The Sun in More Detail.
A Point-Form Summary.
This section of the course notes, and the associated PowerPoint presentations, make the following critical points:
the sun is 'on our doorstep' and can be studied in intricate detail -- far more than we need for this course
the granular structure is caused by convection, the churning motions of the outer parts as the internal heat makes its way out
the visible part of the sun is surrounded by a very hot but very tenuous corona, visible during eclipses. That gas is so thin that it emits very little total light despite the million-degree temperature.
sunspots are seen to come and go in regular fashion, in roughly an 11-year cycle
at the start of the cycle, the first sunspots appear in mid latitudes. As time passes, the older spots vanish but we see new sunspots which appear at progressively lower latitudes (i.e. nearer the equator)
after about 11 years, the face of the sun is 'wiped clean' and the cycle starts again
the sunspots look dark because they are somewhat cooler than the surrounding gas, and emit correspondingly less light
being cooler, these regions of gas should quicky collapse under the pressure of the surrounding hotter gas, but do not. This tells us that there must be some extra source of pressure support in the sunspots
that pressure is provided by a strong magnetic field in the sunspot: the ionised gas is not free to move with complete liberty in the magnetic field, and this resistance provides an extra 'buoyancy'
the presence of the magnetic field is deduced from the behavious of absorption lines in the light emitted by the sunspot regions. The magnetic field there is several hundred times the strength of the overall field of the sun
we now understand the sunspot cycle in terms of the general magnetic field of the sun getting progressively more 'wound up' as the sun rotates. This happens because the sun rotates differentially, being a fluid. Here and there, the tangled magnetic field 'pops out' through the solar surface, and the local pressure support allows the hot gas to radiate away its heat, cool off, and darken (forming a sunspot pair)
after about 11 years of progressively getting more tangled, the magnetic field simply 'wipes the slate clean' and goes back to a simple dipole field (like a bar magnet), except that the North and South poles have reversed. In this sense, the apparent 11-year cycle is actually a 22-year cycle
similar magnetic field reversals happen in the Earth, on much longer (and less regular) timescales. The magnetic field of the Earth is produced in its swirling molten metal core
one might suspect that the presence of sunspots would have an effect on the Earth, or on life on Earth (if only because the sun might be a little dimmer when its face is flecked with cooler sunspot regions). But the evidence is subtle, and there is no very obvious effect that comes and goes on 11-year (or 22-year) timescales. The problem is that we do not have reliable sunspot records beyond just a few centuries ago, and the skimpy database makes the question a very difficult one to address
there is, however, suggestive evidence that there was a long period (the 'Maunder minimum') during which there were relatively few sunspots overall and the climate on the Earth was atypically cool for some decades. Whether this is a cause-and-effect relationship is not completely clear, however
we have one more way of studying the interior of the sun: we can see how it 'rings' in response to various motions within it that make it jiggle. This is the science of helioseismology
the sun is vibrating in many 'modes' -- just as a musical instrument vibrates in a lot of different overtones. Indeed, it is the complex spectrum of overtones that gives rise to the rich musical nature and distinctive sound of various musical instruments. In that sense, the sun is like an enormous bell, the ringing of which tells us about its structure
in combination, helioseismology and the neutrino observations leave us confident that we have a very good understanding of the interior of the sun -- both the structure and the thermonuclear reactions
Associated Readings from the Text.Please look at: Chapter 15 (especially pages 503-505 and 508-520) of your text.The Face of the Sun.Some astronomers are fascinated by the detailed scrutiny we can give the sun: I am not. I agree that the science can be challenging and interesting, but in the greater aim of understanding stars as a class there is a danger in being swamped with the details available for this nearest of stars (just as our study of the asteroids is not really helped by the fact that we can examine every pebble on Earth in intricate detail). Thus my tendency is to treat the Sun rather more superficially than you will find in, for instance, Chapter 15 of the text. Nonetheless, there are some important things to be said. Here are some of them: On the sun's surface, we can identify all sorts of detailed features including filaments, prominences, spicules, granules, and so on. Perhaps the most interesting surface feature is that of the sunspots, something I will return to later. Close-up images of the surface show that is has a granular appearance, rather like a honeycomb in appearance, with features some hundreds of kilometers across. These features are interpreted as being the tops of convective cells: that