Arno Penzias (1982) - The Isotopes of the Common Elements on the Earth and in the Galaxy

I am going to tell you today about the abundant elements in the galaxy. Because there is so little sand in that glass, I mean only oxygen and carbon. The build up of the elements begins of course in the big bang, if there was a big bang. And at that point presumably the light elements up to mass 4 were produced. And then from mass 4 onward one needed the help of stars. The stars were presumably formed out of the collapse of intergalactic gas for those clouds of gas which had a certain amount of net angular momentum the rotation flattened the object. The picture here, like many other astronomical photographs fools the eye somewhat. If you look rather carefully, you will find that these blue lanes move further and further out into the picture. And in fact, if the hall were darker and the picture better and so forth, you would be able to see this galaxy is in fact considerably larger. The blue colour out there is representative of the fact that the stars in the outer edges of this galaxy are very much hotter than those in the inner portions. That has to do with a very simple physical principle, namely that elephants do not require overcoats because they have so little skin. When a star gets to be larger and larger, it has much more trouble getting rid of its heat because it has so little outside. A very simple physical calculation that you may all wish to make is that each one of you is producing as much energy per gram as the sun is. But the absence of skin on the sun makes it very much hotter. The sun has very little outside for great deal of inside. And as the stars get yet larger, their problem becomes increasingly more difficult. They become so hot in fact that they burn their helium, not just their hydrogen. And so in this helium burning of the short, very eventful lives of these extremely hot stars, the helium, mass 4, can be turned into mass 12 and mass 16. I am taking you back with this kind of nuclear physics to a much gentler, easier, earlier age, when we were comfortable with just a few immutable particles like protons and neutrons, electrons perhaps and if you are very fussy, positrons. And then at least for the benefit of this talk you can restrain your contact with this morning's speakers to social occasions. If one builds up these very simple reactions, one can in these massive stars immediately therefore make carbon 12 and oxygen 16. Very quickly this material is blown off again into interstellar space where new generations of stars form. In the new generations of stars there is—like with every generation of stars—a certain fraction of smaller stars. Stars which lead rather humdrum, uneventful lives like our own sun fortunately. We have enough economic trouble without having to worry about the sun blowing up. So under those circumstances at least the astronomers can give you better news than the economists, our sun will last for billions of years yet, not German billions but at least American billions. Under those circumstances then what we find is that very quickly, very quickly as the generations of stars go forward, what one is left with as the process of star formation grows is, one is left with an area as in the centre of the galaxy which has very little gas left in it and essentially no massive stars and only low mass stars. The stars which have not yet finished their lives out, all the gas has already been turned into massive stars, those stars themselves have been blown up and so forth. And as one moves out from the centre of the galaxy, one finds a higher proportion of gas and dust along with it. You can see—as you see these dust lanes here are not at all the absence of stars but really just tracers of interstellar gas. And under those circumstances the proportion of material, the proportion of contribution from the massive stars diminishes as one moves outward from the centre of the galaxy. Sorry, the other way around. Backwards: The proportion of material from massive stars increases. Sorry. Blame it on the translator if I got one. The proportion of gas from massive stars increases as one goes outward. One of the things one learns very early in childhood unfortunately is that you're better that somebody else. And people like to think that physicists are better than chemists, as I've already said earlier in this meeting. And so, what you try to do when you do physics is try to avoid chemistry somehow. And so, if for example one wishes to take a nucleus which is characteristic of a low mass star and compare it with a nucleus from a high mass star, the nicest way to be able to do it is to forget chemistry. And so the way to do it is to compare nuclei which have the same, hopefully the same chemical process, the same chemical properties. And so, what one can do if one studies the elements and I guess because the sand is running out, I'm only talking about two, as I said oxygen and carbon, oxygen is the more abundant. What one then ought to do is just compare the massive star isotope of oxygen with the low mass star isotope. Or in any case study those isotopes compared to one another in which case one should be able to trace stellar processing outward from the inner portion of the galaxy. And then also learn something about one particular spot in the galaxy which is of particular interest to us, namely our solar system. I'm dealing in this case with five isotopes. Oxygen as you probably know has three stable isotopes: oxygen 16, the most abundant which is made from helium burning, then oxygen 17 and 18. In the case of carbon there is carbon 12 and carbon 13, the former again being a massive star product and the latter a product of a secondary process that is taking this helium burning material into the low mass star and sticking on some hydrogen. And so, if one were to look at the ratio of carbon 12 to carbon 13 as one moved out from the galactic centre, one would see a rapid increase in that proportion. You could get the same thing between oxygen 16 and 17, the idea being that in here there are a lot of low mass stars. The low mass stars are beginning to have their own solar winds and they're throwing off their own product. And in their own product they are producing a good component of hydrogen burn material, that is to say material in which they get their energy by sticking one proton on to either, in this case a 4 alpha particle or a 3 alpha particle nucleus and you expect this kind of general increase. When you get out here to 10 kiloparsecs, that is to say 30,000 light years from the galactic centre, you ought to be below another value here. And the circle dot there is the value that we expect on the earth. This line in this case want not to intersect that dot because the earth after all was formed at the time of the birth of the pre-solar nebula some five billion years ago. So under those circumstances, 5 times 10 to the 9th years ago, presumably the interstellar medium had not yet received as much low mass star product as it has now. So we should have a natural progression in these simple cases from a very low abundance in the centre of the galaxy a little bit less here and then considerably less here. This interval reflects a time variation having to do with the fact that the stars haven't been around as long. And this slope reflects the fact that the inner portion of the galaxy is more agitated and more dense so that the stars are formed more quickly. There are obviously more generations of stars here than here and more generations of stars here than the material which