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Interview of Jan Oort by David DeVorkin on 1977 November 10, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/4806
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Discussion of Oort's family background and childhood interest in astronomy. Stimulated further through study with Kapteyn at University in Groningen (1917-21). Early work with Kapteyn and Lindblad, leading to postdoctoral work at Yale (1922-23). Return to Leiden (1924), and discussion of stellar research. Other projects of interest to Oort, include radio astronomy and establishment of ESO.
Dr. Oort, I know that you were born in 1900 in Freneker. Is that in the Netherlands?
It is an old city in the Northern province of Friesland which is famous among other things for its having had a university in the old days. But that stopped nearly two centuries ago. It is still well known in Holland because of the Planetarium, which the wool comber Eyse Eysinga completed in 1785. He had a gift for clock making and such things. The planets were attached to the ceiling of a room in his house. It was quite accurate and a real work of art.
Did you see this as a young child?
No. When I was three years old my parents had moved to the neighborhood of Leiden and that's where I got my education.
What was your early education like?
Just lower grade school and high school in Leiden; the very commonest thing one would have. But I certainly got interested in science and astronomy already in my high school years. I don't remember exactly where my first interest in the subject came from. I suppose it was by reading books by Jules Verne who was very popular in those days. Around the World in 80 days, the Voyage to the Moon and the Excursion of a Comet.
Well, these were fiction, of course, did you appreciate that?
That was the sort of science fiction of those days, but in a way perhaps of a more literary nature than the present science fiction. Jules Verne was quite a witty French author. But he is still being read today, my grandchildren enjoy his books as much as I did.
That's marvelous. Did you read anything like Flammarion?
I suppose very soon after I got into reading Jules Verne.
Did someone direct you to these books or to these interests? I would like to know the background of your family, if I could.
My father was a medical director in a sanitorium for nervous illnesses. We lived in the director's house of the sanitorium, in a small forest which was very nice for the children, of course, to grow up in.
This was upon your move to Leiden?
Then for how many years did you live there?
Until I went to study in Groningen which was in 1917.
I would be very interested in a little more about your family. What their general interests were and what family life was like?
My father was interested in science. He made some simple brain experiments and was one of the first in the Netherlands to use psychological tests. I think these had been introduced by the American Army in the First World War. He was also very much interested in nature, especially botany and. plants; those interests were inherited by my younger brother who became a naturalist already from his school days. I think in a way he was much pronounced in his direction than I was. My mother had undergone a severe operation and had to go to Italy for a recovery period after that. She got letters from the children. In letter's from me arithmetical problems were sometimes scribbled in the margin. When she got a letter from my youngest brother it was always about flowers, birds and mushrooms he had found. In fact, when he was still in the lower grades he discovered a mushroom in the woods which had never been found before in Holland. He was at that age already quite an expert. He later became a professor of plant diseases at the University of Wageningen.
How old were you at the time?
About 10 or 11.
How many brothers and sisters did you have?
One older brother who died of diabetes when he was a student and two younger sisters.
What was your youngest brother's name, the naturalist?
Your mother was away about a year in that time.
Yes, a little more than a year, I guess.
Were you raised during that time by your father or were there other people?
One of my mother's sisters took care of us. Already in my high school days I got a small telescope from friends of my parents and so I became interested in the stars but not in such a very pronounced way. I liked physics in school just as much and it was perhaps, a more exciting subject matter than astronomy in those days. When I went to the University of Groningen I went there partly because a well known famous astronomer, J.C. Kapten was teaching there.
Did you know of this before you applied? This is why you had gone to Groningen rather.
Yes that was the main reason.
Had you talked to your father or anyone else about your career?
And what were their ideas?
They were very liberal. They thought that if you really liked the subject very deeply you must follow your calling and try to study that subject. But when at Groningen I wasn't quite decided yet whether I would specialize more in physics or astronomy. It was the personality of Professor Kapteyn which decided me entirely. He was quite an inspiring teacher and especially his elementary astronomy lectures were fascinating.
What do you recall about them? Do you recall anything particular in his character or personality that made him fascinating or was it the topics he chose?
No, it was his enthusiastic personality and the way he could discuss things.
He was accessible to you? You could talk to him as an undergraduate?
Oh yes. He was the type of man that always liked to talk not only to students but to people who he met in the train for instance, he tried to tell them about astronomy.
What would he say to a person on the train in 1917 who he found was interested in astronomy? What would have been on his mind?
I've never heard him do that. But I know he did it. I suppose they began talking about what the other man was doing, and he told what he was doing, so he would come to his subject. He was a man who insisted very much in his teaching on seeing very clearly through a problem to avoid going into mathematical arguments at an early stage of the problem. He had the fee1ing that he first had to try to grasp the problem more or less in its entirety before going to mathematics. He considered mathematics as an indispensible tool but not an ideal tool for approaching physical nnd astronomical problems. And it seems this spirit was taken over by many of his pupils. In this connection I remember the attitude taken by his pupil W. de Sitter, who worked in the very early days on the expanding universe, and was one of the first to actually apply the general theory of relativity to astronomical problems. He was very much more of a mathematical mind than Kapteyn or I, but he had the same feeling that mathematics was indispensible but contained a danger that it might sometimes lead you away from the actual insight into the problems.
That's quite fascinating especially from what I know about Kapteyn's research, that he was very interested in physical matter of things, looking for absorption, for all of his life literally, in space. During that period when you went in 1917, of course, World War I was very much around, and I know from periods before that Kapteyn was severly hampered in his ability to go over to the United States and observe and work with the people at Mt. Wilson. During that time he was building his model universe and bringing a lot of things into the open so to speak, his long time work on absorption. While you were with him when did you first actually work with him on research as opposed to taking classes?
Rather early in my third year, I believe. The first two or three years usually are taken up by a kind of general physical mathematical education at universities and it is only after the first examination, Candidates examination, that research is taken up by the students.
What was the formal research the students would normally take up? Was it the particular observatory project that everyone participated in?
No, they were left very free in this. My own earliest research was on the phenomenon of stars of high velocity which certainly fell outside the regular domain I would say of research of the Observatory at that time.
But it was certainly close to Kapteyn's interests.
So you must have had detailed consultation with him and begun to know his interests quite intimately by the time you started that research. Was that reasonable to say?
Yes, I knew fairly well what they were doing at the time at the laboratory and had some discussions with him on this subject, but only at the end when it was finished and he could give an opinion on the possible interpretation which was valuable to me at the time. But I certainly don't want to imply that there was any difficulty in approaching him. It was just perhaps, ordinary shyness of a young student.
I can well appreciate that. In your work on stars of high velocity, was it necessary that you analyze the Z motions, the vertical motions perpendicular to the galactic plane?
That was much later. The stars of high velocity had. no direct connection with the Z motions. They turned out to be a subject of rather fundamental importance which I didn't foresee when I started to work on them at that time. I was just puzzled by it. But in essence they foreshadowed the rotation of the Galaxy which at that time had not yet been conceived. The thing we grew up with in Groningen was the so-called Kapteyn system which was supposed to be more or less symmetrical around the Sun. This later turned out to be quite erroneous because Kapteyn and his co-workers had not realized that the absorption in the galactic plane was as bad as it turned out to be. They had good reasons for their underestimate of the absorption. They had investigated it for objects too far out of the plane such as globular clusters, where the absorption would generally be small. It led to wrong conclusions when extrapolated to directions in the plane where the absorption is indeed very large. But anyway these high velocity stars did not fit in at all into the Kapteyn system and he thought at the time they might be just outsiders moving through the stellar system such as we knew it at the time and would be more or less unrelated to it. But later on it turned out that they were members of the large galactic system as it had been outlined by Shapley in the globular clusters. And so they were outsiders in a way to our region of space but not to the galactic systems as a whole. It was Bertil Lindblad who first suggested this.
How did you feel when you first heard of the Kapteyn model putting the Sun in the center of the universe? Did you wonder whether this was a peculiar place to be? I'm only taking these feelings from some words that you wrote much later. reviewing the history of our knowledge of spiral structure in the Galaxy. You went way back into the 19th century and mentioned that there had always been this feeling that the Galaxy might have spiral structure. Did you ever consider the possibility that the Kapteyn universe was too special, too symmetrical about the Sun and that one should look and try to combine these things?
