[Hendrik C. Van de Hulst]
Hendrik C. Van de Hulst (Photo courtesy Harm Habing/Leiden Observatory)



NATIONAL RADIO ASTRONOMY OBSERVATORY ARCHIVES

Papers of Woodruff T. Sullivan III: Tapes Series

Interview with Hendrik Christoffel Van de Hulst
At Leiden
6 September 1978
Interview time: 56 Minutes
Originally transcribed by Pamela M. Jernegan (1979), 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 114B of the 1978 interview.

Begin Tape 114B

Sullivan

Professor Henk Van de Hulst on 6 September 1978, continuing after five years, that is. You mentioned that when you were visiting the States at Yerkes after the War that there were many things going on in astronomy and you feel that that may have somewhat put radio astronomy into a lower priority. You mentioned the lead sulfide cell, now, were there other developments or how did you see the scene in astronomy at that time?

Van de Hulst

Well, that is very hard for me to answer like this, because I came from a country which had been, in which science virtually stopped during the War time. So for us, the jump up was an enormous jump. For the Americans, the jump may have been less, and it was less. Although, they also had lots of hardships and impediments to the scientific enterprise. But itís hard for me to see what their situation was exactly, and I mentioned the lead sulfide cells simply because itís such an outstanding example Ė something coming straight from a military development. And because, also, I was at Yerkes Observatory where Gerard Kuiper and, got that cell and in fact, in the beginning, it was always referred to as the Keshman cell from the man they got it from.

Sullivan

I see. Were there other examples that astronomers were sort of excited more about technical developments that would expand their horizons?

Van de Hulst

I remember that in 1948 I attended a meeting of the AAS; or was it another meeting at Harvard? Well, one of the meetings where many American astronomers were together, and I wrote a popular article for a Dutch popular journal and seeing what the outstanding news was, again, in my impression, and I remember two things immediately. Namely, Stebbins telling about the photomultiplier cells; he with his long standing knowledge of the old fashioned cells and really calling in a kind of an optimistic mood Ė from now on, astronomical research will be only limited by the imagination of the astronomers. Not by instruments. And in a way, itís true, you see. And the development after that has shown how true he was in the photometric field. And I think at the same meeting there was a detailed explanation of the possibilities of the use of punched cards for astronomy. And actually, in the popular paper, I had an illustration showing a punched card. And I did not (?)

Sullivan

Was this by that fellow Eck-something?

Van de Hulst

I think it was Eckert, yes.

Sullivan

Eckert, yes. Which journal was that?

Van de Hulst

(?)

Sullivan

I think Iíd like to check that. You mentioned another think while I was asking you about why people did not like Struve and Kuiper want to support this astronomy, you said, ďWell, it was a gamble and they had other gambles that they chose to put their money on, so to speak.Ē Do you remember specifically what they would choose to spend their money on rather than radio astronomy at that time?

Van de Hulst

I donít know. This is partly hearsay on my part of what happened, for instance in the council of the Yerkes Observatory, these different strong personalities got together Ė at which I was, of course, as a postdoc was not a member. I would guess that this is just a general dilemma that every scientist has, whether finishing well or starting on a new thing. Certainly Struveís own work on spectroscopy of stars was not nearly finished. It was more than a lifetimeís work.

Sullivan

Ok, well, moving on. You, I didnít talk with you before about your impressions of other radio astronomy facilities in the States besides Reberís dish. Did you visit Cornell or any other places?

Van de Hulst

Yes, I certainly visited Cornell; I even was invited to take up a professorship there in 1948 after consultation with Oort and others, I finally declined.

Sullivan

Which department would that have been in?

Van de Hulst

Electrical engineering. And -

Sullivan

Was that part of you felt maybe youíd be uncomfortable in that respect or did they assure you that you did not have to be an electrical engineer?

Van de Hulst

Oh, no, no. I was not a Ė because it was a well thought out scheme in which weíd sort of say in which this would be the first seed of a larger development. In which, of course, astronomy would be drawn in. But in fact, the tradition in astronomy at Cornell at that time was pretty slim, and whether the scheme would work at all or would really remain in its first seed stage, that remained to be seen. And the situation in Leiden looked far more favorable in that respect.

