Oral History Transcript — Dr. Milton Humason
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Milton Humason; circa 1965
ABSTRACT: Focuses on Humasonís interests in observational astronomy. Origins and extension of velocity-distance relations in the 1920s; his work with Edwin Hubble, Walter Baade, Alan Sandage and Nicholas Mayall. Own spectroscopic work. Comparison of observing conditions at Mt. Wilson Observatory and Mt. Palomar Observatory; the most distant object observed at Mt. Wilson; living conditions and relations between astronomers at Mt. Wilson. Description of life since retirement.
The velocity-distance relationship started after one of the IAU meetings, I think it was held in Holland. And Dr. Hubble came home rather excited about the fact that two or three scientists over there, astronomers, had suggested that the fainter the nebulae were the more distant they were and the larger the red shifts would be. And he talked to me and asked me if I would try and check that out. Well, our trouble was that our spectrographs were extremely slow -- that was back in about 1927 or Ď28. We had prisms in the spectrographs then and they were made of, a lot of them, of yellow glass which didnít let the ultra-violet light through and the exposures were extremely long. I agreed to try one exposure, and as I remember it lasted over a period of about two nights. I exposed the plate for two nights and got one of the brighter nebula whose velocity wasnít then known. Of course Slipher had taken the brighter ones -- youíre probably aware of that in Flagstaff and he had velocities of some -- I think -- 25 nebulae that heíd gotten in Flagstaff or red-shifts. Some of them were negative, some of them positive, and the largest one was about 1800 kilometers per second.
So I took this one and I got a very poor spectrum but enough to know that the velocity turned out to be over 3,000 kilometers. Right here, here I am I donít know what that was. I could get it for you but, well, it turned out to be over 3,000 kilometers which was quite exciting at that time. And they wanted me to continue with that work. It wasnít very happy kind of work because it took long exposures -- I mean exposure ran into hours and hours and the objects were hard to see on the slit of the spectrograph and the spectrographs were slow but Dr. Hale was still alive at that time and he got quite excited about it and called me down to the Hale Laboratory in Pasadena there, and talked to me, and told me if I ever was willing to go ahead with it that heíd do everything he could to see that we got a faster spectrograph. Well, Dr. Anderson who was at Mount Wilson at that time and had not yet, I think this is correct, had not yet taken charge of the 200-inch down there but was at Mount Wilson, and he got interested in the problem of a faster spectrograph. He worked on the design of a lens in his office there and came up with a modified design something on the order of a microscope objective.
Our spectrograph and camera was built along the lines that he designed and it turned out to be very fast -- at least we thought it was fast compared to what I had worked with before. So it cut the exposures down to 4 and 6 hours -- these same things that were taking me two to three nights to get before. Well, we went ahead from there and for a long time I worked with that camera and that spectrograph and got some probably 80 red-shifts and as predicted they did increase. The red shifts increased or the velocities got greater as the nebulae became fainter. In other words the indication was they were more distant. And then Don Hendricks was in the optical shop at that time and the first designs of the Schmidt telescope had been made and used as a telescope but Hendricks decided it could be adapted to a spectrograph -- to a camera lens for a spectrograph. So he went along after a lot of argument about whether he should do it or not but Dr. Hale felt as though he ought to go ahead on it. He designed a camera lens which we called the Schmidt lens or Schmidt spectrograph and again we made a big step forward in the speed -- I mean then we were able to go to extremely faint nebulae and the largest velocity I got on Mount Wilson with that camera turned out -- just as the two hundred inch started -- on the order of 46,000 kilometers per second. Then, of course, Dr. Hubble did the direct work on these objects, measured their brightness, and if that were plotted against the red-shifts, well, then you got a linear relationship between the red-shifts and the brightness or magnitude of the nebulae. And thatís the way it started. Well, of course, the first one was.
Shapiro:Could you describe that?
Humason:Well, I donít know how, I donít remember the NGC number of that anymore and I donít remember the exact velocity, I can get it.
Shapiro:Well, just describe your feelings about it, if you could.
Oh! Well, something like Byron, I mean Iíd been working in spectroscopy doing spectroscopic work for years and you develop a plate and you grab a magnifier and look at it and there it is. All I can say is that I was a little surprised and quite happy to see that it did have a large red-shift and it was shifted to the red and not to the violet and so on, and then as we went on, all of them were shifted to the red. And it turned out then that only the very near nebulae have a negative shift that is to the violet like the Andromeda Nebula -- the very close ones where it doesnít act on those that are too close to us. But after you leave those why everything -- there has never been a single instance when they didnít go to the red and always kept on this linear relationship -- stayed right on there. I called Dr. Hubble up the next day and he was very happy about it and anxious to see the plate. When I came down he took a look at it and then started talking about doing more and thatís when I was undecided whether I wanted to go ahead with it or not because of these tremendously long exposures. But I finally did, and I enjoyed very much working with Dr. Hubble -- for one thing -- and as I said, the largest velocity we got at Mount Wilson was about 46,000 kilometers per second.