No, not at that time, because before one realized sufficiently how serious the interstellar absorption was there was no sufficient reason to have great doubts about the general size of the galactic system. Although at that time we thought that the galactic system would be like the spiral galaxies we didn't know either where the large spirals were and how large they were because we didn't know the distances. But from the earliest days of which I can remember my own thoughts, I think I was convinced that they were of a similar nature. There were many different galactic systems in the world so to say and the fact that the Kapteyn system did not show distinct spiral structure was not so difficult to understand because you see it exactly edge on and everything would lie on top of each other so it would be very difficult, still is difficult, at the present time, to unravel the spiral structure. So that was no matter of great concern at this time. But what was a matter of great concern was the system of globular clusters which Shapley first investigated. But nobody at that time, Shapley included, had anything like a clear idea of what the connection was between the stellar system and the system of globular clusters. I think the idea that more or less, was that the Kapteyn system would be a big cloud of stars drifting around in a much larger system that was outlined by the globular clusters. Now I believe that Kapteyn himself and van Rhijn probably also did not worry sufficiently about this. For one thing at that time it was still doubtful whether the distances that Shapley had derived were correct and that they might be much closer than Shapley thought. I have no clear recollection now of what I thought myself about that.
Nevertheless at least your first research held the basic ingredients of not only finding an alternative way to determine mass distribution in the galaxy and thereby detect the presence of non-stellar material, a subject Kapteyn was still interesting in doing, but from what you just said about your first work which was published in 1922 I guess, that there were more high velocity stars than you would expect statistically and therefore you had to account for them in terms of a unified system.
It was not so much their quantity as that they moved in a very systematic way. Their main characteristic was that they moved all towards one hemisphere of the sky. As became clear soon after, this is due to the fact that the galactic system as a whole in the vicinity of the Sun is rotating around a distant center. Therefore everything that is not connected with the direct vicinity of the Sun — and the high velocity objects were such objects — would have a systematic motion relative to the slow moving stars in our neighborhood. But this only became really clear by Lindblad's research which pointed out that these high velocity objects formed a system of much slower rotation. He had the idea that the galactic system was very much larger, it contained on one hand objects like, the globular clusters which had only very slow if any rotation around the center of the galaxy and that there were other intermediate groups of high velocity stars which also had a very much slower rotation than the bright stars around us. But he knew that these would form complete systems. You see before that time there was always this idea that the Kapetyn System would be a real entity drifting around in something that might be very much larger but which was not one continuous system.
Maybe we should add just one sentence to what we were talking about because these ideas of Lindlad's led directly to the verification of the galactic rotation hypothesis by more direct measurements. When I got impressed by what these ideas of Linblad's meant I remembered at the same time that the O-type-stars in our vicinity had some systematic motions which I had not been able to interpret, but which turned out to be the consequence of the differential rotation of the galaxy. Lindblad supposed that these various systems would more or less move as solid bodies with differing angular speeds. But I realized at the time that it would be difficult to have any real system in which the mass would not be concentrated toward the center so that the inner part should move around more rapidly than the outer parts. This effect showed up very clearly in these O-type stars and distant early B-type stars. And so in that way one was able to give a very direct confirmation of this rotation hypothesis of Lindblad's.
You had done that in 1927, at least this is when the published record came out. Lindblad's work was about one year previous.
Something like that.
But when did you first learn of Lindblad's work? Was it before publication, at a seminar or talking with people?
I think de Sitter first drew my attention to this. Lindblad had a somewhat obscure way of expressing himself. De Sitter drew my attention to these papers by Lindblad because he knew I was interested in high velocity objects and their interpretation.
So it was after the actual publication of the papers. There was no general undercurrent at Groningen or later on at Leiden where you would be looking for evidence of rotation. Lindblad's work clearly was the beginning of that.
I look for certain trends and certain underlining assumptions that people many times were working on that turned out to be very fruitful. It seems to be that the papers and topics you chose were extremely fruitful.That's why I was trying to reveal it.
Yes, these things were in the air; through Shapley's investigations as well through the work on the high velocity stars. But Lindblad's articles were a very large step; a step which also Shapley had not made at all to go from the idea of the limited swarm of stars which was the Kapetyn system — to go from that to the idea that this is only just an arbitrary section of a very much larger system. It was a revolutionary step at that time. One didn't at all realize at that time that the absorption was so strong, that the size of the Kapteyn System was entirely limited by that. Afterwards it is perhaps hard to understand why one hadn't thought of that right away. Well, in a way I should have and I've always accused myself that working on these high velocity objects I didn't then realize the correct interpretation.
What was happening in your own life during those years, let's say in 1920 you would have received Candidates Degree, is that correct?
So in two years you received that. Did you continue at Groningen with Kapteyn?
I finished my study in 1921, when I got my final degree which is in Holland a doctor's examination. After that one can start writing a doctoral thesis, on the basis of which one gets the actual degree of doctor of science. This will usually take several years, but the formal study itself is finished with the doctor's examination, after which you have just your thesis to work on. I continued to work after that doctor's examination for one more year in Groningen. Kapteyn had reached the age of 70 years and so I became for one year assistant to his successor, Professor van Rhijn.
And you continued working in very much the same area?
Yes more or less. Professor de Sitter who was director of the Observatory in Leiden wanted me to take a position at Leiden. He thought it would be good for my general education to go for a certain period to a place where they did fundamental positional work. He, at that time, was rather befriended with Professor Schlesinger at Yale and so I got a research assistantship at Yale Observatory for two years during which I learned a lot about various subjects which I would never have entered upon without that and which were quite useful to me in my later life but were not directly connected with the sort of work I had started out on at Groningen and that I really wanted to continue in. At Yale I worked on the problem of latitude variation, for instance, which is quite far away from the subjects I had so far been studying. But I consider these years as having been very useful. They gave me an interest of problems of fundamental. astronomy that I felt was capitalized on later, and which certainly influenced my future lectures in Leiden. A few weeks ago I received a letter from Dr. Westerhout, who, as you mny know, has just become Director of the U.S. Naval Observatorv. He wrote among many other things "you may be surprised that I took this position at the Naval Observatory but it was really due to your lectures in the early days where you always stressed the importance of fundamental positions in astronomy."
Very interesting. The question I have though is that I know that Kapteyn earlier in his work, of course, in his association with Gill and his later work was all tied in with positions and proper motions. Why was Schlesinger picked out in particular? I know that he was well known but his techniques in photography were new and yet de Sitter thought this was the best place you should go to learn the techniques.
Well, Schlesinger was an exceptionally good man in this line. Of course Kapteyn and other workers at Groningen were also interested in proper motion work. Schlesinger was more specialized in getting the utmost precision. But in a way it was somewhat an arbitrary choice you might say.
How did you feel about Yale and the United States when you got here for those two years? Were you married at that time?
No. I felt somewhat lonesome in Yale in those days, it was not like today, I mean there weren't so many foreign students in New Haven except Chinese. There was a "Yale in China" and so they got quite a number of Chinese students at the time. However, New Haven was very hospitable to all foreign students, and some of my very best friends were made in these years in New Haven.
Who did you meet in astronomy primarily?
Principally Professor Schlesinger and his wife. But the friends I was just now thinking of were a professor of oriental languages and his wife and daughter. I'd been introduced to them by a cousin of mine who was a professor of oriental languages in Leiden. They became like second parents to me when I was in Yale. His name was Charles Torrey.
Did you correspond with him after you went back to Leiden?
Especially with his wife. His wife was an author and we did a good deal of reading over there in the evenings together. We read aloud to each other and I learned a lot from that.
Do you recall where you lived? Did you live with them?
No, I lived at the Observatory. Between their house and the Observatory there was a street lined with beautiful elms; the elms are no longer there, I suppose.
No, that's Dutch Elm disease.
Which we can't help.
I know. Did you know that you were going to go back to Leiden after the two years?
Yes, that had been the idea from the start. The job problem was easier in those days.
There was never any question then as you were growing up that your interest would be some intellectual profession, and that once you did there was never any question there would be support for your interests. Was this the basic situation?
Well, that depends, of course, on whether you are a good student or not. Not all the students could find jobs in science. But on the whole it was very much easier than now-a-days.
Had you ever considered any alternative to a course in physics or astronomy?
Yes. One thing I hesitated about is that the profession in pure science might alienate one a bit from people in general. In concentrating on purely scientific subjects one might not develop the human factor sufficiently and I did like to have contacts of that nature. Now I look back on my life as it has become, the human contacts were certainly plentiful and as director of an observatory one has of course, a large task in human relations. But that was a thing I remember worried me a little bit at the time so I wondered whether I had not better become a medical doctor or an engineer or something like that. But I certainly liked science better and so the specialization in this came naturally.