is, hot material is bubbling up, radiating away its energy to space, and falling back down, just as a pot of soup churns and boils. You may remember our discussion of the interior of the Earth in Physics 015. There, we considered the ways in which heat can escape from the interior of an object. When there is limited conductivity, the heat gets dammed up inside, and the most efficient way to get rid of it is to start convective motions - indeed, that is what drives continental drift. The outer part of the sun is in convective (churning) motion. We see flares and prominences, big tongues of hot material which come off the surface of the sun on short timescales. For other stars, we cannot see this kind of detail, but the way some of them vary in brightness shows that they flare up considerably more than the sun does. During an eclipse, we can see the corona of the sun, a distended halo of very hot material around the sun. Ordinarily, we cannot see this feature because the face of the sun itself is so bright. This raises an interesting question. The center of the sun is at a temperature of ten million degrees or more, but the surface is only about six thousand degrees; the corona, however, is at a temperature of a million degrees or more. If the corona is so fantastically hot, why is it so very faint? Why does it not dominate the sun's appearance? The answer is simple: the corona is of very low density. Although the individual atoms are moving at very high speed (i.e. it is hot), there are so few of them there that the total energy emitted is actually quite small. The photosphere, what we see as the `surface' of the sun, is not a discrete solid surface like the surface of Mars. It merely represents the layer from which the photons emerging from the sun's center are at last free to shoot straight off into space (and thus into our eyes). As noted, the outer parts of the sun (roughly the outermost 15 percent of its radius) are in convective motion. The inner parts are not, which may surprise you. Surely, you may think, there is so much heat in the very core that convection is the best way to get it out? Why are the inner parts not in full bubbling, convective motion as well? But that is not how it works. It is not how hot the material is that matters: the crucial factor is how transparent the material is. The deep interior is quite hot, but the radiant energy can flow through it fairly efficiently. In the outer parts of the sun, it is cool enough for electrons to recombine with atoms, and these provide an extra amount of opacity (blocking of the light) so that the radiant energy cannot readily pass through. It would be completely dammed up if it were not for the fact that the material is in convective motion, which allows the heat to escape.Sunspots: Their Appearance and Phenomenology.One of the interesting things about the appearance of sun is that it occasionally has blemishes: sunspots. Before we ask what causes these, let us consider how they behave. The first point is that they come and go. There are times when the sun has lots of sunspots, and other times when there are few. If we count the number of sunspots (taking into account the fact that one very big complex one is `worth more' than a couple of tiny ones), we discover that there is a distinct periodicity to this effect. As the figure on page 514 of the text demonstrates, the numbers rise and fall about every 11 years, a trend which can be traced back a couple of centuries at least. In medical terms, it is as though the sun has recurrent episodes of chicken pox every 11 years. Please note, however, that no individual spot lasts for 11 years! Each one vanishes after a while, to be replaced by others, but the total numbers behave in this periodic fashion. The spots behave in a strange way; they do not appear just anywhere on the face of the sun. As panel b of the figure on page 514 makes clear, at the start of the sunspot cycle -- say, in the year 1945 -- there are very few spots anywhere on the sun. Then some appear at middling latitudes (i.e. not at the equator, nor at the poles, but somewhere in between) - consider the year 1949, say. As the years pass, we find increasing numbers of spots, but the later ones are more likely to be found near the equator (look at 1952). Finally, the spots vanish more or less completely (by about 1955) and the cycle starts again. (This behaviour explains why the figure on page 514 is sometimes called the `butterfly diagram': look at the figure sideways to understand the origin of this expression.) As you may know, the first spots in a bout of chicken pox appear on the chest and back, with later spots showing up on the arms and legs. (I think I have the sense of this right!) But the individual spots do not migrate from your torso to your limbs; rather, new ones appear in these remote locations. Similarly, individual sunspots do not migrate towards the equator. Instead, a number of spots appear at middling latitudes early in the cycle; after a while, they vanish, but new spots appear at locations closer to the equator. Before we can begin to explain this complex behaviour, we need to understand what the spots are.Sunspots: Their Nature.Why do sunspots look dark? The answer is that they give off less light than the surrounding parts of the sun, and consequently look darker by comparison. In fact, however, the