[Oort, 1953]
Jan Hendrik Oort, 1953



NATIONAL RADIO ASTRONOMY OBSERVATORY ARCHIVES

Papers of Woodruff T. Sullivan III: Tapes Series

Interview with Jan Hendrik Oort
At Leiden
6 September 1978
Interview time: 65 Minutes
Originally transcribed by Pamela M. Jernegan (1979) as typescript only, retyped to digitize by Candice Waller (2016)

Note: The interview listed below was originally transcribed as part of Sullivan's research for his book, Cosmic Noise: A History of Early Radio Astronomy (Cambridge University Press, 2009). The original transcription was retyped to digitize in 2016, then reviewed, edited/corrected, and posted to the Web in 2016 by Ellen N. Bouton. Places where we are uncertain about what was said are indicated with parentheses and question mark (?).

We are grateful for the 2011 Herbert C. Pollock Award from Dudley Observatory which funded digitization of the original cassette tapes, and for a 2012 grant from American Institute of Physics, Center for the History of Physics, which funded the work of posting these interviews to the Web. Please bear in mind that: 1) This material is a transcript of the spoken word rather than a literary product; 2) An interview must be read with the awareness that different people's memories about an event will often differ, and that memories can change with time for many reasons including subsequent experiences, interactions with others, and one's feelings about an event.

Click start to listen to the audio for tape 115A of the 1978 interview.

Begin Tape 115A

Sullivan

This is a follow-up interview with Professor Oort on 6 September 1978 at his office in Leiden. Well, first letís just cover these points from the previous interview, which was over six years ago now. I was interested in the very earliest idea that you had after the War. Was your plan just to get a general purpose instrument Ė were you thinking only about microwaves or rtzberg you thinking about all wave lengths? Can you just tell me what your idea was at that time?

Oort

We certainly were thinking only of microwaves. As you know, I was especially inspired by the results that Grote Reber had obtained and from those I realized at once that this would be the given ideal means of investigating the galactic system. Of course, Reber had started already something in this line, but it was clear that his beam was much too large for a real investigation of the galactic system. And so the idea was to build an instrument of 25-meters aperture, with a ½° beam or so.

Sullivan

Right from the very beginning?

Oort

Yes.

Sullivan

I see. And where did that number come from? Why 25-meters? Why not 50? Or 10?

Oort

That is a question I canít answer. I mean, it was clear that we needed much better resolution than Reberís instrument gave and the 25-meter dish would give ½° resolution which should be enough to measure the thickness of the galactic layer even at distances of 10 and 20 kiloparsecs. That was the general idea with the 25-meter.

Sullivan

I see.

Oort

But we started off by using much smaller instruments. These Würtzberg reasons which had been an inheritance from the Dutch army.

Sullivan

But let me ask you now, did de Voogt bring those to Kootwijk even before you asked him to do anything?

Oort

Yes.

Sullivan

And what was his purpose for doing that?

Oort

He was also interested in astronomy in a general way, but mostly in solar astronomy in connection with the general problem of interference with the telegraph service. So thatís also the way he obtained permission from the General Directorate of the PTT in Holland to do this kind of thing.

Sullivan

I see. And you just found out that he had these dishes there. How did that go, can you tell me?

Oort

I really have to think. I suppose that Professor Minnaert from Utrecht Observatory, who was a famous solar astronomer, had been in contact with Mr. de Voogt or the other way around, Mr. de Voogt had taken up contact with Professor Minnaert, and it was through him that we got this contact. Professor de Voogt was really interested in pursuing general astronomical research and he was extremely liberal in putting means at our disposal making these instruments so that we could actually use them for our purpose. So in this way, we are greatly indebted to him for this beginning.

Sullivan

So this sort of research was obviously not really necessary for the PPK by any means?

Oort

No. Of course, Kootwijk was not a very favorable place for these observations because -

Sullivan

Indeed.

Oort

It was full of transmitters. On the other hand, of course, they had a big laboratory and that more than made up for the disadvantage.

Sullivan

Well, and another point that Lex Muller has made to me is that he learned how to very carefully shield equipment, because he had to operate under adverse conditions, he became much more skilled in these things.

Oort

That may have been an advantage. Still, we were glad to escape to a quieter place.

Sullivan

Oh, sure. Was the idea to look at both the continuum and the line radiation from the beginning?