You indicated that you entered Groningen as interested if not more interested in physics than astronomy and indicated that it was Kapteyn's excitement that won you over to astronomy.
That swung the balance in the direction of astronomy. As a matter of fact one didn't have to make a choice in the first two years. The curriculum of the first two years was perfectly the same for physicists as astronomers. Students could thus still make a free choice after those two years.
Did you have any contacts with the other professors there in addition to Kapteyn, van Rhijn?
This was before Kapteyn's retirement.
I was interested in mathematics. I even went to lectures given by a famous French Professor Denjoy in Utrecht on point ensembles. But that was an interest of a more aesthetic nature and not an interest that would ever lead me to take up mathematics as a profession. I was too interested in nature itself. There was one professor in Groningen who certainly had considerable influence on my education. This was Professor Zernike, a theoretical physicist and also experimental physicist. He received the Nobel Prize for the ultramicroscope that he invented. In a way he was the worst teacher imaginable, he never prepared his lectures and often repeated the same stuff over again the next week because he had not prepared anything new. Many people found him difficult to follow in lecture but I learned more from him than from almost anybody else.
It's important to be able to identify the influences of your teachers outside of the nucleus of astronomy itself. If their names come up at any time that's alright. But maybe we should move on and talk about just one other question during your Yale period and then get back to Leiden. If that's alright? When you were in the United States for two years did you visit other observatories or other astronomers?
Yes, that was customary in those days. I did go to California at the end of this period of two years to visit Mt. Wilson Observatory.
Had you known A. van Maanen already?
Not before that, I think, but I did meet him there.
What were your impressions of Mt. Wilson?
An exciting place! In particular by talking with E. Hubble, he was very enthusiastic about various peculiar forms of galaxies, and showed me a good number which he thought were particularly interesting.
This was right at the time that he may have been realizing some of his "novae" were, in fact, periodic, or was it just before that time because it was just about 1924. Did you realize any of this at that time, did he show you any of this?
That I find difficult to remember but I think that already at that time it was quite probable that the Andromeda Nebula and other spirals were at very great distances and that you could get at least a lower limit for their distance from the absence of any bright stars. But I don't recall whether the Cepheids were actually already in the picture. I would have to look that up. I suppose they were.
The date was very late in 1924, if I recall correctly, that these items were made public even though Hubble had circulated them privately for some time before then and I was just wondering if he had talked with you about them.
He certainly wasn't secretive about things, Hubble wasn't. But what I remember most are the pictures he showed at that time as well as at later visits. One thing that startled me very much was his picture of the jet in M87.
Just that it was such a peculiar object?
Yes, something so peculiar that it looked like part of the big marvels of the world. I've always afterwards wondered that the astronomers in Mt. Wilson didn't wonder very much more about this thing. You see they had tried to observe it spectroscopically, but not much appeared. It's a queer thought to realize that if they had made any polarization measures at that time, which they could very well have done with the existing instruments, they would have discovered synchrotron radiation, and with that the whole basis, so to say, of the radio galaxies. But it was only thirty years later that they actually did this.
I know that Kepteyn brought many many people; Van Rhijn, van Maanen, Hertzsprung and others from time to time to Mt. Wilson. Was he establishing or did he ever talk to you about his ideas of cooperation in research where astronomers at Leiden, on a regular basis, would be able to enjoy the clearer atmosphere of the West coast or the Southern Hemisphere or something like that? What was his view of the future progress of astronomy along those lines? Did he ever express them to you?
Yes. At the time that I studied at Groningen the war was going on and there wasn't any actual exchange. After the war, Kapteyn must have felt that he was too old to return to the United States. But the idea of world-wide cooperation on the problems concerning the structure of the galaxy was, of course, very foremost in his mind. He had been the initiator of the so-called Plan of Selected Areas. It was aimed at a coordinated attack where many observatories entered the problem of the structure of the galaxy.
He had. had, though, some rather unfortunate problems with Walter Sydney Adams who was young at that time over the propriety of spectroscopic parallaxes. Was there any regret or second thoughts about his association because of that? Did he ever confide in you?
No, I don't think so. I never heard anything about that and I doubt that. Kapteyn was certainly not the type of man who would ever feel bad about anything of that nature.
How do you feel about the problem that was brought up? If I recall it was brought up by Adams? Do you know anything about that difference that they had? It involved. Anton Kohlschutter to some extent too. If you could add anything to what I've said.
No, I'm afraid that I don't remember that. It probably didn't go very deep with Kapteyn.
That was the important thing to know. Let's go on to Leiden, when you moved to Leiden Hertzsprung was there, if I'm correct and it must have been a pretty active and interesting place. Could you describe what Leiden was like in your first years, 1924 and 1925?
It wasn't nearly so active as it is at present. In that period astronomical science was much quieter than it is today. There was not the marvelous development at various observatories which we have seen later. The staffs were more or less stationary, there was no possibility of extending them greatly. However, the director, de Sitter, was a very active and enterprising man. The first great step was that he got Hertzsprung to join the Leiden Observatory. This meant clearly that he wanted to develop the Observatory into new channels. You see up to that time the Leiden Observatory had been concentrating entirely on positional astronomy, meridian circle work and some proper motion work. But no astrophysics or anything that looked like that. No structure of the galaxy, no dynamics of the galaxy. There was no one else in Leiden who was interested in these problems in which I was principally interested, so the first years I worked more or less by myself in these projects. De Sitter was interested, but his main line of research was celestial mechanics; at that time the expanding universe had moved away from his direct interest.
Before it was rediscovered observationally. So really during the 20's there was little of what we call today cosmological interests at Leiden and with your coming then...
De Sitter had. worked on this subject for several years before I came to Leiden, but he more or less finished it so far as it could go at the time. He was not interested in purely theoretical developments, although he was quite capable of doing that. He, as I said before, was only interested in mathematical developments in so far as they connected up directly with nature and with observations. And new observations were lacking at that time on the universe. It was only after Hubble got his distance/velocity relation that new impetus was given to this subject, and at that time de Sitter had drifted too far away, I think, to undertake new research, and there wasn't so much fertile research to be undertaken by a theoritican at that time.
Did he see that bringing you to Leiden he could almost start new, let's say, with further studies with galactic structure? Did he have you come to Leiden specifically to work in galactic structure?
Did I see my future very clearly then?
And did he see your future that way too? De Sitter?
No, he was an extremely liberal man and he rightly thought that the best science can be got out of people by letting them be entirely free in the choices of their work. He was ain ideal. director to work under. Although the first years he asked me to clear up some of the old material that was still laying around unpublished on positional astronomy, which I surely did but wasn't really deeply interested in.
I was aware that you also prepared a number of papers that were based upon data that you collected while you were at Groningen; work that you had started at Groningen and pretty much completed in Leiden in 1926. This was the stars of high velocity and you continued to work on those through 1926 and. 1927 and you came out with your paper on the observational evidence confirming Lindblad's hypothesis of rotation of the galactic system.
I was completing my doctorate's thesis.
So you look on that whole period as continued study leading to the doctorate degree?
At that time I failed to realize what Linblad realized. That these high velocities fitted. into a picture where the galaxy is supposed to be a very much larger unit than the Kapteyn system. Looking back on it, I see this as a lack of insight of which I feel ashamed to some extent. (But one never can tell.). It is sometimes so difficult to bridge a certain.gap. In my case this was partly due to the fact that I had been educated into the Kapteyn system a little too much and so I couldn't easily get rid of it. Although I did get rid of it after I saw these ideas of Lindblad's.
It was within that one year then you must have done an extraordinary amount of work and it seems as though, at least from history's standpoint, your paper is far more useful in that you set up the theory in terms of paremeters wbich were observationally derivable — the Oort constants A and B.
Yes in a way Lindblad's ideas were very instructive and they were vague to a certain extent. That's also why I didn't realize at once when these papers appeared what the significance was.
Oort constant A, if you don't mind talking about it in those terms, pretty much has embodied in it the idea of differential rotation. How were you led to these simple formulations of A and B? Did you have a model in your mind. and then realized that you wanted to express it in as simple form as possible?