sunspot regions are still very hot by terrestrial standards. If you were to pull a chunk of sunspot material away and look at it in isolation, you would discover that it still gives off a great deal of light. Why does it give off less light than the immediate surroundings? It is because it is cooler than the surrounding material of the sun, by about 1000 degrees. (Hot material gives off more light per unit area than does cooler material: remember the radiation laws that we learned about in Physics 015.) But if that is the case, what maintains it? The hotter surrounding material must have a high pressure because the particles within it are moving very rapidly, and bump vigorously into other things. Why does the surrounding high-pressure gas not cause the cooler sunspot material to collapse inward? The answer is that there must be some other source of internal support or pressure in the sunspots. Astronomers have determined that this arises because the sunspots are regions of relatively strong magnetic fields. (The measurement of the magnetic fields comes from observations of a distinctive 'splitting' of the absorption lines in the spectrum of the light from the sunspot, as shown on page 510 of your text. The details don't matter, but this is yet another indication of the rich variety of information available to us through the study of the spectrum of a star or the sun.) How does the magnetic field provide the needed support? Well, remember that the material in the sun is essentially completely ionized. That is, it consists of negatively-charged electrons and positively-charged nuclei all moving about. In the presence of a magnetic field, charged particles cannot move just any direction they want. (See the figures on page 511.) For reasons well understood in modern physics, charged particles can move more easily along the direction of the magnetic field (`along the field lines') than at right angles to it. (This explains why the Earth has the so-called van Allen belts, where charged particles are trapped in the Earth's magnetic field. See page 308 of the text.) Gravity would indeed succeed in pulling the cool, low-pressure sunspot material down, and the surrounding hotter gas would indeed compress it, except for the fact that the magnetic field makes it hard for the particles to move. In other words, the magnetic field provides a kind of buoyancy or pressure support which maintains the sunspot even though the material is quite cool. This is not merely idle speculation! The spectrum of the sunlight from the sunspot regions tells us that the magnetic fields there are about one thousand times as strong as the Earth's magnetic field. (The global, or overall, solar magnetic field is only a few times stronger than that of the Earth.)Sunspots: The Patterns.Given that an individual sunspot has a strong magnetic field, how are we to interpret how the entire pattern of sunspots arises and evolves? You may readily believe me when I tell you that some rather complicated physics (magnetohydrodynamics) goes into this! Essentially, the story is as follows. At the start of the sunspot cycle, the magnetic field of the sun is rather simple, like that of a `bar magnet' with a North pole at the top and a South pole at the bottom. (This is like the Earth's fairly simple magnetic field.) See the figure on page 515 for a depiction of the starting configuration of the sun's magnetic field, along with schematic indications of the subsequent stages. An important difference between the Sun and the Earth is that the Sun is in differential rotation: that is, material near the equator moves at a different rate than material nearer the poles. Since the magnetic field is `frozen into' the material - that is, it moves along with it - the magnetic field lines get more and more stretched out as the sun rotates. This effect leads to the magnetic field lines (which represent the strength of the field) being packed closer and closer together as they get progressively stretched. For reasons which I do not want to elaborate on here, this is not easily accomplished -- the magnetic field resists being compressed and locally intensified in this way. I described an analogy in class: imagine trying to push a jack-in-the-box on an extremely strong spring down into its box so that you can just close the lid. As you force the jack down, the spring may pop out `sideways' and leave you with a kink in it. Basically, this is what happens in sunspots: the magnetic field `pops out' of the material as shown in closeup on page 511. You can see that this model (so easy to describe in words but so mathematically complex in fact!) explains a couple of things very naturally. First, you expect more and more sunspots as the winding up of the field progresses, because there will be more regions of strong field where kinks will form. Secondly, it can be shown that the sunspots will be found closer to the equator at later times, thereby explaining the `butterfly diagram.' Finally we can understand why sunspots commonly occur in pairs, as shown on page 511. The pairs are simply the two poles (North and South) of the magnetic field. But why does the cycle come to an end? The answer is that the magnetic field gets so completely snarled that it `is simpler' for it merely to wipe itself out. It disappears and gets re-established as a simple