About science; About dissecting a star

This is about science produced by the California Institute of Technology and originally broadcast by station KPCC in Pasadena California. The programs are made available to the station by national educational radio. This program is about distracting a star with host Dr. Albert Hibbs and his guest Dr. Robert Christie. Here now is Dr. Hitz of all the stars in the sky our own private one the sun is about average. There are quite a few that are considerably smaller and quite a few that are very much larger. Many of them come in pairs that is stars going around each other. There are some systems that have three or more stars all in orbit around each other. But perhaps some of the most curious and interesting ones are those that are called variable. They change in brightness apparently also in size in a very regular manner. And it is these that have caused quite a puzzle among stellar astro physicists for many years. Our guest tonight Professor Robert Christie has recently won an outstanding

award the Edington medal from the Royal Astronomical Society of Great Britain for work that he has done in illuminating the problem of the variable star. Bob I'd like to start out by asking you what is a variable star apart from the fact that it simply varies. I think that's the only definition that would really satisfy the astronomers who are the ones that that make these definitions. There are actually in many different kinds of variable stars the astronomers come to know them by the way they vary and give them different names. What's observed is just a pulsation intensity of light we receive from more in this particular category of variable star that I've been interested in. There is a regular variation in the intensity of light and many other variations that the astronomers have learned to measure. There are other kinds of variable stars however in

which the variations are. On the whole very irregular sometimes sporadic flashing and sometimes. Just an irregular fluctuation but the one you've been working on have a regular periodic Yes I am a theoretical physicist and theory on the whole deals best with phenomena that satisfy certain laws and regularities and it's these that I've been concerned with. You say there are many different types as do the ones which pulsating regularly with a regular period are those are classed as a single class of variable stars. No the astronomers have again subdivided these into quite a number of different kinds. There are. Probably half a dozen different types that they have named. Even within the category that can be generally called the Cepheids variables which are the kind that

I have worked on. But in addition there are regular variables that are so different that there is no doubt that they follow different rules and laws. Is it the principle difference just in the length of time that it takes to vary or do other things show up. I should make it clear first the commonest kind of variable star is one that I I do not intend to talk about and that is the double star where one star revolves around another and perhaps one of them is periodically eclipsed as the moon eclipses the sun or or vice versa. This kind of variable star has long been known astronomy but it is not a type that I have been working on here the kind you're talking about then is something which is inherent within a star itself rather than just a geometry of two things going around us. In fact at first

these stars had been known to astronomers as variable stars and the first ideas of them had been that they were in fact just double stars. This all of them were considered that way I'll consider that class. They were all considered in that class all the regular ones simply because this was a simple picture that people understood and in addition to the variations in a light intensity. The astronomers knew the stars varied in their velocity with respect to us as they were periodically approaching us and periodically receding from us. And this is just what they expect for a double star range with a line around each other. The what's the length of time we're talking about for these variations. The periods the shortest ones that perhaps well known are about an hour although in some

cases old dwarf stars the periods may go down to a few minutes. And the longest periods of this type are around a year. Quite a wide swing is a tremendous spread in periods. And yet within that range the similarities in the behavior are very striking. Are there many of these. Is this a common type of star in the sky. I think in terms of all the stars there are not so common. But the bulk of the stars actually are very faint intrinsically and among the brightest stars the Giant very luminous stars variability of this type is really fairly common. I might guess that one in the thousand of the brightest stars was a variable so you get a little bit of observational bias then just

due to the brightness of this particular Yes that is the ones that you can see naturally tend more to be the brightest ones. And in these variability is not so rare. Are there any By the way that. Naked eye stars are variables of no you know you touch on a difficult point in that I am a theoretical physicist not an astronomer not an astronomer I don't know the skies the way an astronomer and I'll get off that one go. I think I think I'm told that Polaris actually is a star of this type. But I say I am not an observing astronomer. Of course for the for the naked eye you'd have to have an awfully good memory and reliable atmosphere to notice variability that stretched over many days and weeks I suppose has done much to talk if you measure the brightness of a star for the comparatively techniques include the naked eye work that is the visual observer's form a very important part of the

observational network for studying variable stars. There is a network just there is a network of visual of star observers it's one of the perhaps most popular things for amateur astronomers to carry out and they report observations on the stars and there are certain types. Where perhaps the bulk of the data comes from visual observers with small telescopes. The one that is probably most famous Assateague. Variable is this is this one of the types that you have devoted your attention to. Yes the the types that I've been concerned with primarily because I was able to make calculations regarding them have been ones that are either the Suffield variables or very closely related to those namely operating on the same physical laws but sometimes called by different names by the astronomers

for historical reasons I present historical reasons and because I have some different slightly different characteristics. One of the other groups are called our library stars and they are known to have periods between a few hours and a day. The Sufi adds are known to have periods from about a day to a year. I see I see. And there are in addition some other groups with definitions that are a little bit more obscure. What are apart from the double star notion what. What have been some of the theories suggested to explain the pulsation of a star. The as far as I know the the principal other theory was the double star theory and it had to go undergo very many modifications simply because it didn't fit the facts.

