learning from him, but there was an agreement that I was going to do that. I could have done other things, too, to his liking, but this was not my basis for coming. And then he took it all away. I was on the observing list on one day. The next day I was off. There was a new list: Kuiper, always in my place.
Well, what did he want you to do?
And so I went to Struve because Struve made the list. I said, "What happened?" I said, "I had six nights and now I haven't got any dates." I said, "What happened? I mean I haven't even been informed about it." I said, "I know I'm only a guest here, but it was given to me." He said, "Kuiper is so busy. He has to go to Europe this summer and wanted to finish his program (first)."
We were talking about your differences with Kuiper. What did Kuiper want you to do instead of observe double stars?
I don't know whether he had a very clear program of what he wanted me to do, but after this Struve wanted me to do more spectroscopy. He started me on Beta Lyrae.
Taking you back into double star work again.
Right. But then I brought up the matter of the gravity effect, and I didn't have the formula, see, which I produced later when I was back at Leiden, and I needed it for the work on Beta Lyrae. So that was not very successful.
Why couldn't you generate the gravity term there? Why did it have to wait for Leiden?
Well, that I do not know, but I thought in this atmosphere I'd rather do something on my own, and I started working on relaxation oscillations.
And you talked to Chandrasekhar about it at that time.
Yes. And then I got that letter from Hertzsprung and then I published this note. It took me a lot of trouble to word it just as I wanted. Kuiper offered to help putting it in proper form.
Was he more interested in this type of work that you were doing on relaxation oscillations rather than double star work?
I don't know, but I think he had everything he wanted. He had the observing time, and by the end of my stay he felt that we should get on I think, and we left more or less as friends.
During this time Kuiper and Strömgren were working in different lines but they were both working on the interpretation of the HR Diagram and primarily finding that there were these lines of constant hydrogen abundance. And this was about a year approximately before you were there, but still I guess was working very actively during the time that you were there. And you were there at Yerkes also during the time when Bethe and others came out with their work on the sources of nuclear energy in stars and also it was the time that Chandrasekhar was doing some very provocative work, so it must have been a very exciting time. Did you have contact with all these new directions?
Yes, very much so. I mean I was also very impressed with Struve's discovery of hydrogen in emission in the general field. I mean of course we knew emission nebulae already. They were already known. But it was general galactic hydrogen emission that you could observe. That was only possible with the nebular spectrograph at McDonald with Greenstein, Henyey and Struve. And that happened when I was there.
That must have been quite exciting.
That was very exciting.
That was that very big instrument that was on the side of a mountain?
Why did they do it down there rather than at Williams Bay?
Well, I think even Williams Bay had too much disturbance from Chicago lights.
Yes, well, that certainly is the answer, yes.
And I had contact with Chandra. He wrote a book on internal structure and constitution.
Right: The Introduction to Stellar Structure. You had something to do with that book?
I did some proofreading for it.
You had an interesting story about that?
You want to have it on tape.
Yes, certainly. These aren't libelous stories; they're just important.
Well, the point was that he asked me whether I liked the book, and I did say that I did like the book very much, and then he said to me: "You should buy one." And I thought that he should give me a copy.
And that's where it stayed. That didn't sour your relationship with him.
It's just a very small but interesting kind of an association. Was Chandrasekhar a very formal person?
Well, yes, at home. I liked Chandrasekhar very much, but at his invitation — you couldn't just go over to his house and knock and come in. He did invite me quite a bit and I sat at his table. In the Indian way they dressed up quite often in Indian costumes.
Lalitha did too?
Lalitha did. And I always liked them tremendously.
By that time, of course, he was already very well established. But did he have a tremendous poise? Was he very self-assured with other people?
Yes, I think he mellowed later. The students were quite afraid of him. His exams were hard. He was a difficult examiner.
But what about his relationship with Struve and with Kuiper?
I think with Kuiper it was good in those days, very good. And lots of Kuiper's work was done in collaboration. All the work that Kuiper did on mass/luminosity relation, for instance, was nothing else than updating the experimental knowledge on the mass/luminosity relation — so important for Chandra's book. And Kuiper's interest in white dwarfs directly interested Chandra.
Exactly. And here we have relativity coming in again very strongly.
Yes. Also in the bolometric correction papers by Kuiper in 1938 was of course important for the mass/luminosity relation. So I mean there was quite a lot of interaction going on between staff members. Then, of course, you had Henyey there. He was my age. He was a brilliant theoretician I thought, and Chandrasekhar thought quite highly of him. He was doing his work in the reflection nebulae together with Greenstein, and I think they did very well in explaining the phase function of the particles in the forward scattering ovals of small particles. That was done at that time by Henyey and Greenstein when I was there. And then you had Ebbinghausen, who was a student of Kuiper's. He was measuring proper motions on old and new Yerkes 40-inch plates for the color-magnitude arrays Kuiper was interested in for his evolution theory of the HR Diagram — his explanation of the HR Diagram.
