Donald Osterbrock

Gingerich:

This is January 21, 1982. Owen Gingerich with Don Osterbrock at his office in Santa Cruz. I was a graduate student at the time when the spiral arms in the Milky Way were found, and I think I was working with SKY AND TELESCOPE and I helped prepare some of the illustrations, because I remember Charlie Federer came back from the meeting where it was reported. And the article which went into SKY AND TELESCOPE^[1] was for a long time a fairly basic reference for it, because the rest of the paper wasn’t published that quickly after it had been at the American Astronomical Society meeting. I remember at that time, at least, of seeing it very much as a race against the optical observers and the radio observers who were just beginning to get going on the 21 centimeter line. Now, tell me the circumstances of it from your point of view, how you got involved in it, and whether you sensed there was a race to be won there.

Osterbrock:

Sure. I’m glad to tell you what I can remember. Of course, it was over 30 years ago, and it’s not all that clear in my mind now. I was a graduate student at Yerkes Observatory. And I came there as Professor Chandrasekhar’s student. He was working largely in theory, and my own work was largely on theory. But I was interested in observational work. My best friend was a graduate student, Stewart Sharpless, who was working with Professor Morgan on observational problems, and I got very interested in spectral classification. I think I did fairly well in the spectral classification course that Morgan taught. At any rate, Morgan asked me if I wanted to work with him and Sharpless on some minor astronomical problems. I was glad to have the chance to do it, and that’s what got me involved in this thing. Morgan, at that time — just after World War II — had gotten interested in the idea of using HII regions as tracers of the spiral arms, to establish the spiral structure of our Galaxy, and certainly all of the ideas and scientific initiative — and most of the work — came from him. Sharpless and I were his helpers.

At that time, as you know, Baade, at Mount Wilson, had just made his wonderful photographs of the galaxy M31, which showed the HII regions there, the spiral arm tracers. It was the analogy with M31, and with mapping the spiral arms in M31 from HII regions that gave Morgan the idea of doing it in our Galaxy. He was an expert in spectral classification. He realized that O stars and B stars are in HII regions. His method of spectral classification, combined with colors, was the best way to get distances over large distances in our Galaxy of things that were associated with spiral arms. And in his mind a big part of the problem was to find HII regions. He had under his control the Greenstein-Henyey camera which was a very wide-field camera that had been developed during World War II, at Yerkes Observatory, actually to use the other way round, as a projection system to train aerial gunners. But it could be used as a camera. And Morgan got Sharpless and myself to use it to photograph, with narrow-band filters, the Milky Way, and see what HII regions we could find. And that was really my involvement in it. We found quite a few HII regions, many of them ones already known (but by seeing them on these pictures, I think that Morgan realized that they were big, important objects) and others that were new. We were very much interested in it, and talked with Morgan about it all the time. I myself never had the feeling of a race with radio astronomers, and I don’t know if Morgan did or not. I sort of doubt it. I think perhaps part of our problem is that we only take our own work seriously, and to us the radio work did not seem likely to change anyone’s thinking. We did have, I think — I sensed at least, in Morgan — a kind of a rivalry or race with Bart Bok. At that time, Bok was working with, I guess, the Harvard Schmidt telescope in the southern hemisphere. I’m not sure, but he was working on HII regions, and I think Morgan probably regarded him as the main competition, rather than the radio astronomers.

Gingerich:

Well, of course, I was a graduate student with Bok, but I didn’t sense that he was trying to solve that particular problem. Maybe he was, but if so, I wasn’t in on it. But he was already beginning to think quite a bit at that time about what might be possible with radio astronomy. Because your paper was at the December 1951 AAS meeting, I think.

Osterbrock:

Well, I wish I’d looked it up, but — yes, that’s correct. That’s absolutely correct.

Gingerich:

And that summer, or else that spring, was when Ewen had found, working under Purcell, the 21 centimeter line. And coming into the graduate school at Harvard that year, was Ed Lilley and Dave Heeschen, both of whom already had MA’s that were involved with electronics, and also Tom Matthews, who had been an MA student with Nassau, at Case. And these people were all sort of naturals for getting involved with 21 centimeter work, which Bok was beginning to appreciate its potential. And so his efforts were, I think, already put very largely into the schemes to get a radio telescope built at Agassy Station.