And people kept on making modifications such as suggesting that the double star was one of the stars was moving through a resisting medium and thereby as it moved through the resisting medium resisting medium brushed away the outer surface of the star and on the front side exposed the hotter aspect of the star. The idea was very complicated it was very complicated and the attempt was to explain something that was foreign to the to explain an observed fact that just didn't fit into the double star picture and that is that the intensity varied in such a way that the star appeared brightest when it was coming toward the observer and was faintest when it was receding from the observer. This is the geometry would put him on 90 degrees out of phase with that's right this simple fact just didn't fit in with the simple idea of a double star. And no matter how people

tried they really couldn't fit the facts into this picture. It was I think first in 1914 that the famous American astronomer Harlow Shapley put forward the suggestion that some sort of pulsation might be the explanation of these stars and that is a periodic dilation and contraction of the whole star. Did this seem to fit the facts better when it was worked out as it got worked out. It seemed to fit the facts a lot better. Eddington I think was perhaps the the principal responsible for working or attempting to work out more details of this picture. The does do things like the expansion of the gases when it is an expanding state and compression when it's a retracting

state. The physical principle is getting brighter and dimmer the same way. Not quite that as at first sight. This picture was very helpful because it it did seem to have the sum of the necessary requirements not away from the hang up of the double star idea as well one of the principal problems with the double star idea was that since we know the force of gravity and we know about how massive stars are from the period of the star you can calculate the size of the orbit just as from the period of the earth you can tell how far the Earth is from the sun if you want to go that way. So from the periods of these stars they could tell the size of the orbit and yet they had a fairly good idea how big the stars were. And it turned out that from the double star point of view the one star would have to be

within the other star. In fact at only about a tenth the radius of the big star the other star revolving around this didn't make much sense to us. Whereas the simple ideas of pulsation of a sphere of gas you could from those ideas it was possible to relate the period to the mean density. Because it turns out that given the mass and the radius of the star that tells you the force of gravity acting on the other on the surface of the star and from that you can calculate how rapidly the material at the surface will fall or will will contract under gravity. And in this way you can calculate the period with which a star would vibrate if it were started into a fire into a spherical vibration. Well how does the driving force behind us because without something it would just damp out and that was the point that Edington raised

immediately in this connection. It had been originally proposed that perhaps some catastrophe on the star would start raining you might say and it would just rain. But Eddington pointed out that the conduction of heat in the star is in general. A very important mechanism it's the way the star shines. And he pointed out that in general you would expect. In the compressed phase more heat to be conducted and less heat to be conducted in the Star when it was expanded and in that way you would expect the star to lose to lose energy if it started to pulsate. And this would doubt the pulsation. Basically by the same mechanism that a sound wave reverberating might be damped if the walls were perfectly reflecting. And that is that the air is a damping mechanism in itself.

So Edington pointed out how important it was to understand why the star would continue to pulsate. And he examined really many different physical mechanisms that might account for this. Could he figure out what it was or has it been figured out yet. By the way. Or is this actually the nature of your own work. It is the nature of my own work to get to go into this question. Eddington essentially enumerated most of the possibilities and was able to show in all of those that he could analyze carefully he was able to show that they wouldn't account for the known phenomenon. He was able to deal with the fairly deep interior of the star because there the behavior followed fairly simple physical laws and he showed that the two mechanisms that

could act there wouldn't work which were the two that he. One mechanism is the one that initially got me interested in this question and that is the mechanism of nuclear energy. I was a nuclear physicist when I started in on this and since we know that the stars shine because of the nuclear energy liberated in the center it seemed to Addington as a reasonable possibility that. When the star if the star were imagined to pulsate but in the compressed phase there would be more energy generated in the center because the nuclear fires are very sensitive to temperature and become much stronger when heated and that this would then supply energy to the center of the star just at the phase when it needed it in order to

expand more. And then the nuclear fires would decrease when the star was expanded and that would cause it to contract more and so there would be a couple in between the nuclear energy generation and the pulsation. And you say this didn't work. Yes the mechanism is an excellent mechanism. The reason it doesn't work is simply a matter not of principle because the principle is right. But of numbers namely a star an ordinary star like the sun even is very much more dense at the center than at the surface. It is of density around 100 grams per cubic centimeter more than 10 times as dense as iron at the center. And yet at the surface it is less dense than air. A star built like that. If you move the surface

will move very much less where it is dense and heavy at the center core being responsible and that's right and this is even much more so in a giant star. The surface density of the stars that are acts that are of the Sufi of variables is actually a very high vacuum from our standards of 10 to the minus one hundred thousandth of the density of air whereas the central density of the giant star is maybe 10 kilograms per cubic centimeter that is a thousand times the density of iron. So these are very unresponsive in the center and the result is that although the principle of the mechanisms like the wind blowing over the earth making earthquakes the same idea just doesn't respond. And what about the second idea that the second idea was one that is brought to mind