Sullivan

It was clear that Oort was going to back this for a long time.

Van de Hulst

Yes. And of course, nobody can prove afterwards whether it would have germinated.

Sullivan

If you had gone. Well, we wonít blame you for the reason Cornellís not becoming a major center -

Van de Hulst

One of the person I met at Cornell, of course, was Charles Seeger who later came to Holland for a number of years. My most vivid impression is that I have terribly hot it was in the cabin, because it was a hot summer day and I think it was about 100° or 120° Fahrenheit inside the place where the measurements had to be done.

Sullivan

Youíre not just talking about the temperature of the sun Ė youíre talking about the air.

Van de Hulst

Right.

Sullivan

What was Seeger actually working on at that time?

Van de Hulst

Itís hard to remember the exact chronology, but around that time he was doing, of course, a lot of solar research. Solar research was a going concern, and I think he had already started on the fast bursts of the sun, the very fast recorder because later we got not only Seeger but also certain records from there and that was the beginning of the development in Dwingeloo of the very fast solar -

Sullivan

Right. I think De Jager worked on some of that.

Van de Hulst

Yes. And (?) also worked on some, but its, so this was one thing which was a full body, but at the same time, they had made a survey, he and Williamson of Toronto had made a survey of the galactic plane to determine where the exact center of symmetry was along the galactic plane Ė so they determined whether there was something like Gouldís belt in radio sun and -

Sullivan

To get the radio Ė

Van de Hulst

Well, no, the fine first developments.

Sullivan

I guess itís also a very early example of cooperation between optical or traditional astronomy and radio astronomy.

Van de Hulst

Yes, yes. I think thatís a good example.

Sullivan

This fellow Williamson, do you know if heís still alive or where he is? I see his name, but Iím not able to locate him anywhere.

Van de Hulst

The two places to look, the first to look in the membership list of the American Astronomical Society.

Sullivan

Itís not there.

Van de Hulst

No, and the other thing is to ask Charles Seeger whether he still knows.

Sullivan

Well, thatís a good idea. So at Cornell there was some activity Ė was there any other place that you visited in the States where there was? There as a bit going on at NRL, but probably you didnít visit NRL.

Van de Hulst

Well, yes, I had contact with NRL, also. I donít, I still have to remember whether I visited them or not. Of course, Hagen and Haddock were the two main persons at that time, and I have visited there a number of times, but whether I have exactly visited during those years or in 1951 or 1953, I donít, I wouldnít know. Let me say how that connects up with me. I was quite impressed by Hagenís consistent efforts to do something the actual photometric calibration, what I would call photometric calibration in radio astronomy. Everybody who observed sources put a label on it Ė this is the flux density, but just where they got it from, that was usually a kind of uncontrollable pushing of numbers and often done after the whole thing was finished, and then it was very hard to know, to be sure by factors, of even two whether the answer was correct. And Hagen was one of the first ones to insist on the fact that if this were to be real radio astronomy, weíd better do some real calibration which was exchangeable between different institutes, and controllable, and that to get a bumps, out of, to see which bumps in the spectralcurves were real and which were just calibration. And in that context, I have, at that time he gave a lot of attention to what was necessary and discussed with Charles Seeger at length the possibility of putting up a horn antenna and doing a (?) had one up for some time, so it didnít come very far, and that was also one of the reasons why in this publication of the BAN [Bulletin of the Astronomical Institutes of the Netherlands] 472 -

Sullivan

Is that van de Hulst and Oort?

Van de Hulst

To know that the van de Hulst, Seeger, or Westerhout. I donít know which order Ė the 400 megacycles -

Sullivan

Oh, yes.

Van de Hulst

There we have a longish appendix on defining the gain and defining the matter to measure flux density, and what to do with the backlogs, that was, I wrote, mostly wrote that appendix, but in close association with Strauss, Seeger and Westerhout and it was mostly inspired by these talks with the NRL people. But again that is a few years later.

Sullivan

Thatís okay; thatís very relevant. Well, in fact you had an article specifically on how to correct 21 cm. profiles with Ollongren in 1957 Ė did this get into the calibration problems?

Van de Hulst

No, thatís not a calibration -

Sullivan

That was correction.

Van de Hulst

No, not even. That was even, in a way a student exercise, but what would now be called, deconvolution. But the word deconvolution didnít exist at that time.

