Interview with Jan Hendrik Oort on 6 September 1978

Jan Hendrik Oort, 1900-1992. Interviewed 6 September 1978 at Leiden, length of interview: 65 minutes.

Papers of Woodruff T. Sullivan III

NRAO/AUI/NSF

Oral History

Sullivan, Woodruff T., III

Oort, Jan Hendrik

Audio cassette tape

65 minutes

1978-09-06

Follow-up of 1971 and 1972 interviews; a few research topics missed before (theory of galactic background, Crab polarization); how Leiden was run and research topics chosen; early Stichtung history; views on radio astronomy and optical astronomy.

Working Files Series

Individuals Unit

Originally transcribed by Pamela M. Jernegan (1979) as typescript only, retyped to digitize by Candice Waller (2016).

Begin Tape 115A

Sullivan

This is a follow-up interview with Professor Oort on 6 September 1978 at his office in Leiden. Well, first let’s 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 – 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 can’t answer. I mean, it was clear that we needed much better resolution than Reber’s 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 that’s 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, “Okay, we’ll do the hydrogen line first, rather than doing the continuum.”

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

I’m 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, let’s just say here in Holland, was it looked upon as sort of a strange activity that didn’t 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 don’t 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 you’re 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 shouldn’t 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, “Now what does this lead to for the next project?” 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

I’m 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 you’re 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 don’t remember there was ever a serious competition between the two – there was enough time for the continuum also.

Sullivan

Yes, let’s take as an example. For instance, Westerhout’s continuum survey of the Milky Way, was this an idea that a student would come and say, “I don’t want to do this” 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 don’t 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. It’s 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

That’s the way I see it, yes.

Sullivan

Okay. I think this is perhaps why you’re able to make so many contributions – that it all comes naturally to you. Okay, let’s -

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 – 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 don’t think I ever, there was ever a conflict there because we were never that short of observing time at that stage that we couldn’t do all that we could find time to reduce and to discuss.

Sullivan

Okay, let’s move on. Another question which may be difficult for you to comment on, but I’ll 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 – 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 don’t know whether I’m correct in supposing this, but -

Sullivan

That’s an interesting comment.

Oort

But for instance, Maarten Schmidt was of that kind, and I think Westerhout also. But for the rest, I think it’s 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 they’re short of students now. There is fluctuation. There’s perhaps no deeper meaning behind that.

Sullivan

But you don’t have any particular things about the way a graduate student is trained here that you think are important?

Oort

No, I don’t think we’re 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

I’m sorry, I should have asked this earlier, back to de Voogt at Kootwijk – 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 don’t 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 don’t know at the moment.

Sullivan

I don’t think there is.

Oort

You’d have to ask Mr. Fokker perhaps he would -

Sullivan

In fact, I’m going to talk with him on Friday, so I’ll ask him that.

Oort

He would know.

Sullivan

Okay. Let’s 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 I’m 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, didn’t 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 don’t 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 couldn’t, 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, let’s see, it’s - [shuffling of paper]

Oort

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

Sullivan

See, here you’re 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 don’t 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 don’t know now. Probably also Cornell, but I don’t remember if that was during this year. Maybe not. But at Harvard they had started some – 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 don’t remember exactly what year that was.

Sullivan

Okay. Let’s 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 it’s a bit more chatty than one can afford to have in today’s papers, I think, wouldn’t you agree?

End Tape 115A