Oort

As I said, we had vague hopes that this line radiation would be strong enough to measure, but certainly these hopes were not sufficiently founded that we would ever have felt responsible to propose, to make an extensive instrument just for that. And so the sound foundation for the instrument was the measurement of the continuum radiation which we knew was sufficiently strong to be measured.

Sullivan

I see. However, of course, you could build a receiver which could do a front end anyway which could take care of both of these things, and so it was obvious that you wanted to try to build -

Oort

Yes, you need great sensitivity in any case.

Sullivan

Now, however, the first observations, I believe, were of the hydrogen line. There were no continuum observations carried out before the hydrogen line. Is that correct? Except the test observation on the sun, but there was no galactic mapping.

Oort

No, I believe that is correct.

Sullivan

So at some stage, you must have changed and said, ďOkay, weíll do the hydrogen line first, rather than doing the continuum.Ē

Oort

Of course, we were more interested in the hydrogen line than in the continuum because it promised more, but I would have to look up the old records to see exactly what the plans were. Because the receiver that was being built by Mr. Hoo was really the first receiver we have built and this could be for both purposes. But at the time when this receiver was almost getting into operation, we had apparently decided to concentrate all efforts on the line first.

Sullivan

Right, so somewhere in there, there must have been a shift. Let me ask a general question; you mentioned Mr. Hoo and then of course, Muller came along and was much more successful. The general Dutch philosophy of engineers and scientists, it seems to me that there is a more well-defined role for the two categories of people than there are in other countries. Would you agree with that?

Oort

Iím not quite sure that I understand your question.

Sullivan

Well, that scientists do not get so involved in building equipment as in other countries.

Oort

Oh, no, certainly not. And that was partly due to our isolation during the war years. By which no radar equipment was being built here, and partly also due to the fact that the only people who were building equipment were people connected with industry and with (Philips?) in particular who had no direct interest in science or in astronomy. And they had their hands full, so to say, with their own industry. Because they were also behind when the war ended. Also for the same reasons, it was difficult for us to find quite competent people to start this. So in that way we started at a very low level, as I say, technically.

Sullivan

Yes, but you are saying then that this is not particularly the way that science is done in Holland. This is something that was more peculiar to radio astronomy. I was wondering if there was a Dutch way of doing things in science.

Oort

No, it was, I think the way radio astronomy started was somewhat different in Holland from the way it started in other countries because in Holland it started very specifically by astronomers who were interested in certain astronomical problems. And they just wanted to engage engineers who could work for them, so to say. While in other countries it started the other way around, and there were technical people who then became interested in the question of whether they could use their techniques for astronomical purposes.

Sullivan

Eventually they engaged astronomers to - A related question is the relationship between the radio astronomers and the optical astronomers. First of all, letís just say here in Holland, was it looked upon as sort of a strange activity that didnít really have much relevance to photometry and traditional astronomy, or was it realized that this could say something too. What sort of attitudes did you run into amongst your optical, more traditional astronomers?

Oort

No, I donít remember any conflicts of that kind. Conflicts arose only at a very much later stage when so much of the money that was available went to radio astronomy and relatively so little to optical astronomy. But in the early days most of the astronomers certainly Leiden and also in Utrecht were very much interested in extending their possibilities by radio astronomical methods. In Utrecht they were interested in the sun mainly and the problems that the sun presented. It was evident that also there the radio research could open new vistas.

Sullivan

Now you say at a later stage that there was some conflict. Are you talking about the Dwingeloo dish or are you talking about even later, the Westerbork era?

Oort

You mean at later times?

Sullivan

Yes, you said at later times.

Oort

Oh, that was only much later. I would say only during the last five or ten years.

Sullivan

Oh, in the seventies youíre saying. Okay.

Oort

In the early day and in the development it was clear to everybody that this was such a fertile field and that we had already at an early stage a rather prominent position in the world (?) and that we should pursue that if possible. So there was general agreement on that, I think. There was no fighting among astronomers.

Sullivan

Now what about on an international scale? I have heard some people who have talked about disparaging remarks about, unkind remarks that some prominent optical astronomers made about radio astronomy, saying it was a waste of time shouldnít be worried about.

Oort

In the United States, you mean.

Sullivan

Well, specifically, yes. But did you run into any of these attitudes amongst your international colleagues?