Yes, more or less. It was clear that the observations we had at that time did not extend much further than one or at most two kiloparsecs from the Sun, which was only a small fraction of the distance to the center; so at that time there was not much point in thinking out the whole model for the gravitational field of the galaxy. So one had to confine oneself to, on one hand, the total gravitation force in the neighborhood of the Sun, and on the other hand its derivative as a function of distance from the center. Now, of course, that gave already some kind of model; it showed that the mass of the system should be strongly concentrated towards its center.
Right, this is what I'm trying to identify because you very well could have had that constant A in there but if you wanted solid body rotation you just make it equal to zero.You could give it another value and come up with a centrally condensed, completely centrally condensed object. Did you have any particular model in mind or were you just trying to express the general characteristics of the galaxy, of a rotating medium?
No I didn't work out a real model, I mean I was satisfied to get one parameter, which in essence was a concentration of mass towards the center of the galaxy and the rotational velocity itself.
Was it at that point that you began to realize that examining the vertical motions could give you an estimate of the distribution of mass in the system.
No, that existed much earlier, because Kapteyn. in his dynamics of the Kapteyn system already considered the same problem.It doesn't matter so very much for this vertical force whether you have a Kapteyn system with a radius of two or three kiloparsecs or a very much larger system with a radius of 12 kiloparsecs because most of attraction in the vertical direction comes from the immediate surroundings. So for that problem the new idea of a large system wasn't at all essential. That just came separately. But my general interest was very strongly in the dynamics of the galactic system as a whole. Already from my young days, I think, I had the idea to apply dynamical theories to the motions of the stars. This was stimulated. by Kapteyn's dynamical investigations of the Kapteyn system, and certainly also by Eddington's book on STELLAR MOVEMENTS AND THE STRUCTURE OF THE UNIVERSE, which I admired very greatly; it was a very good book.
That book came out about 1914, do you recall when you first saw that book?
During my studies in Groningen. It was mostly concentrated on the motions. Another book which certainly fascinated me at that time was Jeans' book on cosmology. ASTRONOMY AND COSMOGONY.
When you first, this would be the later '20's, began determining various values of the constant. A, there was, some disparity in the values through the '20's and '30's, of course, because it is a very difficult quantity to arrive at. I read in Struve's ASTRONOMY OF THE 20th CENTURY, that over the years there was some controversy as to what the value of A actually was. While we are talking about it now could you sort of give me your impressions of about how well it is known today and what the advances have been?
I don't think it was quite as uncertain as what now would appear from the numbers Struve quoted. If I had to make a guess, the range of values that various determinations had would have been about 14 to 20 or 21. As to the present value, the generally adopted value at present is 15 and I think that is about the best one you can have at the present time. Of course the problem of determining this constant has changed considerably since the spiral wave theory appeared on the scene, because formerly it was supposed that on the average the stars would move in circular orbits around the center and this is now not necessarily true because there is superimposed on the circular motion a spiral wave motion, which is fairly large. You can only get rid of if you extend your observation over a large region, lets say up to three kiloparsec by which you would average out the effects of the spiral wave to some extent. But, I should. say, by luck it appears to be so that the phase of the spiral wave is such that it does not affect the determination of A very much in our vicinity. The introduction of the spiral wave has therefore not made a big change in the value of A. But you have to first get the proper definition of the value of A. The definition was simple as long as you thought only of the circular motion, but with the spiral wave it needs a better definition; but I don't think we should go into such details.
These are two very fundamental properties of the galaxy that have survived quite strongly through some fifty years of use and so I'm fascinated with your recollections of their general utility. Not too many things have survived that long.
They are still being used.
Now your first rotation studies were based upon radial velocities and then later on you supplemented them with proper motion. And you were clearly gathering data from every available source yet there must have come a time when you decided wouldn't it be nice if you could gather your own data and without your complete bibliography in front of me I'm not aware of' what you have done in the 20's and 30's as far as collecting observations yourself. But I do know that from time to time you did trave1 to other observatories. Could you give me your recollection of what your activities were like?
I did not make observations related with this problem because that would have been too far outside my capabilities and lines of practical work. But I did try to persuade others to get relevant observations like velocities of distant Cepheids. I corresponded with Joy, for instance, on Mt. Wilson and with Adams also. And this work was, done — they got these determinations. Now for the rest I never took up the courage to devote myself to experimental work in this line. In the first place I wasn't interested very deeply in observational techniques and I thought other people could do that better. That, of course, may be in a way an attitude which seems somewhat strange to you as an American because you always like to do everything yourselves, in this country.
Possibly true but I would be interested in some further impressions from you. Did you actually visit the observatories from time to time in the United States?
Yes, more to talk with people about things. Especially people like Hubble, Baade at Mt. Wilson, also Minkowski at Mt. Wilson. (O.C. Wilson was not so directly connected with the work. I knew him quite well. These talks were quite inspiring to me, in particular with Walter Baade. We have many long talks and much correspondence also about many problems. In particular on the Crab Nebula for instance but in a somewhat later period.
Could you contrast Hubble and Baade, as you knew them both personally and professionally. We would be most interested to know how you see their roles in the development of modern cosmology.
That is an interesting question. They were both exceedingly inspiring people. Both were fascinated by the phenomena, with a fascination such as I have rarely seen. At each visit to Pasadena it was a treat to be shown by either some of the new treasures he had collected; by Hubble new peculiarities in the galaxies of his tremendous collection, and, for instance, on the way dust and. absorption correlated with other characteristics; by Baade on a tremendous variety of new phenomena which his careful observations had revealed. They had much in common, not only in science, but also for instance in their appreciation of material things. Yet, they were also very different. I know both quite well, in their work as well as in their houses, and I liked both very much. I don't know whether they ever worked together nor whether they discussed much with each other. The styles were different, in observation as well as in life. Baade had been trained in Germany to make observations with the greatest possible attention to detail and precision, such as Hubble may have found unnecessary. Hubble was mainly interested in the great lines. Baade also, but he interspersed his work with observations of smaller things as well, such as the rapidly moving filaments in the Crab Nebula, or observations of the shells around Nova Persei 1901 and Nova Aquilae 1918. As regards personality they were very different, Hubble formal and somewhat stately, Baade as informal as could be and full of jokes. Both were great smokers, but yet different: Hubble with his pipe and Baade with cigars which he even smoked in the 200-inch telescope when waiting for better seeing. You ask specifically about their roles in cosmology. Hubble certainly made by far the largest contributions: his discovery of Cepheids in the nearest galaxies and his establishment of the distance-velocity relation. The great impact by Baade on cosmology came in the optical identification of Cygnus A, and, more indirectly, but perhaps no less important, his careful studies of the Crab Nebula and the jet in M87. His ideas on the existence of different stellar populations constituted a big step in our understanding of the structure of galaxies, while his work on the occurrence of different types of galaxies in different types of galaxy clusters initiated the great development in the research of these clusters that we see today. But these discoveries were only indirectly related to cosmology.
Well let's try to concentrate, for awhile at least, in the 30's, particularly, what were your primary goals in your own research and your own feelings for the future of Leiden? Was Leiden developing at all in any different directions in the 20's and 30's?
Yes, it was, especially through the efforts of Hertzsprung it developed very strongly in photometric research, in particular in research of variable stars. Already in the early years when I was there an agreement was set up between de Sitter and Innes, the then director of the Union Observatory, Johannesburg, for a regular exchange. De Sitter realized that observations in Leiden, especially photometric observations would remain so much inferior to those that you could do under good skies that he set up this cooperation and sent out quite regularly young people to Johannesburg, at first to work with the instruments they had there, while later we got our own instruments in South Africa. Actual returns from these instruments were about ten times higher than a similar instrument in Leiden. The photometric sky was so much better.
I know that Leiden is well known for cloudy skies like Princeton.
Yet, the polarization of Crab Nebula was first investigated in the middle of the town of Leiden with many city lights around and quite successfully investigated at that.
What type of support did Leiden have, was it government support?
Yes. The University is a government institution and the Observatory was an institution belonging to the University. But it was mostly a research institution, and the educational part was just a fraction of its activities.
When I turn tape over we can move on to the early 30's and that is when you did begin working on the stars motions perpendicular to the galactic plane. We can move into the 1930's and of course Trumpler's work in 1930 must have been of great importance to you, the final determination that interstellar obscuration did exist. How did you feel?
Yes, already at that time of course it was quite clear that it should be there. I mean it was a necessary consequence of stepping from the Kapteyn system to this large system and also it was clear from the distribution of the globular clusters. The fact that the globular clusters were absent in tbe narrow zone around the galactic could only be due to absorption and already at the time it was quite clear. So I didn't doubt that there was strong absorption even before the Trumpler measures came along, though it was, of course, quite important to get some numerical data. But it was no longer a revelation at the time.