How did he develop this theory? Was he still developing and talking about it when you were there? Did he talk to you about it directly?
He did talk to me about it.
Did he ever talk specifically about the place of the giants in the evolutionary scheme?
Did he worry about the giants a lot?
He did worry somewhat I think, even then, about the Hyades Giants. People still do.
That's right. Well, what was his problem? Were they time scale problems? Were they mass/luminosity problems?
No, I don't think they were very specific. Of course, the evolution theory which Demarque has now was of course very much in its infancy. People didn't know much about evolution at all.
Oh, no. That's certainly true.
They didn't know much about it.
Okay, this is after lunch, and we may spend a few more minutes completing your early period while at Leiden. I'd like to know the dates when you got your Doctorandus and your Ph.D. and what the defense was like for the Ph.D. and then maybe we will conclude the session with that.
Right. I got my Doctorandus in 1935 and I got my Ph.D. only in 1938.
So this means that you went to Yerkes just after completion of your Ph.D.
Just after completion of my Ph.D.
Right. So this is like a post-doc.
Right. That is correct. Hertzsprung was my thesis adviser throughout.
There was never any question about that?
No, never any question. There was one peculiarity different from other thesis advisers: he didn't want to read my thesis until it was printed.
Until it was printed. Now, at what stage did you have to defend that thesis — before it was printed or after?
After it was printed.
Okay. Now, "printed" means simply put into final form but not published necessarily.
It was printed at your own cost in Dutch, and you did produce a rather small volume and all the details of the individual measures were left out just because it would cost too much. And then later on the observatory would finance an edition in English. That's how it went.
I see. But that later would be after you defended it.
Now, what was the nature of the defense?
The defense was rather interesting. The defense is a very formal thing in the most beautiful room of the University, all decorated with all the paintings of all the old professors — a very long table with Greek carvings and gold pots and things like that. All the professors were sitting there in their togas. It was a very formal thing and it lasts one whole hour. I recall it as a rather harassing event. It went quite well. With Hertzsprung it went quite well, easy. With Oort I had some discussion, and I had some discussion with Kramers also on occultations of stars. You have to make a number of statements. I made in all about a dozen different statements. You have to furnish the University with these statements because the faculty must have something to attack you on. You had to defend these statements, particularly the professors other than astronomical professors. They would want these statements to have something to talk about with you. And there was one statement on occultation of stars by the moon, and it was a rather negative statement by me saying that because of the diffraction of the moon limb, it would never become very useful to derive diameters of stars by that method. I am wrong because now because this is happening. I mean Evans' papers have shown you that.
But this required Fourier Transform spectroscopy in order to do this.
Right. All these things didn't exist. And I still remember that I was very rude in answer to Kramers because he said, "I want to take that up, that statement that you made. It's obvious to me that you can't derive diameters of stars because of the diffraction. It's completely obvious. So I don't understand why you ever produced that statement. Please explain." And I answered like this. I said, "I'm here to defend my statement. If you think the statement is obvious, there's nothing for me to defend." I was so sorry that I said it.
Because obviously the faculty did not like that. They thought it was rude and unmannered and arrogant, all kinds of things, they thought. But I thought it was the only honest interpretation that at that very moment I could produce, you see. I was there for defending, and how can I possibly defend something that someone else finds obvious?
They probably felt you were pulling a trick on them.
I don't know. But in my opinion, of course, at that time, I knew it could never be obvious, because you had to work out formulae and you had to compare the dimension of the diffraction ripples in seconds of arc with the actual dimensions the stars' diameters had, because the relative size of the diffraction pattern and sizes of the stars as they are — and I'm sure Kramers didn't have them ready in his head because he was not an astronomer; he was a physicist. He knew that the stars had a certain finite, but small diameter; but you had to know exactly how large they are and to compare with the diffraction pattern to say just what my statement said.
Did he say why he thought it was obvious?
To me he didn't say. It was not for me to say, "Why do you think it is?" I mean I was very strict in the procedure of that meeting. I mean I took that very very strict, too strict.
But that had nothing to do with your actual thesis, that particular statement.
The interesting thing is that you were required as part of this process to develop a number of statements that you would defend. Did any of them have anything to do with your thesis?
They were all independent of it.
Not necessarily to do anything with the thesis.
I see. Well, what about the thesis itself? Did you ever have to defend that?
Yes, I had to defend that to Hertzsprung.
Just Hertzsprung alone.
Yes. But that went very smoothly.
That's very interesting. Had Kuiper written to you by that time asking you to go to Yerkes?
I think so, yes.
So you knew at least what you were going to be doing for a year after that.
Okay. Well, possibly one final question and then we'll turn this off. When you went to Yerkes, did you have it in your mind that you might have stayed there if asked or were you thinking primarily of coming back to Leiden? What did the people at Leiden say? What did Hertzsprung say? Did he expect you to come back?