Oort

No, not at all.

Sullivan

Really. Okay, another general question: how was it that the research directions were determined here in the Leiden Sterrewacht? Was it a matter that you finished one project and you said, ďNow what does this lead to for the next project?Ē Was it a matter that you made the decision which way to go? Or were there round table discussions about which way to go? Can you just tell me something about how this decision making process would go on?

Oort

You want to know whether I was a dictator or not? (laughter)

Sullivan

Iím just curious.

Oort

No, I always had the feeling that these things grew in a natural way. That was never a moment in which important choices had to be made. Because the progress of the work depended largely on what could be made in the way of receivers, could be made technically, and our technical staff was limited. And was largely guided by Muller during the long time of the Stichting. He was a very practical man, and I had sometimes disputes with him that I would like to have things made more quickly, but that was about all.

Sullivan

Sure, these things are natural.

Oort

(?)

Sullivan

But what youíre saying is that -

Oort

As far as programs were concerned, it was clear that we had to concentrate mostly on the hydrogen line work because there was still an unlimited amount of work to be done in that field, but we did make in the meantime, we did make also continuum observations at several wave lengths. But I donít remember there was ever a serious competition between the two Ė there was enough time for the continuum also.

Sullivan

Yes, letís take as an example. For instance, Westerhoutís continuum survey of the Milky Way, was this an idea that a student would come and say, ďI donít want to do thisĒ or did you have a list of these things and students would choose which they were most interested in? Or did you say, more or less, do a promotional job to sell it to a student?

Oort

Yes, again, I donít think there was any very definite plan. I mean it was natural that we would do continuum observations, and it was also natural that those should be discussed. Itís natural that you find students who are willing to do this. The survey of discrete sources was, of course, a natural outcome of that also. So I -

Sullivan

Are you saying that -

Oort

I see things (?)

Sullivan

Are you saying that there are no difficult decisions, that they all come naturally? At least in the way you see it.

Oort

Thatís the way I see it, yes.

Sullivan

Okay. I think this is perhaps why youíre able to make so many contributions Ė that it all comes naturally to you. Okay, letís -

Oort

It may be if you talk to Professor Blaauw. He may give a somewhat different view. Of course, that was at a slightly later stage when Ė you see in the early stages, it was principally the Leiden Observatory, which was engaged with the galactic research. But later on, when Professor Blaauw returned from America, he also entered the game and he quite naturally wanted to design his own programs, and so he started concentrating more on the individual structure of clouds and high latitude observations.

Sullivan

In Groningen.

Oort

Yes, yes. But again, I donít think I ever, there was ever a conflict there because we were never that short of observing time at that stage that we couldnít do all that we could find time to reduce and to discuss.

Sullivan

Okay, letís move on. Another question which may be difficult for you to comment on, but Iíll try anyway, it seems that if one looks especially in the era of the fifties that there were large number of very good students who came through the Sterrewacht here, and many of them went to other countries and are still leading astronomers. Can you give any, shed any light, on why there were so many good students Ė what do you think were the policies that you had that led to this success in training astronomers?

Oort

Well, I think there was a difference, for instance, between the Netherlands and perhaps I should say more in general Europe and might include England, for instance, and the United States. On the other hand, at that time, my impression was that in Holland for instance, the young students we got in astronomy were, had a stronger vocation for this special subject; while my impression in the United States was often that they took it more, they chose a profession, so to say, and this could be physics in on case or mathematics or astronomy.

Sullivan

By vocation do you mean a real love of the subject, when you say a strong vocation?

Oort

Yes. So the love that they had already in the school years (?) from (?) and (?) and so on, that played a larger role in Holland than in the United States. I donít know whether Iím correct in supposing this, but -

Sullivan

Thatís an interesting comment.

Oort

But for instance, Maarten Schmidt was of that kind, and I think Westerhout also. But for the rest, I think itís largely a matter of accident also that at some time, some period you have many good students and during other periods, there just happens to be less. Right now we have a good many very good students here. Groningen used to have, or had a many very good students a few years ago but theyíre short of students now. There is fluctuation. Thereís perhaps no deeper meaning behind that.

Sullivan

But you donít have any particular things about the way a graduate student is trained here that you think are important?

Oort

No, I donít think weíre any better, certainly not than many places in the United States.