for example we have eaten for lunch. As in that material as we mentioned earlier the carbon 13, if you had whatever it was, the carbon 13 in whatever you had for lunch was 1% unless the caterer was somehow an extraterrestrial kitchen of some sort. So, those are the two expectations. Now there's a 5th isotope, oxygen 18. And oxygen 18 is a peculiar one because the normal simple nuclear physics processes in the low mass stars which make these two don't make that. There is a suggestion however that another kind of low mass star process, not the same process however, a nova explosion for example, a different kind of process from the ordinary stellar wind could account for oxygen 18. And so, the first question one asks oneself is: Do oxygen 17 and oxygen 18 have the same source as far as population is concerned? That is: Do they both come from the same population? Not necessarily from the same star I remind you. That is you can have one kind of low mass star for example giving you oxygen 17 and another kind, maybe it's a double star which explodes after a period of time, some other process giving you the other. But as long as the populations are equally distributed in the galaxy, one would not see a time variation and one would not see a spatial variation. That is in that case the line would go like this. If on the other hand oxygen 18 had something to do with a massive star, then you might expect a displacement and a slope. So obviously the nice thing about this field is its certainty. When one starts, there are only two possibilities. And all one has to do is turn on one's receiver and try to do the experiment. Well, in this particular case it's a particularly easy experiment if one uses carbon monoxide. Because all one does is look in space, nature has produced as Bob told you a little earlier, a great deal of carbon monoxide. And so, if one just compares the oxygen 18 isotope of CO with the oxygen 17 isotope of CO, divide the two abundances, they are both very under-abundant in space. So that these problems of line formation of optical depth don't come in. So very straightforward measurement. And all one has to do is then do the measurement as I did about a year or two ago. In fact the young woman who took most of the data in this experiment for me, is in the audience. So for those of you who have a social interest might want to meet her later, her name is Lauren. Anyway, what happens however is having taken this data. And I think everything is okay with it. All we had to do was take the data, reduce it and then see which of these two lines lies upon and here is the result. The result is that it lies on neither line. What one has instead is a dead horizontal line, exactly what one would expect if, as the nuclear astrophysicists have predicted, that oxygen 17 and oxygen 18 have a common population, have a common origin as far as the population source is concerned. If there is one other point all the way down here, but that is actually envelop of a specific star, a star which is undergoing mass loss from hydrogen burning. And that star as we know does not have the process to make oxygen 18. But presumably because of the flatness then in the average population in interstellar space, there are equal numbers of stars which make oxygen 18 instead of 17. So the fact that this is down here is very comforting. The horizontal line is comforting but the thing that makes us uncomfortable is that way up here. Far beyond our noise exists the solar system by itself. So that the nice progression from the centre of the galaxy to the outer portion of the galaxy, to the solar system doesn't work. That is since there is no spatial difference between oxygen 17 and 18 in their relative abundances, one also does not expect a temporal difference. As time goes on you get the same amount of both. Which means unfortunately or fortunately, depending on your point of view that there is something funny about the place we live. Presumably at the time of the formation of the solar system we were given a rather unusual nuclear composition. This turns out to be in terms of physical volume several parts per million of the mass of the solar nebula. So somehow there has to be associated or very close to the origin of the solar system some sort of tremendous nuclear process, dare I say it in this land of protesters. But apparently in the beginning of our evolution, long before mankind ever appeared, there was this tremendous nuclear explosion or very energetic nuclear process which somehow coincided with the birth of the solar system. Now, whether that was a necessary coincidence, in which case perhaps there aren't very many planets out there because presumably coincidences don't happen that often. Or whether it's very common, whether it would not have been necessary. And had there been no nuclear explosion, we would have had a solar system anyway. We can at this point only speculate. But it certainly tells us that the simple picture I had before doesn't work in the simplest most straightforward case that we can imagine. The next question of course is—all we have is the relative abundance. We don't know: Is the earth enriched in oxygen 18 or in some way depleted in oxygen 17, compared to the interstellar medium? But in order to do that one then encounters a lot more problems. One has to, when one wants to take these rarer isotopes and compare them now to the abundant species, one then starts dealing with line formation. And that is the second topic that I want to cover this morning. What I want to say repeats. It's very fortunate that Bob spoke first because he explained much of this to you. The idea is that the most abundant material here is saturated. Radio astronomers use mathematics too, anyway I'll show you one equation. And it merely says that this is the observed temperature at infinite optical depth and one approaches asymptotically and this is just the opacity. Well, the point is: Since we know that the carbon monoxide has a very high optical depth, one can from the ratio of observed temperature to the temperature of carbon monoxide at the same velocity infer the opacity. And one can actually get around, as long as one doesn't want to look at the carbon 12 directly. As long as one uses this as a tool to correct the opacities of these other species, one can in fact make measurements, if not with this species, at least with this one, this one and then even yet rarer ones. Some data which I don't have time to show you today in fact uses the yet rarer isotope which has both the carbon 13 and the oxygen 18 nucleus in it. And such measurements are possible, but because this line is partly saturated, one has to do something fairly sophisticated. As Bob mentioned also that as one moves to the wings of the line, the carbon 12 to carbon 13 ratio gets much larger, indicating that this line is no longer saturated. So, some years ago when we began this seemingly endless series of experiments attempting to understand interstellar isotopes. Or another way of putting it, some years ago when we had much less information and therefore understood the problem much better, the way we used to do this experiment was to say, let us throw away the central portion of the line. In other words that is to say, let's not make any measurements in between these two areas where it's saturated, make them only on the outside. Now, that's logically consistent because you don't make the measurements only in a place where this ratio is high, you determine where to make the measurements on a criterion based on these two and then throw the rest of it