Osterbrock:

I’m sure you know, and I agree with you, yet I think that what Morgan thought is what I’m telling you — not necessarily what really was happening. He was, of course not doing it out of a sense of rivalry, but because it was a very big, important problem. Yet he wanted to be first with its solution. I don’t think he ever thought that radio astronomy was that much competition. And in fact I think that the radio astronomy shows spiral arms over a very large scale, much further than optical can show that there is structure. On the other hand, in Morgan’s mind, getting fairly accurate distances was a very important part of it. That’s the part that we thought then that radio astronomy can’t do, and the radio astronomers now themselves, I think, agree with that.

Gingerich:

It was not at all clear to me at that time — I took Bok’s course in stellar structure at that time, and of course that was still filled with all sorts of things like m—log Π tables, and star counts, and so on. And Bok had spent a long time trying to find the spiral structure. But I don’t think he had tumbled to the idea that you would look for it with the brightest stars rather than simply star densities. So that we all sort of felt that there was a poetic justice that the optical people had succeeded in getting the spiral structure first, because so much optical work had been put into it, mostly in vain, by chasing the wrong thing. ‘Cause then it was, you know, very quickly on the heels of that that the radio astronomy work seemed to be so much more comprehensive.

Osterbrock:

Right. It certainly covers a much larger area. Yes, I think that’s true, everything you just said… I think Morgan wanted to do it, he wanted to do it himself, and I guess part of his fear was that somebody else would tumble to the idea before he got it done in what he considered the right way. Morgan always believed, and still believes in, in absolutely first-class data, in not cluttering up your mind or your papers with a lot of not very accurate data, but instead doing everything he could to get the absolutely best. And he regarded star counts and sort of large numbers of stars as a waste of time.

Gingerich:

Well, that’s what it was in retrospect.

Osterbrock:

Morgan realized that O and B stars are high-luminosity objects and his scheme was to use accurate spectral classification of these objects that you could both identify at large distances by the HII regions, and then once you identified them get accurate distances by the spectroscopic classification.

Gingerich:

So when did you actually begin taking the photographs for that?

Osterbrock:

I can’t remember. I think it was about a year and a half before that paper. Sort of in the summer of 1950, but it’s so long ago that I can’t exactly remember. We worked on it quite a long time, and we continued to work on it after the paper came out. At that time I think that Morgan realized that the paper was going to be recognized as very important when he presented it. He asked Sharpless and myself if we wanted to go to the AAS meeting when he gave it. And I would liked to have gone, and intended to go to the meeting. That meeting was I believe between Christmas and New Year’s, as most of the AAS meetings then were. My father, though, was quite seriously ill at the time, and when I broached the idea, he was not at all keen on my going. Probably he realized that he wasn’t going to live too long, and wanted to see me again. I didn’t want to go to the meeting once I found that out. In fact my father died less than a month before the meeting. So I went home then. I never saw him again alive, but I went home at that time, stayed with my mother and never did go to the meeting. Sharpless did go. The paper was never written. Again, my memory may be wrong, but I believe that not only was it not written for a long time, but it wasn’t written at all. The SKY AND TELESCOPE article is the only record that goes back to that time, although Morgan published other versions of it later.

Gingerich:

With Nassau, maybe. Nassau wasn’t involved in the very beginning.

Osterbrock:

Well, he wasn’t involved in that part of it. Morgan had a very long term association with Nassau. At that time Nassau was at Case, at Cleveland, and had the Schmidt telescope with objective prism on it. And the collaboration basically was that Nassau would take the plates and search them for OB stars. That would then enable Morgan to take, at Yerkes, spectra of new OB stars that were fainter than those known from the Henry Draper catalogue. The main problem with using OB stars for distances in our Galaxy at that time was, and still is today, finding faint candidates. Once you found them, with the 40-inch it was possible in those days to get spectra down to about tenth magnitude. Now of course with large telescopes and modern detectors, we can go much fainter, but the quest — the problem is to find the OB stars in the first place. Morgan had collaborated with Nassau and essentially they put out joint lists. I think it’s fair to say that Nassau’s main contribution was to see that the plates were taken and preliminarily surveyed. Morgan’s main contribution was being able to recognize OB stars down to very faint magnitudes.

Gingerich:

Oh, that’s interesting because when e did the SOURCE BOOK IN 20TH CENTURY ASTRONOMY AND ASTROPHYSICS,^[2] I guess we reprinted the SKY AND TELESCOPE article for that. Because it was the first, and a clear article. But you say then there really was no paper that had the three of you as authors.

Osterbrock:

I don’t believe so. I didn’t do my historical research before you came but that’s my memory.

