The Isotopes of the Common Elements on the Earth and in the Galaxy
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30:31
Cougar
Transcript: English(auto-generated)
00:36
So, I'm going to tell you today about the abundant elements in the galaxy.
00:48
In that I mean, because there is so little sand in that glass, I mean only oxygen and carbon. The buildup of the elements begins, of course, in the Big Bang, if there was a Big Bang.
01:04
And at that point, presumably, the light elements up to mass four were produced. And then from mass four onward, one needed the help of stars. The stars were presumably formed out of the collapse of intergalactic gas.
01:20
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.
01:42
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 color out there is representative of the fact that the stars in the outer edges, the stars in the outer edges of this galaxy are very much hotter than those in the inner portions.
02:06
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.
02:24
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 a great deal of inside.
02:41
And as the stars get 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 these short, very eventful lives of these extremely hot stars, the helium mass 4 can be turned into mass 12 and mass 16.
03:08
I am taking you back with this kind of nuclear physics to a much gentler, easier, earlier age, before we had to, when we were comfortable with just a few immutable particles like protons and neutrons,
03:21
electrons perhaps, and if you're 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.
03:47
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,
04:04
stars which lead rather humdrum, uneventful lives like our own sun, fortunately. And 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.
04:27
And 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 center of the galaxy
04:47
which has very little gas left in it and essentially no massive stars and only low mass stars, the stars who 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.
05:02
And as one moves out from the center 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
05:28
diminishes as one moves outward from the center of the galaxy. I'm sorry, the other way around. The proportion, I guess I had that right, the proportion of, no, I had it backwards, the proportion of material from massive stars increases, sorry, blame it on the translator if I got it wrong.
05:50
The proportion of gas, the proportion of gas from massive stars increases as one goes outward. Now, one of the things one learns, one learns very early in childhood, unfortunately, is that you're better than somebody else.
06:06
And the people that physicists like to think they're better than is chemists. I already said earlier in this meeting. And so what you try to do when you do physics is try to avoid chemistry somehow. It turns, and so if, for example, one wishes to take a nucleus which is characteristic of a low mass, of a low mass star,
06:31
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.
06:47
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, carbon and oxygen, or 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.
07:09
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.
07:22
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.
07:40
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.
08:06
And so if one were to look at the ratio of carbon 12 to carbon 13 as one moved out from the galactic center, one would see a rapid increase in that proportion. You'd get the same thing between oxygen 16 and 17, the idea being that in here there are a lot of low mass stars.
08:25
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 onto either a four alpha particle or a three alpha particle nucleus,
08:47
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 center, you ought to be below another value here, and the circle dot there is the value that we expect on the Earth.
09:03
This line in this case ought 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, American billion.
09:21
So under those circumstances, five times ten to the ninth 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 center of the galaxy,
09:44
I'm sorry, well, a lot of, let's put it the other way, of this low mass star product, a lot of low mass star product here, 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,
10:03
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. 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.
10:20
As in that material, as we mentioned earlier, the carbon-13, if you had whatever it was, the carbon-13 in whatever it is you had for lunch was 1%, unless the caterer was somehow an extraterrestrial kitchen of some sort. So, those are the two expectations.
10:41
Now there's a fifth isotope, oxygen-18, and oxygen-18 is a peculiar one. Because the normal simple nuclear physics processes which make, in the low mass stars, which make these two, don't make that.
11:02
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
11:27
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,
11:40
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,
12:02
then you might expect a displacement and a slope. So obviously, the nice thing about this field is, 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
12:20
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 underabundant in space
12:47
so that these problems of line formation, of optical depth, don't come in. It's a 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
13:02
is in the audience, and so for those of you who have social interests might want to meet her later. Her name is Lauren. So, 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,
13:21
and then see which of these two lines it 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,
13:45
that oxygen-17 and oxygen-18 have a common origin as far as a population source is concerned. If there is one other point all the way down here, but that is actually envelope of a specific star,
14:01
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 number of stars which make oxygen-18 instead of 17,
14:24
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 center of the galaxy
14:43
to the outer portion of the galaxy to the solar system doesn't work. That is, since there is no spatial difference, no spatial difference between oxygen-17 and 18 and their relative abundances, one also does not expect a temporal difference.
15:01
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.
15:24
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.
15:43
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.
16:08
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 a very common,
16:20
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, the simple picture I had before, doesn't work, does not work in the simplest, most straightforward case that we can imagine.
16:44
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.
17:03
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, I was very fortunate that Bob spoke first, because he explained much of this to you.
17:22
The idea is, the idea is that the most abundant material here is saturated. You know, radio astronomers use mathematics too, so anyway, I'll show you one equation. And it merely says that this would be the,
17:41
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
18:01
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, one can in fact make measurements,
18:22
if not with this species, at least with this one, this one, and then even yet rarer ones. In fact, 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,
18:43
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
19:06
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,
19:22
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.
19:41
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 was a convenient way of doing it. The graduate student at Princeton who worked with me on this project used it.
20:01
He got his thesis, fortunately, and it 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 realized at the time, but what one normally does is in such a paper,
20:20
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 onto a core.
20:45
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,
21:01
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 the galaxy,
21:20
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 ionized material. And under those circumstances,
21:40
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 liter of beer, the carbon-13 and the carbon-12 taste the same because they're chemically identical. On the other hand, in interstellar space, the stability of the carbon monoxide molecule
22:01
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
22:23
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 imperceptible amount, in our experience, this molecule becomes slightly more stable.
22:43
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 favored
23:00
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 ionized, 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,
23:21
and replace the carbon-12 atom in the molecule and give off a bit of energy. So it's energetically favored in the absence of things which would restore equilibrium. So in the edges of these clouds, it is actually possible, we thought, but we hope not. In the edges of these clouds,
23:42
it seemed possible that the chemists 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.
24:02
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
24:20
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,
24:42
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,
25:06
get them so noise-free 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
25:23
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.
25:45
There are ears on the side, wings which increase, and except for the center, the wings are quite high. As one moves out, the central minimum is constant over a number of positions.
26:00
Now that's important that the central minimum is constant because had I done the saturation correction wrong, then presumably if this were 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 center of these one gets exactly the same number in the minimum
26:21
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.
26:43
We look in the center of the cloud. Now let us assume that the edge of the cloud, the edge of the cloud has extra carbon 13 in it. Then if one looks through the center of the cloud, what one is going to see is at the high velocity edge
27:00
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,
27:21
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,
27:42
and Bob mentioned that these clouds are not in free fall, if the core in exactly the center is agitated, 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,
28:01
and so the ears then in the middle ought to be diminished, and in fact they are. They're 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.
28:22
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
28:41
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 and trace them again and see if in fact the deuterium has similar behavior. The other cloud is a little less interesting.
29:03
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 center 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.
29:23
One sees in the second example as well. So having done that, let's see, I'll turn that off. I don't know if Bob took a drink, but we're friends.
29:57
I'd like to thank you very much for your time today.