Sullivan

Are you talking about in angle now, or velocity?

Van de Hulst

No, in velocity. So we wanted to like many people have done, and actually I had at one time a card index about a deconvolution problem all over fifty titles. Because it was a mathematical hobby for me, and this was a practical application and a practical application calls for a practical decision on what to do and what not to do. So Ollongrenís exercise was to show where the practical limits were. (?) was based on.

Sullivan

Okay. Now back to the calibration business. Was it you, then particularly, that was pushing for the tremendous care that was put into the accuracy of the profiles from the first Kootwijk survey, and in great length, this is described exactly how everything was done.

Van de Hulst

No, that would be wrong, if I would claim that that was me pushing. It was more that I was also reared in the tradition and this was, in which this was the natural thing to be done.

Sullivan

So it was just the thing to be done by everyone.

Van de Hulst

It was the thing to be done about everything. I had learned that same care as a first year student in working with Minnaert on profiles of the solar spectrum. I wouldnít think of doing it with less care.

Sullivan

Okay.

Van de Hulst

But in the same thing, Muller who was really the prime author of that work had been one man who knew all the technical things and he was reared in exactly that same tradition at Delft University to do things right. And so, no that really was obvious that we ought to do it.

Sullivan

You mentioned a discussion in the previous interview about whether a large radio telescope, like a 25 meter, might make a map of the sky and that would be it, and there was concern about this, but it wasnít clear from our previous conversation when this discussion took place. Was this in the late forties Ė long before even (?) or was this later on?

Van de Hulst

No, it must have been somewhat later on, but itís conceivable that we can find it back in the minutes of the board meeting of the Sitchting. Thatís quite conceivable because the Kootwijk was, radio astronomy had at Kootwijk was established more or less by the private initiative of de Voogt who was at that time the Director of the Radio Receiving Station and part of the telecommunications program of Holland. In Kootwijk he could do as he pleased. And he got, partly for testing purposes, he got this radio telescope there and so that was his private initiative, but he offered it to Oort to do radio astronomy with. This is the very start, nucleus, from which the Stichting started.

Sullivan

Let me just confirm that. It was not a matter of Oort asking de Voogt if he could find a dish or something, but de Voogt had already brought them to Kootwijk.

Van de Hulst

I think that is the correct one. But Oort could tell you better.

Sullivan

In fact, I intend to ask him this afternoon.

Van de Hulst

Thatís my impression, and certainly there was another thing. De Voogt had another thing, which was simply a hole in the ground which was covered with chicken wire and that, I think, we all felt a bit far-fetched to try for radio astronomy.

Sullivan

But did he put it there for radio astronomy?

Van de Hulst

He put it there for radio astronomy, and my impression is that he did that without asking anybodyís advice. At heart an amateur astronomer and wanted to have some fun.

Sullivan

Did any observations ever come of that?

Van de Hulst

I donít think much came out of that.

Sullivan

It was not supported at all from the observatory here?

Van de Hulst

No, no it was, of course, discussed and then, but the main emphasis came to the mobile mirror of, it was smaller but little bit more flexible. But, of course, that must have happened already, or started to happen already in the year í46 or í47, immediately after the War. And I think in 1948, I came back and about half a year after I came back, they - the foundation was formally established and I was asked to sit on the committee and later the (?). And already during that time, we started to discuss a large radio telescope, for which later the site of Dwingeloo was chosen. Now the sites (?) can certainly be traced in the minutes, and it may have been somewhere in 1951 or so, because I think the decision was taken just before the actual detection of 21 cm. line.

Sullivan

The decision for the site?

Van de Hulst

No, to go ahead with the larger telescope. But this discovery, of course helped a lot to get it pushed. But the precise date can be checked easily; itís a matter of finding the right documents. As part of that discussion, it must have been around 1950-51, thereabout or perhaps í49, I remember personally that conversation was saying what do we do if the sky is mapped. But that must have been before the discovery of the 21 cm. line.

Sullivan

But in any case, the upshot of this discussion was that well probably, there would be a lot of things to do.

Van de Hulst

Yes, right.

Sullivan

But I guess though it still wasnít all that clear though.

Van de Hulst

No, no.