Gingerich:

There was a Nassau and Morgan paper in the Curtis volume of University of Michigan Publications.^[3]

Osterbrock:

Yes. I don’t think that was about the spiral arms, I think it was about finding the OB stars.

Gingerich:

It was about finding the OB stars, and I remember that as a related paper.

Osterbrock:

Right, right, right.

Gingerich:

And I don’t remember —

Osterbrock:

No, I don’t think there was any paper by Nassau and Morgan about the spiral arms. There was also a later paper by Morgan, Whitford and Code, which was a full—length paper.

Gingerich:

On the spiral arm.

Osterbrock:

On the spiral arm, yes.

Gingerich:

I see. So that was then the only formal publication of —

Osterbrock:

I think that’s right. But the fact that it was given at that meeting, and the SKY AND TELESCOPE article, made it very well known, and I think that for many years after both Sharpless and myself were best known to other astronomers because we were collaborators in that paper, which as I say, I think never actually appeared.

Gingerich:

You went on though to do your thesis with Chandra.

Osterbrock:

That’s right.

Gingerich:

And Sharpless?

Osterbrock:

Sharpless did his thesis with Morgan. On observational work, on early-type stars. I can’t exactly remember what it was.

Gingerich:

And then where did he go?

Osterbrock:

Sharpless got a postdoc appointment — a Carnegie fellowship at Mount Wilson, and then he’s been for many years at University of Rochester in their astronomy department.

Gingerich:

Okay. I don’t have a very clear impression of him, so he must not go to meetings very often.

Osterbrock:

That’s right. I haven’t seen him for years.

Gingerich:

What was your reaction when the radio astronomy results came out?

Osterbrock:

it is, you know, very complicated, and again it was a long, long time ago, so I’m not sure I really remember my reaction then. I think it was, that we felt that they could see spiral structure over much much larger distances than could be possible to do optically. On the other hand we felt that the distance method, the correlation between rotational velocity and distance was not really as accurate as the spectroscopic distance method over the distances within which it could be used. We felt that the radio results gave a more of a schematic picture than a scale picture of the spiral arms.

Gingerich:

It’s amazing how much problem there has been subsequently in reconciling the pictures.

Osterbrock:

Yes, well —

Gingerich:

‘cause it seemed so, you know, that it was all going to fall into place very obviously in 1952.

Osterbrock:

Right. I think that Joe Miller, my colleague in the office next door, was a graduate student with me at the University of Wisconsin in the early 1960s. And his thesis was essentially getting optical velocities of HII regions in the nearest spiral arms, and showing that there was not a one-to-one correlation between radio velocity and distance that you needed to make the radio method work on a large scale. I think the radio people were naturally not willing, or at least weren’t eager to accept that evidence for a long time, although I think they gradually came to realize it. Butler Burton I think is the person in the radio astronomy group who, in his papers, emphasized that finding the distances to the spiral arms is more complicated than it looks at first sight.

Gingerich:

Now, going back to the taking of the plates. You and Sharpless took all the plates, or some other people helped?

Osterbrock:

No, we took them all. It was the Greenstein-Henyey camera.

Gingerich:

That was a very wide-field camera.

Osterbrock:

That’s right.

Gingerich:

So you must not have had to have very many plates to cover the whole sky.

Osterbrock:

That’s right. The problem was to get good plates. The Greenstein-Henyey camera was a spherical mirror with a tripod that held something that was very similar to a miniature camera above it. The diameter of the mirror was 12 inches, and the overall size of the camera was about 4 or 5 feet. It was mounted on a equatorial drive on the roof of Yerkes Observatory. There was no way to guide it, you just had to adjust the rate to get it as close to the diurnal rate as you could, and then let it go. We were using narrow-band filters so the exposure times were relatively long, in the order of a half-hour to an hour. We took plates in Hα and [NII] to show the HII regions. We also took plates in the near infra-red that were intended to show not the HII regions, but as much as we could to penetrate the dust in the Milky Way and show the galactic bulge and so on. Those exposures were 2 or 3 hours long. One plate covered 140 degrees diameter at one time, but we kept trying to improve the plates. We probably took 50 to 100 plates during the time we were doing this program. Taking a plate largely consisted in putting a plate in a plate holder and pulling the slide out, and an hour or so later —

Gingerich:

And hoping for the best.

Osterbrock:

— and closing it, and developing the plate. And of course being there every night when it was dark, with the hope that the sky would get clear, which doesn’t always happen.