Sullivan

Okay.

Oort

I think there are many places in the United States where they are trained much better than here, in theoretical ways for instance.

Sullivan

Iím sorry, I should have asked this earlier, back to de Voogt at Kootwijk Ė did he ever carry out solar observations? De Voogt at Kootwijk. After all the trouble of getting these dishes and so forth, did he ever do anything with them?

Oort

Well, in a way yes. For a long time, he had continuous survey of the sun at one or two frequencies.

Sullivan

But was this just after the war also?

Oort

Yes. I donít think that he ever discussed those himself. They were always discussed by astronomers at Utrecht at the Utrecht Observatory. But whether they ever had a combined publication about it, I donít know at the moment.

Sullivan

I donít think there is.

Oort

Youíd have to ask Mr. Fokker perhaps he would -

Sullivan

In fact, Iím going to talk with him on Friday, so Iíll ask him that.

Oort

He would know.

Sullivan

Okay. Letís look at a couple of specific papers that were neglected last time. The first one was the paper you wrote with Westerhout in 1951 on the Nature of the Galactic Background, and you proposed a model where it was due to an unknown type of star which had such-and-such a density per kiloparsec and so forth. Now earlier you call this a sin in that, sin being defined as later on turning out not to be correct. But what Iím interested in is how did you view the subject at that time? Why did you propose, why did you choose to go that way as opposed to something to do with the interstellar medium?

Oort

Well, it was quite evident that these sources were not distributed in a way that resembled the distribution of the interstellar medium. There were far too many at high galactic latitudes. But it was not at all clear at that time that one was dealing with a mixture of very different sorts of things, objects that were actually in the galaxy and extra-galactic sources. The idea of the extra-galactic sources had hardly arisen at that time as I remember.

Sullivan

Sure, right.

Oort

So we tried to find a way of fitting them into the galaxy by hypothesizing about a special type of object that would have this queer distribution, some concentration to the galaxy, on the other hand very many at high galactic latitudes. But I would have to reread the paper to actually remember exactly what we thought at the time.

Sullivan

But there were no identified extra-galactic sources when you wrote this paper. And of course, the radiation you were trying to explain we now ascribe to synchrotron radiation from the (?) so why did it seem -

Oort

That idea had not arisen at all.

Sullivan

Right. But why was it more acceptable to you to propose a new kind of star rather than a new kind of interstellar medium with a different distribution from the -

Oort

Well, I suppose because we were so much impressed by the fact that everything we knew about the interstellar medium indicates the very thin layer and that there were no phenomena at all that pointed to the possibility that it would rise to greater heights above the plain, and then again, the discrete sources of course, didnít make one think of an interstellar medium as the ultimate explanation of these sources.

Sullivan

And they, of course, had more or less isotropic distribution, the ones that were known at that time.

Oort

Yes, which was surprising also for start, of course. But -

Sullivan

This made you have to have a lot of these things, because they had to be nearby and therefore, low luminosity.

Oort

But why we did not think of the possibility of extra-galactic objects at that time, I donít know. Of course, one is apt to forget at the present time that extra-galactic things were something a little bit out of the ordinary at that time. One knew about spiral nebulae and so on, but it was quite clear from these observations already that these objects were not related to bright galaxies. And it would have been quite a novel thought to think that it would be connected with galaxies that were near the invisible like the Cygnus A -

Sullivan

Oh, I agree.

Oort

That was a jump that we couldnít, at that time, be supposed to make.

Sullivan

Oh, I agree wholeheartedly. Also, of course, the luminosities that were implied if you put them at large distances, were rather frightening.

Oort

That was a very big jump. And that could only be made after the discovery of the Cygnus A source.

Sullivan

Right. Okay. Another paper was the AAS Russell Lecture which you gave in December. 1951 on the Optical and Radio Galactic Structure. And this was before the Kootwijk survey had been taken, but clearly you had gone through all the methodology that you were going to use. You had it all in your mind exactly how you were going to do this. It was just a matter of getting the actual numbers. Is that correct?

Oort

Again, I would have to glance into the paper to remember exactly what I thought.

Sullivan

If we look, letís see, itís - [shuffling of paper]

Oort

We were certainly interested in the spiral problem of the galaxy, was of course -

Sullivan

See, here youíre talking about the theoretical profiles at different longitudes and so forth. And you talk about the plans to construct the Dwingeloo dish even though the Kootwijk dish had still really not begun its survey.