Sullivan

Okay. Now looking at a few of your research projects as seen in your publications that we missed before, one thing we didnít discuss was the aspect of the bit of cosmology that you did in your 1944 colloquium paper which was published in 1945. This is based largely on a spurious detection of M31 by Reber, and you were getting into what the integrated background of such, but do you remember how this came to you?

Van de Hulst

Yes, oh yes. I donít feel terribly proud about that part because it was one of the things that Oort insisted must also be in there, look at it, and I did the literature search and the calculations, but I never, that part of it, I cannot say that I really digested very much myself. My own background in cosmology was a bit too slim to, or I was perhaps too awed by the difficulty of all the earlier research. To really say that I had digested, what normally would have happened if this would have caught on with me as a real research problem, I would not have been content with just one page. I would have written a new paper with the proper discussion. But there were other things asking my attention, so that never came far from the ground. But I think that the calculations were correct, and its fun to see that a number of things are as they later turned out to be. The fact that the Olbersí paradox really in a way works out entirely differently in the radio domain as it does in the visual domain is clearly there. So -

Sullivan

Well, to me, itís just fascinating that this technique was so young, to say the least, and yet immediately, you say, Oort was grasping at doing some cosmology with it.

Van de Hulst

Of course, for Oort it was more natural because Oort after all had been in the de Sitterís Institute for many, many years. And of course, cosmology was one of the main drivers and so he knew more of what he was talking about at that time.

Sullivan

In terms of cosmology, yes, but to use this new technique where you didnít know what was producing the radio waves and you didnít really understand the technology of it all and yet youíre still ready to try to apply it to cosmology; thatís what I find so interesting.

Van de Hulst

Yes. Itís interesting to me; itís kind of a daring attitude. That type of daring attitude I would attribute more to Oort than to myself in this particular example.

Sullivan

Okay. Moving on, there was a paper in Nature in 1949 and then in BAN in 1950 about electron density model for the solar corona and how it varies over solar cycle, and you concluded that 50 centimeters was the optimum wave length to see that solar cycle variation in the brightness temperature. Can you tell me about that work? How did you get into the - was it an outgrowth of your study of the solar corona? You said, ďLetís see what we can do in the radio regime?Ē

Van de Hulst

Yes.

Sullivan

Was radio the best way to do it Ė to monitor the corona?

Van de Hulst

It is obvious I had been a student of Minnaert and I mean the corona was one of the things which I learned when I was a student, and I knew about the shape differences and I knew, was also entirely intrigued by the physics of the corona which was, after all, itís only quite late Ė no, letís say the suggestion by Alfven that this high excitement, highly excited or highly ionized ions in the corona might mean or would mean an actual high temperature, an impossibly high temperature of a million degrees. That development occurred while I was a student -

Sullivan

In the late thirties?

Van de Hulst

No, no, it was 1942 or so. It was very late, you see. The identification of the lines by (?) was final only in 1940. And thinking through the physics and thinking that Ė it was a big jump, a conceptual jump to say from all there is something super thermal, particle streams to seeing this is actually gas at a million degrees. That was a big jump, you see. And I think Alfven was the first one to make that jump.

Sullivan

Was this generally accepted in the community at that time?

Van de Hulst

Probably 1942 and there wasnít much of a community.

Sullivan

Just after the War -

Van de Hulst

Oh, yes, after the War it was fully accepted, yes.

Sullivan

So the radio evidence wasnít necessary then.

Van de Hulst

The radio was not actually necessary; no, I donít think so. But that was a topic which had intrigued me a lot and on which Iíd simply done a lot of reading about. And it was one of the obvious topics to (?) when the radio astronomy came. And in fact, mind you, when I gave my first course on radio astronomy in 1949, more than half of it was filled by the sun, because that was the only thing where we had that many data of, a detailed theory.

Sullivan

Oh, sure.

Van de Hulst

And so this paper, I think I was the first to suggest that there might, indeed, be a systematic, up-and-down electron density and the corona during the solar cycle. Itís now such an obvious thing, but at that time, it was not common knowledge that that would happen.

Sullivan

Well, how the corona fit into the lower atmosphere I guess, wasnít that clear.

Van de Hulst

Yes, yes.