So his work was more a confirmation which just allowed you to proceed. In your work in the early 30's on perpendicular motion were you interested in confirming the amount of absorption that he had found in any way?
No, just in determining the field of force and the mass density in the galactic plane because already at that time it was clear that interstellar matter and certainly the dust was contributing very little to this gravitational force.
But yet you could subtract out the star density and what you were left with was the density of the medium.
Yes, but you didn't know the star density well enough to do that.
When was it that you made your first estimates? Was it that you thought fifty percent of the mass of the galaxy was in the interstellar medium? Was it then or earlier than this?
No, I never said that. The conclusion arrived at from this investigation of the density distribution from motions perpendicular to the plane was that you could not account for the total force by the known stars. But it seemed very doubtful right away that this unknown matter that you were left with could be furnished by interstellar gas or interstellar dust. So right from the start it was fairly evident that it would have to be furnished by some unknown type of stars — fainter stars.
Well in this particular paper in 1932 the value that you got for the density of the interstellar medium something on the order of 3 x 10-24 grams per cubic centimeter has been called by Struve and others the famous Oort limit. I was wondering if not only you were aware of that term but what the significance of it was?
It has been used far too much, because right from the start this was extremely uncertain and it still is at the present moment, I'm sorry to say. In a few years from now we'll have better data but it has taken a long time since 1928 to get those better data. But it was not so much the concern, as I said before, with the interstellar matter as with the unknown stellar population.
The clues certainly were there as you look back. There was a Groningen publication where you had shown that you could differentiate the high velocity stars from the low velocity stars partly in terms of spectral peculiarities, now this was in 1926. What caused you to look at the different spectra of these two different groups of stars because that's pretty much what Baade did later on.
Partly metal content or mostly metal content I would say, probably. And so the high velocity stars would all be very old stars and some of them formed before the metal content of the medium from which they were formed had increased very much.
But certainly in 1926 from the state of stellar evolution studies at that time with no idea what nuclear enrichment was all about you couldn't have known that or foreseen that.
No, but there was the critical fact that some types of stars — long period variables or PR Lyrae variables were of this high-velocity type and so you could distinguish them by some properties, but certainly nothing was known about the metal content at that time.
Well you did say something about that in 1926, as you said from an empirical standpoint.
I'll have to go back and look but I had the impression that you did show that some of the stars had spectral peculiarities as you indicated.
Yes that is true. But at that time this was not immediately connected up with metal content. Not yet.
Not in your mind at least?
What were your interests through the 30's? I'm interested also in the development of your social life, your family home life. How did you feel about your life as it was developing in the late 20's and 30's? You were certainly settling in as a professor of astronomy at Leiden. How did the world look to you?
I wasn't a professor yet at that time, that was only in 1935 after de Sitter died. But before that I was lecturer at the university and had a research position. How did I feel about life in general? Well, on the whole I was quite happy, I was and am still happily married and we had three children between 1928 and 1954. 1 think we formed a very happy family.
When were you married?
And your wife's name and something of her background?
The family name you mean?
What's her first name?
Mieke. It's an abbreviation from Maria.
How did you meet her?
At a university celebration where she was. My brother knew her first because she came from Utrecht and he was studying botany or biology in Utrecht at the time. So he formed a link of introduction.
What were her interests? Did they change before and after marriage?
Yes, was she interested in a career?
No. She was much interested in poetry and art in general and still is. She never imagined that she would get a career in that line. Her ideal was more to build up a happy household and a harmonious life.
Was your salary adequate throughout these years, in the late 20's and 30's?
We never had any real financial problems.
What about religious background? We should go back and ask about your own family, your father and mother. Did you have any religious training when you were at home?
Yes. The common religious training, lessons about Bible interpretation. Both my mother and father came from families of clergymen. My grandfather, the father of my father was a Protestant clergyman with quite liberal ideas. He was one of the founders of the more liberal Church in Holland. He later became professor of Hebrew in Leiden. He was one of the three people who made a new translation of the Bible into Dutch, with extensive comments. It was quite a. large undertaking. Several of my uncles (his sons) were pastors. The father of my mother was also a clergyman.
There was an awful lot of clergymen in Holland.
My mother kept up her interests in that, at least in the early years of her marriage. But my father was less interested in Church matters.
Did you follow your father or mother's interests? Did you go to Church regularly or do you go to Church regularly now?
No. But it was not so customary in Holland, I think, as it was in the United States to go to Church. There were many highly esteemed families who thought going to Church was not at all essential. Although religion still plays a very important role in Holland and especially in Dutch politics. It has much less influence than in the past. We have had three political parties based on religion, one Roman Catholic party which was and is very powerful and two Protestant parties. To you it must seem sort of strange to have political parties connected so directly with religion, but this was not a bad system at all because in these parties, for instance in the Roman Catholic party, socialists and employers were combined into one party, and social problems were more easily evened out in those days than they are present..
Was there any difference that you could see in support for the University, for education, for astronomy, basic research depending upon which denomination was actually politically in power?
No. I don't think there was any, except perhaps just during the 1ast few years where the Socialist Party has grown in influence in the government, and this has been to some extent to the detriment of science. Not because they were not interested in science but because they were so much more interested in getting higher wages and more social security. So that all the money available went into income.
Was World War II looming on the horizon by the late 30's, was there any change in direction or feeling that something was about to change tremendously in the everyday life at Leiden, the Observatory? Did people see this coming for a long time?
No, not really.
I know by 1941, of course, in your own recollections and writings, I recall that you said the Germans already pretty much restricted your activity at Leiden, but that you still were able to offer the prize that van de Hulst eventually did receive for the discussion of the nature of particles in interstellar medium. But previous to that in 1939 you were still able to travel anywhere you wanted and you attended the McDonald dedication ceremony and things like that.
Yes, but the occupation came only in 1940. After that, of course, everything was closed but even the work continued to go on more or less normally at the University for some time until 1942 when the Germans dismissed the Jewish professors in Leiden University (and in other universities as well). Among the professors who were dismissed was a very famous, (also internationally famous), professor of law by the name of Meyers. On the day when he got the letter from the authorities that he could no longer teach his classes, the dean of the faculty of law went into his class — it was a big class in the main university building — and delivered a speech in which he started by saying, "I won't talk about his dismissal and I shall leave the people who did this, below us, but will concentrate on the greatness of the man dismissed by our aggressors." This speech made such an impression on all his students that on leaving the auditorium they sang the National Anthem and went on a strike. I happened to be present and was greatly impressed by the lecture. This occasion formed the beginning, in a way, of the active resistance in Holland. The speech by the dean of the faculty of Law was widely circulated during the rest of the war by the resistance groups.
Did you partake actively in any of this resistance, anyone else at the Observatory?
No. We had a little group of professors in Leiden who came together regularly and discussed the problems the university faced in view of the German occupation. Most of the members of this group were put in hostage camps very soon after the speech by Meyers. I wasn't and so we went down to live in the country for the rest of the war, not to take this risk unnecessarily, but I never did any real active work in the resistance.
What was your contact during the occupation? What was your contact with the outside world, with the West? Was there any contact at all, was there indirect contact?
Practically none. There was no way, at least not in the second half of the war, of sending letters to foreign countries or to receive them. There was practically no contact.
From the papers that I've found of your work during that period, there was one paper in 1942 co-authored with N.U. Mayall, and this was on the Crab Nebula. This was the identification of the Crab Nebula with the 1054 star?
How did your interests develop and how did your collaboration with Mayall maintain through the war or was this something which was done in another time and only published then?