Gingerich:

How large were — what was the plate scale?

Osterbrock:

The plate scale … well, …

Gingerich:

These couldn’t have been very large plates.

Osterbrock:

I can’t remember. I used to know it very well, but, it’s something like an inch and a half circle was 140 degrees on the sky. So something like ten degrees per tenth of an inch, or a hundred degrees per inch, more or less.

Gingerich:

Um-hmm. So the plates were miniscule.

Osterbrock:

Very small, that’s right. And only the biggest HII regions showed. The stars that showed were essentially all the stars down to about 5th or 6th magnitude, so more or less the same stars that you see with your naked eye. Even a large HII region looked hardly more than stellar, so to see the HII regions, you had to essentially blink the plate taken in the Hα region with a comparison plate. We also took plates in the nearby continuum, missing the emission lines, and we always had to try to get a matched pair, if we could, on the same night, or if not, one one night and the other the following night.

Gingerich:

I see. So these were blinked under fairly high magnification.

Osterbrock:

Yes. We didn’t actually blink them, but we looked them over very carefully with a magnifying glass, several times. And I must say again that Morgan involved Sharpless and me in every stage of it. Yet our major contribution really was in taking the plates. Most of the HII regions that were found he found, although we had looked very hard for them too.

Gingerich:

Fascinating.

Osterbrock:

I’ve always felt that he gave us an awful lot of credit for two young graduate students whose contribution was quite minor. Many other investigators, I think, would have written the paper themselves. It was an idea that he had had for many many years; he was asked to go to a meeting a deliver a paper on it, and yet he felt it was right to include us as authors. I think we got a lot of exposure or recognition or whatever you want to call it —

Gingerich:

I hope you’ll let me use that quote verbatim when I write this up.

Osterbrock:

I certainly will.

Gingerich:

Cause that’s, does give some feeling for what was going on. I take it after they were found, goodness, you couldn’t have terribly much positional accuracy doing that.

Osterbrock:

No, well, if you could see the HII region on those plates you could recognize approximately where it was, and then go into a larger-scale map and find some trace of it there. At Yerkes we had the Barnard Atlas which consisted of large scale prints of the Milky Way. You could almost always find something like it there. Or find a picture of it somewhere.

Gingerich:

I see. And so then armed with that as the next stage, it was possible to get a spectrum for spectral classification…

Osterbrock:

Well, that would isolate a region in the sky, often three or four degrees in diameter. One example is NGC 2244. It’s a large ring nebula in the winter Milky Way, in Monoceros. It’s a ring about two degrees in diameter. That was a well-known nebula before, although it wasn’t really recognized as an HIT region before our work, although now you would wonder why anybody didn’t recognize it. On our plates you could see in addition to it, a large faint ring that was something like 8 degrees in diameter. That just barely shows on, for instance the Barbard Atlas, and you probably wouldn’t believe it if you hadn’t seen it (?) much better in our Hα pictures.

Gingerich:

Then you can see it in the Palmar Sky Survey?

Osterbrock:

Yes, you can see it very clearly in the Sky Survey. Everything we took you can see much much better in the Sky Survey.

Gingerich:

But that wasn’t yet available to you.

Osterbrock:

That’s right. Much of it wasn’t even taken at that time. Most of the Sky Survey was taken, I believe in the years around 1952, 1953, 1954. Well, this was in 1950, 1951. They were just starting the sky survey at that time. Baade and Minkowski had plates of a few regions, including one very good picture of this NCG 2244.

Gingerich:

But when the HII regions were found, then there was the attempt to identify the O and B —

Osterbrock:

That’s right. The next thing would be, once you knew where it was, to go to a map like the BD and we have coordinates then. And then see what suspected OB stars were known around there, because there were these catalogues, largely in manuscript form, of Nassau and of Nassau and Morgan, and of various other people working with Nassau where they found a lot of O stars or B stars or suspected O and B stars. And then here you’d have a region where to look that there was association. Invariably Morgan would then find there was an unusually large number of them there. He’d have spectra of many of them, and he and his assistants would take spectra of others. Both Sharpless and I also worked at taking spectra with the 40-inch, Sharpless as a paid observer on Morgan’s program; I only very rarely as a volunteer.

Gingerich:

This is the 40-inch refractor.

Osterbrock:

That’s right.

Gingerich:

It was being used at that time primarily for —

Osterbrock:

For spectral classification. At the time we were there it was used half the time for the astrometric program of parallaxes and proper motions, and the other half the time for Morgan’s spectroscopic program on spectral classification.