Sullivan

That just reminds me, did you perhaps go up and visit Covington?

Van de Hulst

Oh, yes; oh, yes; of course.

Sullivan

During that first time - ?

Van de Hulst

Yes, yes. Sorry I have Ė because as a young man you travel a lot and I certainly have seen Covington.

Sullivan

Okay. Moving on, you talked about continuum radiation from the Milky Way at the Astronomenclub Symposium in 1951. I donít remember your publishing anything specifically on the general problem of the galactic background, but could you tell me if the early fifties at this time, what your thoughts were on the nature of this background? Of course it was a great puzzle to say the least. What was your attitude towards it?

Van de Hulst

Well, there is a review paper in the Observatory by Reber and Greenstein.

Sullivan

Yes. In 1948.

Van de Hulst

In 1948. Now while I was involved in the discussions about that review paper because I was at the same institute as Greenstein and so one of my vivid memories of that time, and at that time nobody would really wish to stick his neck out that this measured point by Jansky which came out at a brightness temperature well above the accepted (?) of 10,000° of interstellar gas, whether these were really true. Not that the measurements were not right, but the calibration, there might have had some flaw. And -

Sullivan

Well, this in fact, is what you say yourself in your - and you had even come to this conclusion in 1944. You say, ďWell, the whole point doesnít fit, but then again, the measures may not be right.ď

Van de Hulst

Yes, I say that. But to say the same thing in 1948 four years later is different. And not only me as a non-insider saying it, but really these two people, among them Greenstein as an astronomer and Reber as a technical man, talked to everybody. And tried to find out, because this was a key point. And even so, my impression from the conversations that time that nobody really wanted to say, ďYes, that is right.Ē

Sullivan

Well there were some other points then, I mean, Hey had made a map at 64 megahertz so it wasnít, by that time, in 1948, it wasnít just Janskyís measurements.

Van de Hulst

I donít remember so well, that that was already digested. But otherwise, probably the same doubts could be held.

Sullivan

Yes, but youíre saying, though, that it was very hard to swallow Ė that it could be so hot in some sense.

Van de Hulst

Yes. And then, so I do not know exactly what wordings they used in that review Ė it was their review. But if I would have been asked at that time, I would not have said, ďYes, there is proof that there is a super-thermal mechanism at work, or non-thermal like you would call it now.Ē And I would have kept that question open. And for me, the actual breakthrough came really with the first identification of the radio sources showing that at least the brightness temperature of those bodies was far, far above anything we had imagined. And once having seen that, you were willing to believe anything.

Sullivan

I suppose that fact that the spectra were similar.

Van de Hulst

Yes, the spectra were similar, there was so many, the whole thing was thrown open. In fact, we had perhaps been too narrow minded in what we wished to believe.

Sullivan

Did you ever think, as many people did, in terms of the integrated effect of millions of radio stars of unknown type?

Van de Hulst

I think that would be just one of the possibilities, yes.

Sullivan

Okay, you were the editor of the Jodrell Bank Symposium in 1955 which was published in 1957.

Van de Hulst

Iíd forgotten that, I was the editor. [laughter]

Sullivan

Youíve been editor of so many things! So you probably, well you say you forgot you were editor, but in any case, you probably have a good memory of the meeting anyway. Can you tell me what you do remember about that meeting Ė what juncture radio astronomy was at that time?

Van de Hulst

Well, no, I do not have very distinct memories of that particular meeting, except that it was well-run and they had a big tent, near the big telescope to serve the lunches and so on. But I was so involved in everything going on, with many contacts going on, that the meeting itself did not stand out as a meeting at which I heard many new things. So thatís the reason why itís not -

Sullivan

Itís not like someone whoís been isolated coming to the meeting.

Van de Hulst

Itís in that respect, completely different from for instance, my memory of the 1949 meeting on cosmical dynamics where I heard only new things.

Sullivan

Right. But those were not to do with radio astronomy.

Van de Hulst

Only in a distant way.

Sullivan

Well, is that also true about the Dublin IAU meeting?