Oort

Yes.

Sullivan

But I get the impression that you saw all this happening in the next few years, and as I said, it was just a matter of getting the actual numbers. You could almost write the paper, you just had to fill in the numbers. Is that fair?

Oort

I think that is correct, yes. Because at that time the -

Sullivan

You also talked about the continuum radiation, of course.

Oort

This was mostly on the continuum, which was at that time a surer basis for discussion, because it had been discovered already. But the 21 cm. line had been discovered, so at the end I did go into that, and we already had in our first paper observed some line contours.

Sullivan

Right.

Oort

Various longitudes. I donít remember whether the actual rotation curve of the galaxy had been discussed already.

Sullivan

Well, you have a figure here.

Oort

Oh, yes.

Sullivan

Okay, the dots come from a model based on stellar observations.

Oort

Yes. At that time, we had made one observation already at Kootwijk, and so, and that agreed with the expectation of the rotation curve. We had the interest of course, in the structure and the dynamics of the galaxy for a long time before we had the advent of astronomy and so we had built models, mass models already, before and during the war and it was evidently very interesting to us to try to compare those with what one would expect the line profiles to look like.

Sullivan

Do you remember, you probably went to other places in America when you gave this lecture. Did you visit any radio astronomy installations at that time?

Oort

I certainly visited Harvard, but -

Sullivan

Do you have any reminiscences of that visit?

Oort

I would have to look it up; I donít know now. Probably also Cornell, but I donít remember if that was during this year. Maybe not. But at Harvard they had started some Ė though Purcell and Ewen never continued the work. But it was taken up rather soon by Bok.

Sullivan

Right. I talked with him, of course.

Oort

But I donít remember exactly what year that was.

Sullivan

Okay. Letís move on to the paper on the polarization of the Crab Nebula which you did with Walraven. You presented this at Jodrell Bank and then, of course, wrote it up later in BAN in 1956.

Oort

That gives some of the history.

Sullivan

Yes, it does. A historian loves to see these sort of things you love to give here. Although itís a bit more chatty than one can afford to have in todayís papers, I think, wouldnít you agree?

End Tape 115A

Click start to listen to the audio for tape 115B of the 1978 interview.

Begin Tape 115B

Sullivan

Continuing with Professor Oort on 7 [i.e. 6] September 1978.

Oort

I had been interested in the Crab Nebula for some time. And had written a paper with Mayall and Duyvendak Professor of Chinese languages in Leiden.

Sullivan

This was 1942, I think.

Oort

It was much earlier, already during the war; and after the war, we continued those and Duyvendak in particular looked up several new sources after the war, but that was at the time, was the end of it, except that I remained interested in the object in the Crab Nebula itself, and the observations which Mr. Walraven did at the time when this polarization was discovered were not to discover polarization because neither of us had ever thought about the existence of polarization in the Crab Nebula at that time, but to see whether we could measure the expansion of the Crab Nebula by determining the total light intensity as accurately as possible.

Sullivan

Over a period of several years?

Oort

Over a period of several years. But when this polarization was discovered, it was just the beginning of it, so the first epoch, so to say, of measurement of the total brightness. Then because I was interested in these observations, I joined him at the telescope in, a very cold winter it was, and -

Sullivan

Right, and the whole winter you were only able to get a total of twelve nights. Sounds like a typical -

Oort

It was a very clear winter and very cold, and also the city light at that time were not too bad, so one could make photometric measurements even with an instrument in the center of the city as this telescope was. But Walraven had the idea that well, why not measure polarization at the same time. And we have this separator, he was a very good instrument man, and then he found a very large polarization and became, we became, very much interested because the polarization at once seemed to be larger than one could account for by interstellar scattering. And that was the only thing at that time that we could think of getting polarization.

Sullivan

But let me ask, in your paper you say it was decided to check the discovery by the Soviet astronomers of the polarization of the light, so you had heard about their results. In fact, I believe you visited the Soviet Union.

Oort

Right, yes.

Sullivan

Can you tell me about that?

Oort

When I visited the Soviet Union Ė at the time, that was the time of the dedication of the new Pulkovo Observatory when for the first time a delegation of western astronomers visited the Soviet Union. And at that time I had several discussions with Shklovsky and Ginzburg, I believe also, but at that time I did not know about the polarization results that they had found. I understood only very incompletely what they were telling me. They had developed this idea of the synchrotron radiation to quite considerable extents.