That is a slightly difficult question. Not in itself, but it requires refreshing my memory. The discussion on the Crab Nebula must have started before the invasion of the Netherlands in 1940, and apparently I could get letters through to Mayall in 1941, before the real difficulties at the University started in 1942. My interest in expanding nebulae had arisen before the war from a study of the nebulosity around Nova Persei 1901 and the remarkable discrepancy between on one hand the distance of about 100 pc derived by Kapteyn in 1902 from the expanding filaments observed around the nova in 1901 and 1902, on the assumption that the observed expansion was that of the flash of light emitted at the outburst, and on the other hand the distance found in 1934 when Humason had measured the expansion velocity of the gaseous shell that since 1916 had become visible. The combination of Humason's velocity with the angular expansion of this shell indicated a distance of 600 pc. The discrepancy with Kapteyn's distance showed that the signal of the flash had moved at an apparent velocity of more than five times the velocity of light. I concluded that this was due to the fact that the filaments considered by Kapteyn were not situated close the the nova as he had assumed, but at some distance in front of it where the "reflection" of the flash would displace itself with a much higher velocity than that of light. As a consequence I obtained a picture of the three-dimensional distribution of the interstellar matter in the surroundings of the nova, which showed that this matter was concentrated in thin sheets. My study was never quite completed, principally because meanwhile a paper by A. Couders appeared in which he had elaborated very similar ideas. The essence of my own studies is given in an account written by J.J. Raimond in the 143-volume of Hemel and Dampkring, based on a lecture I gave for the Dutch "Astronomen Club" (pp. 145-153 and 200-206). There are two reasons why I mention these investigations. Firstly because they were never published, but particularly because exactly the same explanation has recently been revived for the apparently "superluminal" velocities observed in nuclear components of quasars. Anyhow, this explains my general interest in expanding nebulae, of which the Crab nebula was, of course, the outstanding one. I had undoubtedly discussed this with Baade during one of my visits to Pasadena, and must have been impressed by his beautiful plates of the nebulae. Duyvendak, who was professor of Chinese language and literature at Leiden, was an intimate friend of mine and was only too ready to be involved in the subject. His careful scrutinies in Chinese and Japanese sources contributed in an important way to establishing with certainty the supernova character of the object observed in 1054. The correspondence with Mayall arose from his determination of better radial velocities, and from my wish to get better data on the expansion. I interested this Chinese friend of mine in this subject. He probably looked again at it after the war after first things calmed down.
In 1944 there was the legendary colloquium given by van de Hulst on the probability that a spectral line must exist in the interstellar medium from interstellar hydrogen and as the story goes, you originally gave him this idea to search theoretically for the existence of the discrete line in the spectrum. Did you develop these ideas from G. Reber's work? Could you give me some kind of feeling for how your ideas developed on the use of radio spectra for galactic structure?
Yes, it was Grote Reber's work which first impressed me and convinced me of the unique importance of radio observations for surveying the galaxy. In a paper which appeared just before the war he published his observations of radio emission evidently coming from the galactic system. There had been papers earlier, of course, but I did not at that time realize the importance of them. The work of Grote Reber made it quite clear this would be a very important tool for investigating the Galaxy, just because it could investigate the whole disc of the galactic system unimpeded by absorption.
Did you realize that at the radio frequencies he used the medium would be transparent?
Yes, that's why I got the idea that we should try to develop this work of Reber's in particular with an instrument of larger aperture so as to be able to get the resolution one would need to study the more distant parts of the Galaxy. What exactly the talks with van de Hulst contained I find difficult to remember. I certainly mentioned the possibility that it would be wonderful if one could get some discrete emission line. But that may have been just a vague remark. I don't know how much I influenced him by that to think of this hydrogen emission line. But I don't think one should attach too much importance to such things. The subject was in the air and it was very fortunate that a good theoretician became interested in the subject and showed that the hydrogen hyperfine structure line might be observable in the interstellar medium. At that time, I think, both of us thought that it would be valuable to build a radio telescope of considerable size if only to investigate the continuum radiation from the Galaxy. Because it still seemed uncertain whether his 21 cm line would be strong enough to be observed. So although we did aim directly after the war to get receivers for this 21 cm line we would not have easily decided to ask for money to build an expensive instrument just for this purpose. But there certainly was the continuum radiation that Reber had observed, so that it would, in any case, would be worthwhile to get a large telescope.
When you first read Reber's papers, this would have been in ApJ, 1940, and began wondering about the implications, were you bothered or concerned at all that you did not seem to have thermal radiation that he was observing?
No. We never thought at that time about the possibility of anything but thermal radiation.
In looking for the line did you have the idea in mind that this would be important in terms of radial velocity observations?
So you had the geometrical idea even in 1944, let's say, or when you talked to van de Hulst first that you could eventually map out the structure.
I know that was quite clear.
How did you feel when you were just barely edged out of the actual observational discovery by Purcell at Harvard after the war?
We didn't mind a bit. Neither van de Hulst nor myself were at all concerned about that. It was not in our line to be. We had an interesting intercontinental telephone conversation soon after the line had been discovered by Ewen and Purcell. Van de Hulst then happened to be on a visit to Harvard. In those years a telephone conversation to the United States was considered very extravagant. It was made possible by the high representative of the Netherlands Post, Telegram and Telephone Service (P.T.T.) on our Board who said "well if you telephone in the evening I can give you any time you like." And so we telephoned for half an hour. We had then just found the 21 cm line at our station in Kootwyk and were trying to determine the galactic rotation.
Marvelous. There was someone on the Board, did you mention, of the P.T.T., that you knew? What was the relationship?
He was on the Board of a special organization for developing radio astronomy in the Netherlands. We had convened a special organization for that because it was an expensive project and several different groups would be involved; not only astronomers but also the P.T.T., the Philips research laboratory and the Meteorological Service.
Could we talk about that Board and the development of support for radio instrumentation in Holland for a little while? How did the Board initially get formed? Who was the instigator? How were the contacts made?
Well, my idea for this large project was quite favorably received. After the World War we were very poor and there were, of course, many more important things to think of, such as building bridges and harbors. But the first Prime Minister after the war was a man I knew well, who was very much interested in science. He was a professor of Geodesy at the University of Delft, and right from the start he was very much in sympathy with this project.
Who was he? What was his name?
What were the arguments that you gave him? He certainly wasn't a specialist in astronomy.
He was sufficiently acquainted with astronomical problems to understand the significance of this, so that there was no need for very much argumentation.
Did you consider this a very fortunate situation? What if there was someone in that position who was not as sympathetic?
That would undoubtedly have made things more difficult.
But of course the Prime Minister is not enough. For a scientific project you have to get the consent of other bodies, more directly concerned with science.
Which were they?
Administration, the Academy of Sciences, and in particular, a body more or less equivalent to the National Science Foundation, which was set up in Holland soon after the war. They were, of course, instrumental in this case. But there again we were very fortunate. In the young days of this new science foundation it was more or less looking for new projects of larger size, and right from the beginning they gave us their full support. This "Netherlands Organization for Pure Research" had an extremely good director who was, himself, keenly interested in the project. So there were lots of very fortunate circumstances. I never had to fight for the project.
That's very interesting. But when you did have to discuss the advantages of radio astronomy what were the arguments that you gave to them at that time? Is this written down anywhere?
It was quite clear, I think at that time that such an instrument would have enormous prospects in just delineating the galactic system in particular, of course, in combination with this 21 cm line. It was not difficult to point out the great significance of such research and moreover, by Kapteyn's work we had a certain tradition of investigating galactic problems. The project fitted in with Dutch tradition.
Well, the P.T.T. how were they brought in? Were they brought in for their expertise?
Yes, mostly for the expertise. But they were of very great help in many other ways. They had under their care all the radar equipment that was left behind by the Germans on the coast of Holland. This radar equipment consisted in part of reflecting telescopes of 7 1/2 meter aperture. In those days that was quite large. They were called Wurzburg-Riesen. Now these were not ideal for the problem of radio astronomy but still they were very much better than anything we had, or anything Grote Reber had. They were very solid structures. So the P.T.T. had saved several of these and transported them to their central transmitting station. Our radio astronomy was really started with the aid of one of these instruments. In addition the P.T.T. permitted us to use the laboratory at the Kootwyk transmitting station. So we had a great deal of help from the P.T.T. during the first years and it was in Kootwyk that the first map of the Galaxy was made.
In talking about the instrumentation and this very important reason for contact with the P.T.T. you must have dealt with particular people. Was this person who would allow this long distance phone call to Purcell at Harvard the main contact at P.T.T.?
Yes; he was not the director general, of course, of this whole big P.T.T. service, but he was in a high position there and was keenly interested in this kind of electronic problem.
What was his name?
De Voogt. We've always kept this contact with P.T.T., also after moving away from Kootwyk. As mentioned before, the Board of our Radio Astronomy Foundation comprised other representatives as well, including astronomers from the Universities of Utrecht and Leiden, while its meetings were generally attended by the Director of the National Science Foundation.
The Dutch National Science Foundation. From the beginning how was the organization set up? What was the participation on the part of the astronomers? Was the idea that the astronomers themselves would learn how to construct better amplifiers, better telescopes or was it always understood to be another expertise?