Van de Hulst

No, the Dublin IAU meeting, I have somewhat more memory of. I had perhaps slightly more active role there. I think I was chairman of the committee on interstellar matter at that time. And the Dublin meeting, I remember who things distinctly from the Dublin meeting. The first thing was a private conversation I had with Pikelíner, who I think, whose first trip outside Russia this was. And we sat together in an empty classroom in that university building, side by side, each with a scratch pad so we talked in three different languages and mostly by the scratch pad, we talked astronomy. And it was a very interesting and difficult conversation because of the language. But that has stuck with me as a fine example of what people can do who both say have studied the same field because there were many, many points on interstellar matter and on radio astronomy, on the galactic structure and on the solar corona which he discussed at that time.

Sullivan

He has a very, actually, similar sort of broad ranging interest as you do.

Van de Hulst

At least he was at that time, he was very much, well I wouldnít say identical, but very much in the same types of research that I was.

Sullivan

Do you remember specifically, any of the points about radio astronomy?

Sullivan

No, thatís a bit much to ask.

Van de Hulst

But the other thing from the Dublin meeting was that in the, at the time, the ideas that there might be in interstellar space a hot phase of the gas or a hot gas, was entirely new. It was connected with possibility of the halo, connected with Spitzerís work on the heating of the interstellar gas, it was connected, and Pikelíner had certain ideas and I think (Zanscer?) had certain ideas in connection with the thermos-dynamics of the gas. And all of these things seemed to come together, and although it was not a formal item on the agenda, I felt it very timely at that time in an improvised manner to call a meeting of the Committee 34 to let a few people ask, blow off their ideas on this hot interstellar gas. And that was one of the most exciting meetings I remember.

Sullivan

How hot do you mean?

Van de Hulst

Millions.

Sullivan

Millions. Now when you say connected with the halo, is this that the cosmic rays might be the heating mechanism?

Van de Hulst

No, I mean connected with the possibility that a halo radio halo might be detected, you see. That has been up-and-down -

Sullivan

Just from the thermal emission, even.

Van de Hulst

Yes.

Sullivan

I see.

Van de Hulst

That has been up-and-down.

Sullivan

Speaking of a radio halo, was it clear to you once that people, well Shklovsky primarily and then Ginzberg also began talking about this, that synchrotron radiation was the way to go for the galactic background, or did it take some more convincing?

Van de Hulst

No, it didnít take much convincing, because of the - because we had seen that it, for a thing like the Crab Nebula it worked and it checked precisely with the polarization, and once seeing that it could work at that scale, the assumption that it could work on a different scale in a slightly milder fashion wasnít very far.

Sullivan

Even though the electron component had never been detected, it was quite reasonable that a couple of percent of the cosmic rays would the electrons.

Van de Hulst

Yes. I mean, that is, it was clear why it was never detected. Because it wasnít detectable from (?) there was no conflict whatever. And in fact, that, we were rapidly progressing now. In 1959 we had the first discussion for setting up our national space program. And we made a list of about half a dozen possibilities of things that we might tackle and one of the, I mean a clear criterion (?) some connection with things we are experts about. And so one of the things on the list was just to go after these electrons in the cosmic rays. Because this would seem to be a very key point in the explanation of radio astronomy. From that decision sprang the working group on cosmic rays, who is one floor below here. And so thatís, since then, has had a further history, it is now one of the top groups in gamma ray astronomy, so itís Ė these things are not accidental connections, theyíre pretty straight connections.

Sullivan

Speaking about meetings, the other one Iíd like to ask you about is the Paris Symposium in 1958. You summarized one session in particular to do with what was really a grab bag of papers about discrete sources and identifications like Cass A and so forth. But what are your main memories of that meeting?

Van de Hulst

I have a feeling there have been two memories Ė two meetings in Paris on radio astronomy, and that this was the later one. There had been an earlier one also, and I now have difficulty in -

Sullivan

Well, not on radio astronomy. The first one in Paris on the hydro-dynamics.

Van de Hulst

No, that I donít mean. I thought there was another one in radio astronomy.

Sullivan

Iím quite sure on radio astronomy in 1958, just before the Moscow IAU.

Van de Hulst

Oh, yes. Okay, that one, yes I know. Well, oh, yes, I traveled from Paris via Helsinki and Leningrad and Moscow. And I remember Helinski was very quiet compared to the noise in Paris.

Sullivan

Thatís still true Ė I was just up there last month.