Sullivan

Had you heard about this at all before you went to the Soviet Union? About synchrotron radiation?

Oort

No, no, no.

Sullivan

Not even the Physical Review articles by Alfvén and Herlofson?

Oort

No. I saw those only later. And the name synchrotron radiation we designed ourselves, so to say, that didnít exist at all before we wrote paper. But although I had not understood the subjects very well when I was at this, I mean it was just short discussions, just during receptions, and so I did remember enough of it to think of the implications in connection with this polarization. So it was only when we got these polarization observations that I, for the first time, started to think seriously about what they had told me in the Soviet Union. And then I studied these theoretical papers but if I hadnít been in the Soviet Union and had not heard these vague reports during receptions, I would not have, I would never have had any idea about it.

Sullivan

Right. And once you began thinking about this synchrotron radiation, was it clear to you that thatís what was happening in the Crab Nebula? This idea must be right?

Oort

Oh, yes.

Sullivan

What about for the galactic background Ė could you make that jump and say, ďWell, that must be synchrotron radiation, also?Ē

Oort

Yes. But that we did very much later, and that was much more difficult. I mean, in the radio waves, -

Sullivan

To actually measure the polarization.

Oort

That was actually measured by Seeger and (?), but that was rather later.

Sullivan

Right, but Iím asking you in terms of what you were thinking. Once you learned about the synchrotron mechanism, did it seem to you that this was the best candidate for the galactic background also?

Oort

Yes, but not at all certain that that could be measured properly. And that it would not be depolarized and so on.

Sullivan

Yes. Youíre talking about some actual observational proof. Well, what Iím saying is not so much the polarization aspect, but just what your feelings were on the nature of the galactic background. I mean, as soon as you learned about synchrotron, did you just flip over and say, ďWell, the galactic background is clearly synchrotron.Ē

Oort

Oh, yes. About the same time, we realized quite clearly.

Sullivan

Was that the general case amongst radio astronomers, do you feel? Or did they -

Oort

I suppose so, after it became clear that the background radiation was only partly thermal and partly quite steep spectrum. It must have been generally realized.

Sullivan

There was no good competing theory, I donít believe.

Oort

No. No.

Sullivan

Anyway, I interrupted you so you measured the polarization here and you also got some very nice plates from Baade.

Oort

Yes, and the year after, it was the year of the IAU meeting in Ireland (1955), we went to a cosmic ray symposium in Mexico and there they were terribly interested, of course, in these measurements. After that, I went on to Pasadena and talked to Baade about it. And he had just the day after I arrived, I think, he went to Palomar, he had a few nights in Palomar and immediately took the pictures. They were really striking.

Sullivan

I see. Which were then eventually analyzed by Woltjer.

Oort

And then I became interested in the general problem of the Crab Nebula and the central star of it and the filaments of it, the moving filaments which Baade had just told me in the central part.

Sullivan

Speaking of that central star, here you mentioned that it had been suggested by Baade and Minkowski that the south-preceding component is the remnant of the supernova. Did you ever consider making special observations of this star, or did you feel that there was not much that could be done?

Oort

Oh, yes. But apparently, at that time, they couldnít do much about it. They had, their best spectra didnít show much and so . . . because they were capable, sufficiently capable observers to have done what they could.

Sullivan

Okay. Letís see what else we have here. The expanding arm which was with Rougoor at the Paris Symposium and the next couple of years. Also with van Woerden in 1957. Can you tell me about those observations? That was with the Dwingeloo dish, was it not?

Oort

Yes.

Sullivan

Was that a complete surprise or did you expect that there might be activity in our nucleus?

Oort

Not at all, no. I didnít expect it at all so it was an entire surprise. I donít know whether you have a reprint of our article in the Comptes Rendus at the Paris Academy?

Sullivan

I do have a copy of that. Hugo gave me one.

Oort

Oh, yes. No, that was an entire surprise. To everybody. Of course, we thought from the first that this was something that was really expanding and was due to activity in the nucleus. Later on, I began to doubt that again, or to feel less certain about it.

Sullivan

Youíre talking now in the last ten years.

Oort

Yes.