Once again there was perhaps a difference between the United States and Holland, in that we never attempted to have the astronomers themselves learn very much in electronics. From the first we drew in specialists who had studied in electronics, the understanding always being that they would make the electronic parts of the instrument and the astronomers would just use them.
So what was the basic communication? Did you tell them the frequency ranges you wanted and the noise levels you wanted to keep under?
And then they would work from there?
That's a very nice operation.
It turned out to be a very efficient way of doing things. Of course, there were other problems, concerning the telescope. The beginning of our radio astronomy was however with these borrowed Wurzburg instruments.
How many did you have?
There were several, but we actually used only one and that's the one with which we mapped much of the spiral structure of the Milky Way. But then the large instrument had to be built, a 25 meter dish which would enable us to obtain a detailed picture of the structure and motions of the Galaxy. It took ten years in building.
This is the large one at Dwingloo.
Yes. At the time it was completed it was, I think, the largest instrument of its kind. Soon after, the Jodrell Bank telescope was completed. But that was not at the time of sufficient quality to observe at decimeter wavelengths, so it could not observe the 21 cm line.
But your very first 21 cm survey of neutral hydrogen mapping out the spirals, this must have been a very large project, very time consuming.
It was, rather. At present we would not consider it such a very large project, but at that time suitable computer programs had not been developed and it was quite an undertaking.
How was it organized? Did you know from the beginning that the dishes would have to be stationary and you would simply allow the objects to cross the meridian?
No these Wurzburg telescopes could be moved. We just took line profiles in different positions along the Milky Way and combined those to get the total picture. So that was a very active period.
This was a period in the early 50's. Was there enough observing so you could actually set up a system whereby students as well as faculty and staff would rotate the observing schedule?
Yes, but most of the observing was done by students. The electronic engineers were responsible for the proper functioning of the instrument, but the observers took a very important part in the whole scheme.
What would they actually do, the observers, the astronomers? They would simply tell the engineers where to point the machine or did they know how to point it?
Yes they knew how to point it. So they did that in the early years. Now-a-days we don't do anything of the kind. We just punch cards and send them to Westerbork where the observing is done entirely in a factory manner.
Hertzsprung, of course, lived through the beginnings of this very exciting period of radio astronomy. But yet his career was totally unlike that other than the fact he was always dedicated to gathering more observations. How did he feel? Did you ever get his feelings for the new astronomy? And how it was being done?
He left Leiden almost right after the war and this development started early after the war. He was interested but he was so much engrossed by his own work on variable stars and double stars that he would not be very deeply interested in these new developments. I don't think he attended the colloquium, for instance; that we were talking about.
The van de Hulst colloquium in '44.
So he was off purely in his own direction of research?
He worked extremely hard and extremely precisely. A very fine critical mind, but especially in his later years his interests became limited and were confined mostly to observational astronomy in direct connection with his life work.
Let's talk specifically about detection of spiral structure then. It sounds like, and it makes all the sense in the world, that you had very quickly grasped the value of the 21 cm line in mapping out the structure of the Galaxy. Did you feel the most effective way to go at general galactic structure, was by the radio techniques or that you should try to support the optical efforts? What was your feeling for the most efficient way to progress in searching out structure like this?
Well at that time we were, of course, deeply involved in radio work, and had not sufficient time to dive also into new possibilities for optical observations. But it's quite clear now that the optical observations of high luminosity stars and star clusters and HII regions is of very essential importance to outline the spiral structure of the Galaxy. In particular, because of the spiral wave motions that are superimposed on the rotation of the Galaxy. Those make the interpretation of the line profiles in our own Galaxy and the outlining of the spiral structure much more difficult than we thought they were at that time. Just because of that it has become more and more important to have optical data to go together with the radio data, to get the proper picture.
When you first began to work on mapping out the Galaxy were you aware of Morgan's work and that optical mapping had been attempted?
Did you try to concentrate on the regions that he had worked on to try to get radio confirmation?
No, I don't think so. The optical mapping was very local and rudimentary at the time, so we certainly had the idea that we could do a very much better job with the 21 cm line. In any case, we could get a survey of the whole galactic system which was quite important. We could also find the rotation curve for the whole system. Morgan could only see some pieces of nearby spiral arms. Now-a-days one can do much more but even at present the optical work is limited very much by absorption effects.
There's a difference in actually amongst astronomers, I guess has been prevailing since the 60's that many of the arms that have been identified look very very circular to most people as opposed to being spirals. What is the present feeling in your mind about these questions?
Well they certainly are spiral-like and if you combine the optical observations with the radio observations you certainly get some kind of spiral pattern. But it remains difficult to connect up the arms in various parts of the Galaxy. It's not at all clear yet how one should connect the features observed on the two sides of the direction towards the center, for instance. And so we're very happy now-a-days that we can turn our attention to other spirals, several of which you can see very much better than our own.
The difficulties in optical observation sort of point to another interest that you seem to have had possibly the late 40's, I'm not sure that I have my dates exact. It was 1953 that you had discussions with Walter Baade on the establishment of a European Consortium for optical research. I'd like to know how these discussions came about because this was exactly around the period that you were working so hard on radio astronomy and yet you saw the need, the tremendous need for the Southern Hemisphere optical work. So I'd like to know how those discussions started.
I had much contact with Baade at the time and, of course, we were now working only and exclusively on the 21 cm line. We also observed the continuum radiation both thermal and non-thermal. So we had interests in radio sources and radio galaxies; I was greatly fascinated by Baade and Minkowski's success with the just optical identification of radio sources. Both Baade and myself had rather wide interests. We had, of course, been much more involved in optical astronomy than the new class of radio astronomers. But Baade's interests were principally, I think, to get good observations in the Southern Hemisphere, in order to obtain an optical survey of the whole galaxy. I agreed very much with him on this and it seemed at that time that the only way of realizing a larger telescope in the Southern Hemisphere would be by having it built either in the United States or by European countries because the South American and Australian observatories, themselves, were not, at that time, sufficiently manned to develop the interests that would be needed and to have the manpower to man such a southern observatory. That was the starting idea of this project of the European Southern Observatory.
The original money for looking into the feasibility of creating this observatory came from the Ford Foundation. I was wondering did Baade get that?
No. B. Lindblad and I worked on that. Baade wasn't so much the sort of man to approach these people successfully. In Europe there was considerable resistance to the new project, as there always is. It started more or less as an agreement of a group of good friends in Europe.
These included many countries?
H. Spencer Jones, then Astronomer Royal in England, B. Lindblad in Sweden, O. Heckmann in Germany, A. Danjon, Director of the Paris Observatory. I had rather intimate ties with all of these persons, and that gave us the hope that we could together persuade our governments to support the project. But it went through very many difficulties the first being that Spencer Jones was pensioned and his successor was not so much interested in the project. England finally didn't participate, to my great regret, and this caused some difficulties in France, which did not want to spend more money on it. This stalled the project for much too long before an international agreement was signed to carry it out. But it's there now.
Were your original plans for a very large telescope, as large as four meters, or what?
Three meters is what we had in mind.
I'm interested in the growth of this observatory because the idea was, of course, quite an advanced one at the time.
There was of course the example of CERN in Geneva which was a luminous example of a good European cooperation. But our project had very much greater difficulties getting into being.
Other than England pulling out what were the problems?
You live in a fortunate country here where all the states are bound together in one union, and you have not these difficult nationalist problems which face Europe all the time. I had confidence, however, that the group of highly esteemed astronomers, who were convinced of the importance of the project would carry more easily their conviction to the governments than actually was the case.
What happened that didn't turn out as you had hoped? Was there anyone who had a specifically different idea than you had?
No, but there's always the same problems in setting up cooperative schemes in Europe. They have been strongly encountered by the organization of space research, also. Each country has its own projects besides the international projects and each is afraid they will suffer if the government would be bound to contribute unduly to an international undertaking.
Who was the most supportive, the Dutch government?
The Dutch government always supported us, and Swedish government also, the Germans from the beginning supported it very strongly.
There were no difficulties in talking to the German government as early as 1953?
No, it was all done by Heckmann, Director of the Hamburg Observatory. He was quite a powerful figure.
He became first director of the ESO?
Yes. I was president of the Board of the organization for a long time and he became the first actual director.
How would you compare the Board and the general ESO group that you helped to organize with the AURA group that we have in the United States as far as their organization? Is there any way to compare them as far as funding is concerned?