away and then make the measurements only in the wings of these lines. Well, that's a convenient way of doing it. The graduate student at Princeton who worked with me on this project used it. He got his thesis fortunately and was off safely out in California before the rest of these measurements made and put that particular method into doubt. The problem with the method which we realised at the time, but what one normally does is—in such a paper—one waves one's hand in the text and says it isn't going to be a problem. And the problem however that we were dealing with was something that Bob also mentioned which is consider the model of a collapsing cloud. This particular symbol is supposed to be the observer. The observer looks at a cloud which is collapsing on to a core. Maybe the core is doing something funny but the outer portion of the cloud is collapsing. So as one looks at this core, one sees some gas here which is red shifted, some other gas which is blue shifted. That is to say the radiation emitting, coming from here will exist at the high velocity, a wing of the line and the radiation coming from this portion is at the low velocity. Now that by itself wouldn't be so bad, except that the chemists may well have had the last laugh. Because in the outer portions of this cloud one has a rather peculiar set of circumstances. What one has in the outer portions is a rather low temperature, a quite low density and a fair amount of ionised material. And under those circumstances the ordinary hand waving experience that I as a simple minded physicist would like to have which is to say that when I drink a litre of beer, the carbon 13 and the carbon 12 taste the same because they are chemically identical. On the other hand in interstellar space the stability of the carbon monoxide molecule depends upon the weight of the constituent isotopes. The molecule, again to use some sort of ancient physics, a dumbbell with a springy connection, it vibrates. The zero point vibration energy is dependent on the weight of the constituent masses. If one makes one of these weights slightly heavier, its zero point vibration energy is slightly lower. And by an inperceptible amount in our experience this molecule becomes slightly more stable. This difference is so small that it only matters at extremely low temperatures. And at extremely low temperatures there is something called an activation energy which prevents anything much from going on. Even things which are energetically favoured in chemical reactions don't go on unless you can get over activation barriers. On the other hand, if one of the two constituents is ionised, if—say you have a carbon monoxide molecule, C12O16, if a charged carbon 13 comes along, it can collide, react, without having any activation energy and replace the carbon 12 atom in the molecule and give off a bit of energy. So it's energetically favoured in the absence of things which would restore equilibrium. So, in the edges of these clouds it is actually possible we thought, but we hoped not, in the edges of these clouds it seemed possible that the chemists we had were in fact having their day. And it was possible then that in the edges of the clouds where we were actually making our determination that somehow there was extra carbon 13 in the carbon 13 isotopes we were measuring. In other words the carbon monoxide was somehow like a biological stain which sucked up carbon 13, gave us a much better contrast, made it easier to publish papers, but in fact didn't give us the right answer. Well, thanks to the splendid equipment which exists now at Bell labs, we're able to make these measurements again. And having been fooled in oxygen 18, I decided to take on the most simple minded project I could. And that was, instead of doing what had been done before which is to measure a great number of clouds at one position each, was to take just two clouds and measure so extremely carefully at several positions in the cloud that one could actually measure the ratio as a function of velocity. That is, what one could do is obtain these spectra, get them so noisefree that having corrected them with the C12 for opacity, one could actually plot the ratio as a function of velocity. Obviously on the edges it gets noisy but in the middle that might be possible. And then plot those ratios as a function of velocity for several positions in the cloud and this is in fact the data that I was able to obtain last winter on one of these two clouds, NGC 2264. Now, these spectra are really remarkable in several ways. The first is that each of them has a central minimum. There are ears on the side, wings which increase and except for the centre the wings are quite high. As one moves out, the central minimum is constant over a number of positions. That's important that the central minimum is constant. Because had I done the saturation correction wrong, then presumably, if this was some kind of saturation effect—because all this data is independent, there are very large differences in relative intensities, one would expect it to jump around. But the fact that if in the centre of these one gets exactly the same number in the minimum at the central velocity, one presumably is measuring something. But then, when one has those wings on the side, we're measuring something else. As we move off, the wings get closer together and also the minimum between the wings rises. Now let's try to understand that in the picture I showed you a moment ago. Let us assume that the edge of the cloud has extra carbon 13 in it. Then if one looks through the centre of the cloud, at the high velocity edge and the low velocity edge one is going to see an enhancement relative to the middle. Furthermore, as one looks off to the side of the cloud, the projected velocities get closer to each other and so the line is expected to narrow. Also as one gets off to the side, the unfractionated core, the material which hasn't been messed up, diminishes and so that moves up as well. So as one moves either to the side spatially or front to back in velocity, one sees exactly the same effect. Finally, if the core of the cloud itself in the middle that is to say there is a high velocity component in the middle somewhat, then a line which goes exactly through the core ought to be messed up somewhat at the high velocities. And so the ears then in the middle ought to be diminished, and in fact they are. They are lower here because in addition to the high velocity wings we're getting from the outside of the cloud here and here, presumably there is an underlying high velocity agitated feature of unfractionated gas which lowers the average ratio. So, from this data we have for the first time unmistakable proof that in fact these clouds are collapsing. And furthermore we have a mechanism now for tracing the chemistry as well as the nuclear physics of these objects. One of the experiments I did some years ago had to do with tracing deuterium in the galaxy. And under those circumstances deuterium is tremendously affected by fractionation in these clouds. And it would be fun to go back as I plan to next year and trace them again and see if in fact the deuterium has similar behaviour. The other cloud is a little less interesting. Exactly the same thing happens. Here unfortunately the cloud is about six times as far away. And as soon as one moves away from the centre, one immediately starts seeing a rise. The ears aren't very big, this cloud is very much agitated. But on the other hand somewhat the same symmetry that one saw before, one sees in the second example as well.