Sullivan

But at that time you were quite clear that it was expanding?

Oort

It was expanding certainly, but we always felt that we couldnít exclude the possibility that it was due to some Ė to the fact of the dynamics of the galaxy in this inner region, might be more complicated that we had considered so far. That you would have the symmetry structure so, in the mass distribution that could account for it. But, so that idea has been existing for all these years, since this discovery, that it might be both things. It might be something that had been expelled or it might be dynamical feature. And for the three kiloparsec arm, Iím not at all sure at the moment which is the right interpretation. Since then, of course, many other expounding features have been found closer to the center, say within one kiloparsec of the center.

Sullivan

Right. But that was not known at that time, right?

Oort

That was not known at that time. Well, not at that time, but rather soon after, we of course, took this up right away and looked at the profile near the galactic center in a more extensive way, and found that there must also be matter behind the center, expanding away from the center which had not absorbed in the Sagittarius A, so this subject developed right away so to say. From the three kiloparsec arm to a wider subject.

Sullivan

Right. You mentioned the absorption, I donít think that you were ever an author on those papers, that was from Muller.

Oort

Which one?

Sullivan

The absorption of the hydrogen, by the hydrogen of various continuum sources, Sagittarius and Cassiopeia -

Oort

No, Sagittarius A, of course, in this three kiloparsec arm, but not, no for the rest. I never worked on specifically.

Sullivan

Right. Did that give you any new insights into the interstellar medium, would you say?

Oort

Not so very much. I mean, of course, it gave some insight into the detailed structure of the interstellar medium that must be quite discrete clouds of considerable optical thickness, and small velocity rates, and so, but for the large scale structure of the galaxy and the dynamics, it did not yield any very new insights.

Sullivan

What about the possibility of measuring distances through seeing whether various arms -

Oort

That, of course, is important.

Sullivan

But I guess that could only come later when there was more sensitivity so you could look at these . . . In the fifties you could only look at the very few biggest -

Oort

It was a very early paper by Muller on absorption in which he found absorption in theCassiopeia source, for instance. And that was already gave some information on the distance of Cas A.

Sullivan

Right. The NRL group also made such measurements. And that had a battle with Minkowski, which you may remember, about the distance.

Oort

Yes, yes.

Sullivan

About Sagittarius A, was it always - what was your opinion as to whether it was at the galactic center -

Oort

(?)

Sullivan

Whether or not Sagittarius A was at the center. There was some -

Oort

So far as I remember, I never doubted it.

Sullivan

There was some controversy though, was there not? Maybe not in your mind.

Oort

And I donít even doubt it now, though some people still doubt it. But itís the exact center of the very compact source at the center.

Sullivan

Well, if I could just ask you about your memories of a few key meetings.

Oort

Of who?

Sullivan

A few key meetings. In Rome in 1952, was the first time, I believe, that radio astronomy results were presented at an IAU meeting.

Oort

Probably, yes. And I just looked at the reports of this meeting and I saw that in the report of Commission 33, on galactic structure, there was no mention of it, though I was president of that committee at that time. But I mentioned in the beginning of this report that I did not consider any papers after 1948 so -

Sullivan

Oh, I see.

Oort

But in the report on interstellar matter by Struve, Commission 34, there was a brief mention of it, just two paragraphs on that page.

Sullivan

And also, I believe, the announcement of the identification of Cygnus A was given at that meeting. Do you remember that?

Oort

I donít remember, but it is very likely.

Sullivan

What about the Jodrell Bank Symposium in 1955, do you have any specific memories about what the important issues that were discussed at that meeting?

Oort

No, of course, that came at a time when our general survey of the spiral structure of the galaxy had been completed, more or less, and that was reported on. But I donít remember any very special things, because the expanding arm signals had been discovered at that time and would have been just a report on the HI distribution and motion in the galaxy.

Sullivan

Yes, well Gart Westerhout has told me about hauling this huge model, which is a big -

Oort

Yes, that was prepared for that meeting; that is true.

Sullivan

And also for the Dublin IAU meeting.

Oort

Yes, that came right after. But that was, of course, also the time when I, myself, was deeply interested in the Crab Nebula.

Sullivan

This is the main thing that you were talking about then?

Oort

At that time, that was the foremost in my mind, Iím sure.