Van de Hulst

(?) Okay. That was, I remember it was a quite lively meeting. There were no issues in the sense of the big things coming to the foreground and big questions or big achievements. It was so to say, a mid-course check. And there was -

Sullivan

There was the controversy over the radio source count, of course, between -

Van de Hulst

Yes, oh yes, but -

Sullivan

Which had been known before. That was not -

Van de Hulst

But do you think that we on the continent take that terrible seriously? [laughter]

Sullivan

Would you like to elaborate on that?

Van de Hulst

No, that type of (?) whipped up so much that I may want to - weíll see the facts sooner or later.

Sullivan

I see.

Van de Hulst

No, everybody has their own feelings of who you trust most, but at least I can never get excited about that type of controversy.

Sullivan

But in any case, your attitude was that you couldnít tell which was the reliable survey and you were just going to have to wait until the experts agreed.

Van de Hulst

Yes, any it was good reason to assume that they would agree.

Sullivan

Eventually.

Van de Hulst

So at least I was not in the research line that I would say I must have these things. I must have reliable data in February because in March it is too late. And I remember for instance, there was this Palmer work on the galactic (?) at that time which was new set of data slightly over-discussed, perhaps, on a known problem. But that is typically, a number of these things, you have to say, ďFine, new data, but where do we go next?Ē And we had the format of the meeting was that we had set one room apart completely with bold displays that people had detailed graphs and drawings and usually, so to say, this personal acclamation with it, and that was a much used room during that meeting. So that was typical for the mid-course work, that everybody knew the problem, and they wanted to see what the data are.

Sullivan

Would you agree it seems that was the last time that one could have a meeting on radio astronomy? That after that, you had to have subfields?

Van de Hulst

Yes, yes. In fact, the Noordwijk Symposium which came afterwards was entirely on galactic, and even that was a postponed for one or two years because it was at the time where it was originally held, was supposed to be held, it was not very timely. I mean, Oort discussed it with a number of other people and we all felt it was unfortunate to draw the people in just before the real data were coming, became available.

Sullivan

It was held in 1966 Ė it was withheld that long?

Van de Hulst

I think two years at that time. I do not know now the precise reason, but there was some correspondence and everybody agreed it was a good choice to say (?). And I think decisions of this type ought to be possible.

Sullivan

Another general question in which you did not specifically work is the identification of radio sources which was going rather slowly throughout the fifties. What were your feelings at that time as to what the reason for this was? Was it that these were really dark things, or was it just a matter of improved positions would do it?

Van de Hulst

I donít really remember what my feelings at that time were, it was not, I knew a number of people were working on it and it was obvious that better positions were always good for that reason, you needed also large telescopes. But the fact -

End Tape 114B

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

Begin Tape 115A

Sullivan

This is continuing with Henk van de Hulst on 6 September 1978. You were saying?

Van de Hulst

Yes. On the identification of radio sources, the fact that the number of firm identifications, Cass A and Tau A and Cyg A and Virgo A, were objects of such a completely different nature, and that you could easily make the calculation of how invisible a normal radio source like Cyg A would be in the optical domain, that did not leave me with a big worry that there was an eternal inconsistency.

Sullivan

I see.

Van de Hulst

Now, of course, a real research scientist should not leave it at that, but should go on and look at alternative models and try to narrow down. But I was not at that time working in that line, so my impression is "itís okay Ė the answer will be found sooner or later."

Sullivan

What about the general feeling about the state of the interstellar gas Ė youíve already mentioned about the hot component idea at the Dublin meeting, but how would you say through the fifties in particular, that radio astronomy changed our view of the interstellar medium?

Van de Hulst

I would say extremely strongly. I mean it has been very important in many ways.

Sullivan

But the cloudy nature, for instance, that came from the Dutch hydrogen line work Ė that had been put forward before from the interstellar optical.

Van de Hulst

Oh, yes, but the question was not at all whether it was cloudy but just how you would describe the cloudiness, whether you could fix real parameters. And I went, for myself, to great lengths in the, for instance, in the meeting of cosmic (?), the second one in Cambridge, Massachusetts.

Sullivan

In 1957, right.