Arno Penzias (1982)

The Isotopes of the Common Elements on the Earth and in the Galaxy

Arno Penzias (1982)

The Isotopes of the Common Elements on the Earth and in the Galaxy

Comment

Arno Penzias and his co-Nobel Laureate Robert Wilson came to Lindau for the first time 1982. In principle, they could have attended the previous physics meeting, in 1979, the year after their prize year. If so, there would have been no less than three talks on radio astronomy at that meeting. This shows that some 270 years after Galileo Galilei constructed his optical telescope in 1609, observations in the radio wave part of the spectrum of electromagnetic radiation has become an important way to study the physics going on in the Universe. At the Lindau meeting, Penzias and Wilson reported on different aspects of a joint project using Bell Laboratory’s excellent equipment. Wilson’s talk was scheduled first, which made Arno Penzias refer to it a couple of times for more general information. In the project radio observations were made of the abundance of the molecule carbon monoxide, CO, in the interstellar medium. With the precision obtained they could actually not only measure the normal molecule CO, but also molecules made up of different isotopes of C and O. While Wilson used their observations to learn about molecular clouds, the scientific subject of Penzias was how this information on the abundance of isotopes of the elements C and O can be used to infer what goes on in space. He first gives a short review of the production of the elements from the Big Bang to recent times. Since only the lightest elements were produced originally, the heavier ones, such as C and O, were produced in stars and blown out into space after supernova explosions. The most interesting result that Penzias describes is probably that the variation of the isotopes in our solar system is not the same as in interstellar space. This seems to imply that there were some special nuclear processes going on when our planetary system was formed. It can also be noted that Penzias both starts and ends his talk in German, at the end thanking Count Lennart Bernadotte for a very nice meeting!

Anders Bárány

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