Sullivan

Well, moving on to the Paris Symposium in 1958 where you did talk about the three kiloparsec expanding arm, but it seems to me that that was an important meeting in that it was a dividing line when radio astronomy was a rather small group of people who could all get together, and then later on it very soon became sort of big science.

Oort

Right. It was a very nice meeting. Especially since it was limited, the attendance was limited and so one could discuss things. And one could discuss things and one could get a fine general survey of the whole subject of radio astronomy, and that was the last time.

Sullivan

Right, in one week you could do that. Just a couple of last questions of general nature. Iím sorry, there is one more specific question. This has to do with the Benelux cross, which we did discuss in the last interview, and I found quite a bit in the archives about it, but I was wondering. There were several different project leaders, so to speak, Christiansen, Högbom, Erickson at various stages. Do you think that hurt the project in terms of continuity, or do you think that things would have gone along at about the same speed if youíd had one person throughout that time?

Oort

It would have been much better if we had had one person, but there was a problem of finding one. We certainly have been extremely happy if Christiansen could have remained for a longer time, but he had his obligations in Australia and his family to think of. But certainly, the changes in the technical staff and direction have been a great disadvantage, but that was not the only problem. The second problem which we discussed, I think, the time before was the cooperation with the Belgians and this did not develop in the way we had hoped it would develop.

Sullivan

I found the minutes of the meetings of the -

Oort

Thatís a lot to look through.

Sullivan

Which give a pretty good picture of how it developed, although Iím sure thereís even more than that. Okay, a couple of general questions to close. What would you say are the fundamental ways in which radio astronomy has changed astronomy, especially through about 1960?

Oort

Thatís a big question. What do you want me to say?

Sullivan

Well, I mean, is it simply that itís a new wavelength region? Is it because it has made it into a violent universe?

Oort

Well, I would say in the first place, the possibility that radio astronomy offered to penetrate to larger distances than could be reached before. Now you may say, of course, that later on the discovery of the radio quiet quasars has shown that we might have penetrated quite as far by optical means -

Sullivan

But they were discovered through radio astronomy.

Oort

They were discovered through radio astronomy, yes. Otherwise we would never have found these. So that, I think, is probably the most important influence that radio astronomy has had on astronomy at large. And the second point was as you said already, the discovery that very much in the universe is of an explosive nature and because before the advent of radio astronomy, this had not at all been realized by astronomers. I mean, they knew there were supernovae and novae, but those were not considered as something that was an essential feature of the universe. I mean, there were things that concerned individual stars, but that explosion phenomena would become of such prime importance as they are now with quasars and in general, nuclei of galaxies, has certainly been brought about largely through radio astronomical methods. Of course, Seyfert galaxies were known and discovered optically as we know now are very grossly related to the more violent things like quasars. But before radio astronomy come to the foreground, these were again, considered more like rather special objects. But that this explosive nature would be such an important element of the universe has only come to be realized through radio astronomy, I think.

Sullivan

What about in the way astronomy is done? Do you think that radio astronomers have had a fundamental influence on the way observations are done, the way instruments are built?

Oort

Oh yes. Especially in these last few years and of course optical observers have begun to use methods that one might say were borrowed from radio astronomy. Certainly the coming of radio physicists into the picture has had a large influence also on the development of instrumentation in other wavelengths, in optical wavelengths in particular.

Sullivan

Okay. One final question. You, of course, have always been very much interested in both radio and optical results as well as the theories.

Oort

And infrared.

Sullivan

Well, now infrared. Iím talking about before 1960. However, most people at that time were either in one field or the other field I think itís fair to say.

Oort

Quite wrongly, I think.

Sullivan

Quite wrongly?

Oort

Quite wrongly, I think.

Sullivan

Oh, okay right. But did you consider yourself a bridge between these two? Were you trying to convince people that they should get together?

Oort

No, Iíve never been very much of a preacher in that way. No, to me, it was entirely natural that you should look at all possibilities that could further the insight into the problems in which you were interested in. If something offered itself at another kind of wave length, it was only too natural to look at that also.

Sullivan

Right. Well, thank you very much. That ends the follow-up interview with Jan Oort on 6 September 1978 at his office in Leiden.

End Tape 115B


Modified on Tuesday, 19-Jul-2016 13:16:55 EDT by Ellen Bouton, Archivist (Questions or feedback)