just by the way sound waves would dissipate in ordinary gas namely the. Gas gets heated when it's in the compressed phase of a Soundwave and the heated air conducts heat to the cooler parts. And this means that energy is being conducted from the compressed part to the expanded part and this takes energy away from the sound wave as woodland and a star facilitated transport of energy in some way. And it would that is providing the star does conduct heat more readily when compressed than expanded. This will decrease the energy Edington examined this proposal because it's possible to imagine a material that will conduct heat more readily when it's expanded and cooled and less readily when compressed which would be the wrong way. Want wrong way around. If a star did that then it would just automatically set itself into vibration. Oh

it would. That would be the way that would be the way. Yes that is you have a really compressed is bottled up the energy and the expanded phase let it out. Then the slightest variation in the star would find it building up oscillations and it would. Automatically build up oscillations by trapping its own energy when it was compressed and then making a bigger expansion the next compression trapping more energy. And yet this idea you say is was also not Eddington recognize that the difficulty with that idea was that if it were true then he would have to explain why all stars were not variable. And so it had to be some very particular kind of star where this could happen. It turned out that in the deep interior of the star where he could analyze this mechanism it didn't work that way it was just the opposite. The material was more transparent when it was

expanded and more transparent when compressed excuse me and the result is that the deep interior of a star actually tends to diminish the energy of any mechanical oscillations. Well what. Where is your own work taking you in this field. My own work started by. By noticing that the conditions very near the surface of a star were quite different from those that Addington had analyzed and in fact they they lead you to think that there might be a trapping of energy in the compressed phase. That is the surface opacity of a star is very peculiar and it becomes very the surface becomes very opaque when it's compressed and it becomes very transparent when it is expanded. But this is not true I presume for All Stars this is a special caring for All Stars in their range from

surface temperature 10 or 20 thousand degrees down to a few thousand degrees. But this behavior is true only for a very very thin layer in the photosphere. The in analyzing this I found that there was. There there was also behavior just below the photo sphere which apparently could generate pulsation. But when I found that out I also found that this was known to others before me which was what was that process. That was a process that was first examined by a Russian named shock and awe. It has to do with a lair at about forty thousand degrees temperature where helium is partly ionized and he found that the

complicated flow of energy in this partly ionized gas led to a Essentially it trapping of energy in ionisation at a certain phase and letting this energy flow out another phase and by a rather complicated calculation he showed that this looked as though it could explain pulsation. The only difficulty with this work was that it was in Russian and although it was first done in 1953 it was not not generally known in other parts of the world. What is this mechanism one that you've also taken into account and yes this mechanism was later explored in this country by John Cox who came to a similar conclusion as she did namely that it looked as though this could account for a pulsation.

My own approach was started not knowing about this other work and I decided simply to put in the best law best treatment that I could of the actual physics that governs the materials there that is hydrogen and helium putting in all of the known properties that they ionize that they have a certain transparency to radiation and how this depends on temperature and density. And then asking given the best treatment we can of the physical properties of these materials how do they behave under an oscillation. As might take place in a star. This is an awful lot of complication to put in. I presume that you made full use of a computer to handle resolve and are able if you had to deal with. Yes the it's a lot of complication to put in and once you put in the actual

treatment of materials and their ionization and so forth then their behavior is no longer simple enough to to express in terms of simple laws. So I tried to do this by means of a computer and I decided to try to do things in a different way than had been done by others namely the others had examined the problem of small oscillations supposing there is a small oscillation of the star. Would this grow or diminish. The actual oscillations we see in the stars are not small at all. In fact there is to me a non astronomer and there are amazingly large. The stars vary in tent city by factors of two to four and in some cases even up to 10 between minimum and maximum light.

It's not a small part of Asian mind and they vary in diameter by a normal layer around 10 or 15 percent from minimum to maximum. But the most extreme cases actually change their diameter by a factor of 2. That is one can imagine what this would look like if we saw the sun suddenly over the course of an hour. That would be the period of the sun this is something we do know. If the sun varied if the sun brightened by a factor of three became three times as intense and then in another hour it diminished again. And if at the same time the sun expanded by 10 or 20 percent of its size and diminished I'd be spectacular show it would be a most fantastic show. Well then did you find in this work that you had both the layer the photosphere plus ionized

layer to deal with that they both come out of the work that you did when you went into the full details. When I went into all the details it turned out that both the mechanism that I had in mind when I started that is having to do with the layer just below the photosphere and the mechanism that back in and Cox had explored both of these mechanisms seemed to be important. There was a the these Part Two portions of the star actually acted like thermodynamic machines in that they transformed the heat energy flowing through them into mechanical energy and this mechanical energy showed itself in the form of pulsation. Well if they do I suppose that with this the variable star field has been considerably advanced but I would guess there are still some more to do in the full explanation of it.

It's been advanced and I've had a lot of fun and I've found a great deal of interest in exploring in detail actually what the stars do. That is the observers have made very lengthy observations showing very complicated behavior in the stars and it's been most interesting to me to try to make use of this behavior to try to interpret the complications that are seen by the observers in terms of actual facts about stars. Bob thank you very much for joining us tonight and telling us about the variable stars. This was about science with host Dr. Albert Hibbs and his guest Dr. Robert Christy join us again for our next program when two prominent scientists will discuss a subject of general interest. About science is produced by the California Institute of Technology and is originally broadcast by station

KPCC in Pasadena California. The programs are made available to this station by the national educational radio network.