As I said before it is very much easier in your country because you have only one government to deal with. And as soon as you get the support of that government everything goes ahead.
Is there any help now with the common market that you have a much more common voice? Have things gotten easier with the common market or stayed the same?
Not much, I'm afraid. There's still, you see, this enormous difference in character between the different peoples. The language, for instance, is much more of a barrier than one might think. I mean, even if all the people in such a set-up speak each other's language fairly well the language barrier is a tremendous thing in enacting the actual cooperation. It often prevents bringing people as closely together as would be desirable.
As you said a few minutes ago it exists and I assume that it's successfully under operation now?
Yes it works satisfactorily now and there's no problem I think, except for its future development. It should, of course, not stay where it is at present. If it's going to be a living concern, it must start new projects.
Is there any difficulty in doing so? The present financial situation?
Now yes. Financial difficulty in all the countries is very much greater now than five years ago.
I know that you considered a number of different southern sites before finally settling on La Silla. But were there political problems involved with the countries involved in Europe all entering into a consortium and centering it in a commonwealth country as opposed to a non-commonwealth country?
Well the commonwealth countries would only be British relations and Britain has left this project. Australia would have been the easiest, of course, as far as political problems were concerned but there are no sites in Australia that are of the same quality as in South Africa or in Chile.
What kept you from South Africa then?
What made us decide not to go to South Africa? Mainly, I think, the very good experience that the American test groups had in Chile with climatic conditions there. They seemed really superior to what we could find in South Africa. At that time the political problems with South Africa were not insurmountable. The Leiden Observatory had had a station in South Africa for a long time and our experience was quite favorable. But also the French observers who went there for testing, and were in general opposed to the South African regime, found the country very agreeable to live in, while they were there. So I don't think that would. have played, at that time, a major role. And at present we have somewhat similar problems with Chile.
Was La Silla affected any differently, let's say, than Cerro Tololo when Allende was overthrown? Were they any particular problems with the Europeans?
So far as I know the condition was much the same in both places. They are very similar and not so far apart. The nearest town for both is La Serena, so they are practically identical in that way.
Then, of course, the Russians, I guess, have a large instrument planned down there too now.
I haven't heard of any decisive steps yet. They had had an instrument for determining fundamental positions of stars for quite a while in Chile. But I don't know any detail about this large telescope.
In the past years how has your contact, Leiden's contact, with the Russian astronomers been maintained, especially with those working on galactic research? Has the contact been relatively free?
Yes. Certainly in writing and also in personal relations. I have had very good contacts, in particular, with Ambarzumian in Armenia and with Ginsburg in Moscow and the group around Zeldovich studying the structure of the universe in Moscow. The only limitation, a serious one, is that they are not free to leave their country and so when we invite them to symposiums or discussions in Western Europe they're often not able to come although they certainly would like to attend. It just depends on a decision somewhere in the Academy or in the government. One can not trace where the difficulties lie. That is a serious limitation and I wish something could be done to alleviate that problem.
It doesn't seem like there's any great hope in the near future?
No I don't think so.
I'd like to ask you some directed questions about the research we haven't talked about yet, your work and interests in the 40's on determining the origin of the particles in the interstellar medium. Also I'd like to ask about your interests in comet clouds, in the origins of comets, again dealing with meteroritic origins and that sort of thing. Could you give me an overview of how your early interests developed? I know that you had set up a prize, in 1941 for anyone who could give an acceptable model for I guess an interstellar medium particle.
How they would originate.
Right, how did you come to decide to give a prize for an essay, to stimulate research or what?
To decide a subject on this subject you mean?
Well it was a natural question to ask if you are interested in the problem of interstellar absorption in connection with the structure of the Galaxy. It was evidently of importance to know a little more of how the absorbing particles would originate and how they would grow. Even if you cannot entirely outline a project that would lead to useful information in interstellar absorption. It was clear that you should begin in this direction. So those were the general considerations. You were mentioning the comets. There is not the slightest relation between my interests in the comets and the interest in the solid particles in interstellar space, although now-a-days some people think that the comets can be formed in interstellar space; I think this is very doubtful.
How did you come about your theory that they were formed from an explosive event between Mars and Jupiter? Was this a popular theory at any time?
No and I didn't stick to that theory very long. I had a student at the time who was interested in problems in celestial mechanics. I don't remember exactly how his interests arose but he was interested to investigate the effect of perturbations by Jupiter on the orbits of comets. And that's how my own interests arose. And so when he finished his doctorate's thesis which was wholly concerned with the special case of the Jupiter perturbations on comet orbits, an important aspect was left unresolved. I couldn't very well let it stand and this brought me on to this subject of studying the statistics of comet orbits in a more complete way. An explosion of an initial major planet in the minor planet region is a possibility for the origin of the comets, but later on I became much more inclined to think that they were just smaller bodies that were formed in the early times of the formation of the Solar System and which did not move in beautifully circular orbits, but in ellipses which brought them under the paternal action of the major planets such as Jupiter, Saturn, Neptune and so on.
Had you talked to Kuiper or read his papers on this at any time? You had contact with Kuiper?
I had contact with him but not directly on this subject. If I would have to select one piece of research with which I've been better satisfied than with other investigations I would mention first the problem of the comet orbit, because in my opinion this gives a complete picture and model such as one can rarely get anywhere in any subject.
Was the understanding that their major axes were such that it took them out to the point where stellar perturbations became important? Was that the element?
Yes, right, but they would have originated in the central region of the solar system, where the big planets are. Perturbations of the major planets would have gradually brought them into more and more elongated orbits until these orbits would have grown to the dimensions of the order of fifty hundred thousand astronomical units. At that time the stellar perturbations come in and instead of what one would think... (tape went blank.)
You were talking about how the orbits are limited by this, what would it be called, a tidal radius almost?
Something like that, yes. At first sight one could think at that point, the effect of stellar perturbations would be added to the effect of the perturbations by the major planets so that they would only disappear even more quickly from the solar system than they would have done without this extra disturbing force, but actually the reverse happens. The stellar perturbations act only on the very distant part of the orbits; they change the orbits slightly in these outer parts but enough so that they do not return to the region where the big planets are, and so, although the stellar perturbations continue to work on them and will eventually evaporate the whole set of them, these external forces are so much smaller than the perturbations of the planets that they can now subsist for a long time. As soon as they get out to this distance, of about 100,000 astronomical units the stellar perturbations trap them in a region where they will no longer be influenced by the big perturbations of the planets and where they can stay for the whole age of the solar system before the stellar perturbations actually throw them out. This was in the early time of the solar system, but afterwards the stellar perturbations will gradually change the orbit and gradually diffuse them and by accident some comets will get orbits which carry them back again into the inner part of the planetary system. Those are the new comets that you actually see appearing. Its kind of a reservoir that has been formed in the early times of the solar system by this somewhat queer combination of planetary perturbations and stellar perturbations and now serves to feed comets into the solar system again in a gradual way.
That is a very nice model. Could you just for a final thing on the tape, give me an impression on how you feel about the history of your research in galactic structure. You mentioned that this comet problem was a very gratifying one. How do you feel about your certainly incredibly influential work in galactic structure over your lifetime. How do you think this research will go in the future?
That I don't know at all. But at the moment I'm more deeply interested in the phenomena that are happening in the nuclei of galaxies including our own Galaxy. That interest was also started by radio astronomy. One of the first things we observed when the 25 m telescope was finally installed in Holland was the so-called 3-kpc expanding arm in the central region of the Galaxy; the question arose whether this had been expelled from the center or whether it was just a peculiarity of, or an assymetry in the gravitational field of the Galaxy. This started us out on the general investigation of all these radially moving features that you find in the central region of the Galaxy. That's a wonderful subject because it brings one into contact with the machinery that must exist in the actual center, possibly a black hole, and which may also exist in other galaxies. And this ties into the research that is going on in Westerbork at the present time on radio galaxies, and their explosive processes, as revealed by their radio jets and the structure of the radio lobes.
Thank you very much for this.
MM 118 (1958) p. 379.
BAN #23 (1922, Sept. 1) p. 133.
BAN III (1977) #120, p. 275.
GRONINGEN PUBLICATION (1926)
BAN III (1927) #120, p. 275.
BAN VI (1932) p. 249.
GRONINGEN PUBS (1926) "The Stars of High Velocity" see p. 55.
(w. N. U. Mayall) PASP (November 1941) p. 95.