Gingerich:

That was essential in order to establish distance?

Osterbrock:

Definitely, yes.

Gingerich:

And this was done because of the luminosity criteria that Morgan had already established on these kinds of stars.

Osterbrock:

Right, right. He was an expert in classification. He had looked at many of them, and found the sensitive criteria to both luminosity and spectral type, which is important for the color. You see, you also had to get the color to get the interstellar ext—

Gingerich:

How did you get the colors?

Osterbrock:

He got various photoelectric observers at that time people like Olin Eggen and Harold Johnson to measure the —

Gingerich:

So other people were contributing in order to —

Osterbrock:

That’s right.

Gingerich:

I’m surprised then that this work was finished so quickly.

Osterbrock:

Well a lot of the data existed already at Yerkes. Probably he didn’t have to take many more spectra. They had spectrograms of many stars. The problem was to find which are the groups of stars that are in HII regions.

Gingerich:

How many HIT regions were located for this altogether?

Osterbrock:

I can’t remember exactly —

Gingerich:

Well, I —

Osterbrock:

Like 30. Like 20 or 30. Morgan had in his office — it’s still in his office, 30 years later — a very large grey board with the sun placed at the centre of it, and every HII region that was found, you already know the galactic co-ordinates of it. This board was a projection onto the plane b=0, and he would very carefully lay off the distance and put a mark there. And then he’d carve a little piece of balsa wood or rubber or something like that and stick it on a pin and put it there.

Gingerich:

This was what was used as the illustration in the SKY AND TELESCOPE, I believe.

Osterbrock:

Yes.

Gingerich:

Let me backtrack to Greenstein and the Greenstein-Henyey camera. That’s interesting, because of course, both Greenstein and Henyey are known for totally different things than that.

Osterbrock:

Right.

Gingerich:

They were both there during the war?

Osterbrock:

That’s right.

Gingerich:

Did Greenstein go directly from Harvard to Yerkes?

Osterbrock:

Yes.

Gingerich:

And then he later left from Yerkes to Cal Tech.

Osterbrock:

Cal Tech, that’s right. During World War II, of course, most of the American scientists were at work somehow or other in the war effort. That was before my time at Yerkes, but many of the people who were involved in that, of course I knew quite well when I got there. At Yerkes Observatory they had a small, advanced optical design shop because both Greenstein and Henyey knew something about optics and were very quick to learn anything. They were very good calculators, and they had a couple of people working in their optical shop that could make things. One of them was Fred Pearson who had been Michelson’s instrument maker years before on the campus of the University of Chicago. The branch of the war effort that they belonged to was the OSRD, Office of Scientific Research and Development. They apparently would get requests from armed services, filtered through some central office down to them for designs of things. This camera was actually designed to use to project from motion picture film onto a hemispherical dome pictures of planes flying in at a bomber to use in training aerial gunners who had some kind of —

Gingerich:

Oh this was used in a planetarium dome —

Osterbrock:

In a planetarium-like dome, that’s right. So it projected from a flat plane the film onto this hemispherical dome, but used the other way, it photographed the hemispherical dome, the sky, onto a flat plane.

Gingerich:

And this was used by Struve, the camera, for doing something on interstellar regions? Osterbrock; I don’t believe so. I don’t think that Struve ever used it. There was a thing that was called a nebular spectrograph.

Gingerich:

Oh that was something totally different.

Osterbrock:

That was roughly speaking just the spectrograph from the telescope pointed at the sky, used in a slitless mode to get the —

Gingerich:

So as far as you know this was the only astronomical use that was ever made of the Greenstein-Henyey camera?

Osterbrock:

It was the first astronomical use, and that general use was the only one. Later it was taken to the southern hemisphere by Code and Houck, working with Morgan to photograph the southern Milky Way. In particular there they could photograph in the infra red with the galactic centre at the centre of the plate instead of just down barely over the horizon as we had to photograph it at Yerkes. They got much better pictures than we did because of that location. They did that before the Code, Whitford and Morgan paper — No, I think it was after it. It was more or less a follow up to it.

Gingerich:

This fills me in rather richly on these kind of details. I suspect I’ll write up something eventually and then send it around to Morgan and others just to look at. How soon I’ll get it going I don’t know, but I suspect it won’t be for, you know, maybe a year.

Osterbrock:

Right. There’s no hurry, I think — [Yes there was — for the tape ended!]