Then we went down to Palomar -- it jumped to, I just have to get my stuff out of there, I guess. (break) Ordinarily with the old spectrographs one used rather small plates but still large enough so that they could be handled easily and developed and all. As you went to the higher speed cameras and spectrographs, especially the Schmidt, the plates had to become smaller, and they became very small until I think the plate I used on Mount Wilson -- the size of the plate that was used on Mount Wilson there with the Schmidt spectrograph was of the order of about 8 millimeters by 8 millimeters square. Or 10 by 10 maybe. They became very difficult to handle. I mean, we had to develop ways to take care of those things in the fixing bath and the developer. If you let loose one of those little things [with] your finger [it] is liable to flop right over and land on the film side and stick to the bottom of the tray. So we made wire holders for them and slipped them in the wire holders first and then they were never taken out of there till they went through the fixing bath and through the wash and dry. Then we pasted them on glass slides so that they could again be handled all right. Those things were then put on a measuring machine in Pasadena. The measurements were made down in Pasadena.
Shapiro:How did you measure one? Could you describe the method of me measuring?
Humason:Well, I mean, you measured it [laughter]. You look in the microscope and of course on each side of the nebular spectrum you have a comparison spectrum. And the spectrum we used was helium because the dispersion was so short that any other would crowd up so you couldnít separate the comparison lines. So on each side of the nebular spectrum was a spectrum of helium put on there. Then we measured the lines in the nebulae with respect to those helium lines whose wavelengths were known and the distances were separated from those would give you the velocity. Probably no difficulties with the cameras except the first ones were slow and we gradually got faster and faster cameras and spectrographs. The problem at Mount Wilson was the Cassegrain platform was very uncomfortable and your feet were on cold iron all night long there and the position would get extremely awkward at times. They will get your eye up to the eye piece of the spectrograph. It became much easier when we went to Palomar and went up in the cage there where you had the spectrograph right in front of you all the time. All you had to do is bend over and look in the eye piece. [Gap] Oh, he at the very beginning, he took a few spectra with the old cameras the slower ones. He was very anxious to get more or get them fast, if you like, to see how far out we could go and whether that thing stayed linear. The relationship stayed linear and he took quite a number of spectra in there at the beginning and there we usedÖ
Shapiro:But did the velocity distance relationship have any meaning to you or special significance. I mean how did you feel about the expansion theory?
Humason:Well, I of course, Iím an observational astronomer, I mean not a theoretical one, but I was greatly interested in knowing the theories that were put out about the expanding Universes or whether the Big Bang thing, and all that business. But I knew nothing about that end of it myself and I have always been rather happy that my end of -- my part in the work -- was, you might say fundamental, it can never be changed -- no matter what the decision is as to what it means. Those lines are always where I measured them and the velocities if you want to call them that or red shifts or whatever they are going to be called eventually, will always remain the same. Pat McDoward who had a lot to do with theÖ Youíre not taking this down?
Shapiro:Yes, I am.
Humason:Oh no, no, not this (laughter)
Unkown female voice:You donít want to record it.
Humason:Whatever we are talking about.
Shapiro:We are talking about the lines, about the Ö
Oh yes I know, well, the position of these lines would never change. Now that wasnít true of the other type work -- the direct work which Hubble did and other people did in measuring the magnitudes of these objects because all of the distances were based on the Cepheids in the Andromeda Nebula in the large and small Magellanic Clouds. And it wasnít long before Dr. Baade came up with the so-called Population II type stars which changed everything. I mean it made the magnitudes of distant things a little different but their distances became somewhat different and that thing is still changing today. I mean as they refine the work which they are doing down there now [which is] mostly electronically, I mean measuring now instead of on photographic plates which had a rather large error. Your measurements had a rather large error. Why theyíre doing it electronically and they get a reading and it is pretty accurate, it goes into the hundredths of a magnitude instead of around a tenth of a magnitude.
Now those hundredths become very important when it has to do with this velocity red-shift problem and Sandage has been working on that a lot. And Dr. Baade did before he died. Well, thatís about the story. I mean what I started to say was that I was always glad that my work could not be changed although one doesnít know what theyíll eventually come up with to explain the red-shifts. Well, one of the problems up there was a small vibration in the hundred-inch telescope for instance. That was because of the fact that it floats in mercury and the clearance between the mercury and the outer tank in the float were extremely small because the mercury became very costly and to put a large bunch in there meant quite a bit of money. In those days we didnít have the money they seem to have to throw around now and so the clearance was small and there was a sort of, you call it a fulminate [foam] that forms on the top of the mercury, that whitish stuff that forms on there, and it would build up and roll up, and it would cause this vibration in there. Sometimes it would be very exasperating and almost cause us to stop work at times. Well, we eventually took part of the lining out of there and it increased the space in there. It helped to do away with that.
Then, the mercury was extremely hard to hold, to contain. It always leaked and I expect that underneath the hundred-inch today thereís almost a mercury mine under there because it always goes down, mercury does. It gets out, it just starts working clear down, it goes through the dark rooms, the lower floors and into the dirt. And we were always sweeping it up to try to save it. Then we had, of course, slower plates then and Dr. Mees of the Eastman Kodak Company was a great help and almost an inspiration in his interest in astronomy. He did everything he possibly could to speed up the plates for us, to make the kind of plates that we wanted, and thatís when other people first started getting the red sensitive, or the color sensitive plates. Up to that time they had to be dyed up there on the mountain. We used to dye it. Eastman started making those and they continued and always worked closely with the astronomers. Sometimes the results have been good, other times they havenít.