Van de Hulst

In 1957 Ė or in the 1953 one, I donít know. At one of these volumes, I went to great lengths trying to see, to make an interim report on where we would stand if you tie all the data from all the optical work and from radio astronomy work together to see what we know about the distance of clouds, about their forms, about their motions. And that was still pretty depressing, there was lots of data, but itís obvious that with different instruments, you see different things, and so the results were not proportional to the effort in a way.

Sullivan

But hasnít this situation in fact, continued, to the present day? And perhaps even worse now?

Van de Hulst

I donít know. You see, the problem, I would judge the present situation a little better because partly through the development in the infrared field and we know of more normal and weird objects by direct name. You see, we can point them out. A couple of years ago, I was asked to give a review paper on interstellar matter at a course which (?) had arranged, and I gave it the title of ďWhere is the Interstellar Medium?Ē because I was not too happy with what I could tell there, but the title was simply to show that I did not know, at that time, I could not identify, if somebody said a molecular cloud, there was a word and two different authors might mean something entirely different by it. Right now with the infrared work maturing a little, I am, and with all the earlier knowledge tied in, I think one could do a better job, and actually . . . but itís not a simple job.

Sullivan

But in the fifties, things were not tying together well?

Van de Hulst

No, no. Of course there was this idea that, for instance, the hope that you could do something by making a turbulence spectrum, a new trick to describe things and I also went at great lengths in trying to use test runs which Lex Muller had made to get, to try to make a turbulence (?) rather than -

Sullivan

Test what?

Van de Hulst

Test runs with Dwingeloo.

Sullivan

I see, with the Dwingeloo dish.

Van de Hulst

With the Dwingeloo dish, but I was neither content with the result nor really with the theoretical basis, so I dropped it. And then later you see similar things in the literature.

Sullivan

Okay, one final question is in the Kootwijk survey in which you were very much involved, there was a definite methodology involved, of course, which goes back to Oortís 1920 work on the differential rotation of the galaxy. How happy were you with that at the time? Did you, in your mind, were they really these streamers, these dense concentrations or were there ever any nagging doubts about non-circular motions and possibilities of being fooled. The model was very consistent, but might be quite inaccurate.

Van de Hulst

I would like to correct your first statement.

Sullivan

Yes, go ahead.

Van de Hulst

I would like to correct your first statement and to, the survey was independent of the model used for analyzing the survey, so to say. So the survey was done with great care and is model-independent. But itís then in the, after all the profiles had been cleaned and put together that you want in essence to start to interpret the density distribution and this velocity longitude diagram, and there indeed, I was quite happy. That happened a couple of years earlier to realize by conversations with Oort how powerful, how far one could get with this simple assumption of circular motions. And so that was really an enormous help to have at least one model which was relatively simple and which could be used, but I never believed the model in the sense of believing that this must be true. The question was whether this was a good first approximation, and that I feel it was, and is. Because, after all our, there was this nice thing, there was very little trespassing in the wrong quadrants, and all of these things together count.

Sullivan

Yes. It looked like a regular motion. The features looked like spiral features.

Van de Hulst

Yes. And of course, we know that spirals exist in other, all of these things helped to give you some confidence, but itís mostly (?) the question that for instance for the near solar, in the optical work, it had worked well, in the near solar surroundings. All of these things together gave a fair complement, and it was also obvious from the very beginning that near the center some things might be wrong.

Sullivan

Well, that section was left out.

Van de Hulst

Well, for observational reason. That you could not interpret in that way, that well, letís leave it initially out. But it was clear enough to me that, and I have later also written that I am probably in the original papers also, that the cloud motion essentially gives a certain convolution which gives an uncertainty in the distance determination, and in the same way as the antenna pattern gives an uncertainty in the sideways position of that of that determination, and one thing which struck me as a beautiful coincidence was that both these uncertainties were of the order of a couple hundred parsecs.

Sullivan

It happened to be compatible.

Van de Hulst

Yes, it happened to be compatible, and we did not design the wave lengths of the 21cm. line nor the size of the German radar dish. And yet they happened to be compatible.

Sullivan

Well, thank you very much. That ends the interview with Henk Van de Hulst on 6 September 1978 at Leiden.

End Tape 115A


Modified on Wednesday, 13-Jul-2016 08:42:04 EDT by Ellen Bouton, Archivist (Questions or feedback)