He wanted me to come back. As a matter of fact, he wanted to send me to South Africa to the Leiden station. That was before the war. But I didn't want it. I tell you why. I said, "I don't want to go to South Africa now because I still have to translate my thesis from Dutch to English." And I wanted to do that, because otherwise there would be a further delay. South Africa meant observing every night because it was clear all the time, and I didn't want to delay the production of the English version of my thesis. So I said, "I won't do that until later." I said, "I have to go back to Leiden first. I have to produce that formula." I needed the quiet of Leiden. Yerkes was not a very quiet place. I don't know what you think. But it was not a very quiet place to me.
There were a lot of people constantly around.
A lot of people, constant commotion. I couldn't think clearly.
Did you have an office there?
I had an office. I had a beautiful office. I had Ross' office. Do you know . . . ?
I know Frank Ross. Which office was it?
Next to Struve's office. It was almost square, a beautiful office. It may be next to Morgan's. You have Ross, Morgan and then Struve. I think I had Ross' office.
Did Struve have the office next to the 40-inch?
That was always Chandrasekhar's. Kuiper's office was on that same side of the corridor next to Chandra's, and Miss Calvert was his assistant — Kuiper's. So that was the set-up. And van Biesbroeck had his office somewhat further on almost opposite to mine. And next to mine on the other side you had Greenstein's office.
Those are big, big rooms.
Yes. And at the end of the corridor there was the office of Henyey. You know the place.
Yes, I know now how it was set up. But I know that people did change offices a lot.
Ross was at Mt. Wilson then during the time I was there. That's the reason why I was put there.
I see. So this was an office that faced the south.
I know where you were, yes. Morgan's office changed while I was there. At first he was off at the very end and then he went to the south. In fact, he had Hale's old office, the one with all the little drawers and everything. What was the general feeling for astronomy in Leiden and various places like that — the regard for the position of a professor and that sort of thing?
Well, in Holland, of course, the professor is always very highly esteemed in society. I mean that is an outstanding position; the professor at a university in The Netherlands is really very important. Even the distance I think between professors and students is quite considerable. I mean you don't approach professors that easily as you do here in the United States. But the position in Society, I think that was never very strongly developed at Leiden. I mean Leiden always struck me as a kind of — I wouldn't say ivory tower because they were real enough into science — but the connection with science was much better developed at Utrecht. I mean Minnaert — you have heard of Minnaert, I suppose, and I have something further to say about Minnaert — was in the middle of Dutch society, giving lectures to the general public about astronomy.
Where was he at that time?
I see. Was he there when you were there?
He was there. He had a room in the physics department when I was there. When Nijland died, he became professor of astronomy, because he knew also a lot about astronomy and astrophysics. He was really a solar physicist, you see. So he developed into one of the best astrophysicists of his time. Well, he was much more involved I would say with the general public and the concerns of the public than ever at Leiden. Hertzsprung was not at all interested in any connection with the public.
De Sitter was not either?
De Sitter was not, only to get his observatory financed by the government.
All the finances were government.
All government. And Oort and Woltjier were not that I know of very much involved with speaking to non-astronomers — put it that way.
Okay. Well, that pretty much describes it, so I thank you for the session.
Before you shut off, maybe I'll have one more discussion with you right now about Minnaert, because Minnaert I think is an important figure in Dutch astronomy. You know, he has produced a book about nature and the physical world.
Is that Light and Color in the Open Air?
Light and Color in the Open Air. That's the title. Okay. He has three volumes of that in Dutch. I have them all. But left and right I get requests from people: why only has the first volume been translated into English and not the other two? And now I'm retired and people think that I have the time to do the translating of the other two volumes. I mean from South Africa Thackeray just wrote me that he wants to know about dust devils and whorls that he's observing there, and he's sure that Minnaert has something to say on that subject. So I read it all up in Minnaert's Volume II and I translated a few pages in English and sent it on to him. But he wants me to do more. He said, "It's not enough. Very good. Thank you. But please translate those books completely."
A tremendous job.
A tremendous job and I feel that I rather want to do other things than that.
It's very difficult. It takes a tremendous amount of dedication. The translation of the Kapteyn biography would be a very nice thing, too, and that's a lot shorter.
A lot shorter.
Yes. You do have that Kapteyn biography.
Yes, I have that. But I told Thackeray that I hope that Dover Publications will have an interest in having a translation of the other volumes of Minnaert's book. I'm going to write Dover, and maybe you could be instrumental in bringing this about.
Very possibly the A.I.P. Center could write something in support of this. Yes, we can have this on the tape. It will serve to remind me more than anything else.
 Bulletin of the Astronomical Institutes of the Netherlands.
 Shapley, H. STAR CLUSTERS (McGraw-Hill, 1930).