field again - but this time in the opposite sense (the North and South poles are reversed). What this means is that the 11-year cycle of the sun is actually a 22-year cycle: it takes about 22 years before the sun is `like it was before.' It is worth saying again that this model is very mathematical and forbiddingly complex to non-physicists; moreover, it is not perfect! There are features of the sunspot cycle which are not understood at all well. In broad outline, however, it is fairly successful.Similar Behaviour in the Earth.You are already aware that the Earth's magnetic field suffers reversals as well: that proved an important element in our discussion of, and confirmation of, the theory of continental drift. The reversals of the Earth's magnetic field are much less frequent than for the sun, however, occurring irregularly at intervals of typically a few hundred thousand years. Since I described the reversals in the sun as being due to the fact that the magnetic field gets all tangled up because of the differential rotation, you may be puzzled as to how this can happen in the Earth, which is a solid body. But do not forget that within its solid crust and mantle, the Earth has a molten core. It is within this core that the big circulation currents are thought to be swirling around, generating the magnetic field we experience. In there, the motions are indeed differential - that is, the entire core does not spin like a solid body - so the behaviour parallels in important respects what we have in the sun.The Earth-Sun Connection: A Hypothetical Scientific Case-Study.Our well-being on Earth depends on a steady and reliable source of energy from the sun. Given that the sun varies every 11 years, it is conceivable that there could be some small changes in the climate of the Earth. At the very least, you might speculate that the presence of a few dark spots on the sun would have the effect of reducing the overall brightness, so that we get a bit less heating and illumination. Would that be enough to matter? Is there any evidence that the sunspot cycle has any effect on the Earth? This question is a good justification for taking some time to consider how one might attack a scientific problem, in a very general way. To be specific, let's consider a quite fanciful and completely made-up example. Note: I would ask you please not to quote this as a piece of real science!! It is an utterly imaginary "case study," invented by me on the basis on no evidence whatsoever! (I very much doubt that any living species displays the kind of behaviour I am about to describe.) Suppose, however, that you, a working biologist, were to find to your surprise that the Australian kangaroo population over the last two centuries has risen and fallen in a fairly periodic way on roughly an 11-year timescale. To be specific, suppose that you find that when there are lots of sunspots, there are lots of kangaroos. (As noted above, I have no idea if this is true, and very much doubt it.) A scientist could think of three possible explanations: This is purely coincidental, and has nothing to do with the sunspot cycle. The natural way to test this is to look at the records over very long timescales. What ups and downs have there been in the number of kangaroos over the last ten thousand years, and how have the numbers compared to the sunspot activity? If, for instance, the cycles are even slightly different in timescale (i.e. one goes up and down every 10.9 years, the other every 10.8 years), a long time ago they would not have been correlated the way they are now - we would have seen lots of kangaroos at times of low sunspot activity. That would seem to rule out any obvious cause-and-effect. The diametrically opposite view is that the sunspots have some very direct effect on the Earth's climate. The most obvious possibility is that the dark sunspot regions give off less visible light than they would if they were hotter, so when the sun is liberally sprinkled with sunspots it is a little dimmer than usual. This could have an immediate influence on the climate and the health of the kangaroos. For instance, perhaps outback Australia is a bit more desert-like at such times, in a way which benefits kangaroos quite directly. The third view is that the correlation is a manifestation of some effect, but perhaps a very subtle and indirect one, depending on a complex interplay of related factors. For instance, you might like to speculate: that the sun gives off just a tiny bit more ultraviolet light at times of high sunspot activity; that this light is especially well-reflected by the moon; that the eyes of kangaroos are especially sensitive to ultraviolet light; that as a consequence kangaroos are better able to see and browse for food on moonlit nights at times of sunspot maximum; and thus the kangaroo population flourishes at such times. This is of course a very contrived example, with a lot of made-up 'facts' that aren't even necessarily true. But my point is a serious one -- namely, we have to remember that the interconnections between life-forms and the environment really are very complex, and the answer to any real correlation (if a persuasive one were to turn up) might hinge on something just as subtle as this -- or even less obvious, given the complexities of the biological world. The bottom line, of course, is that - as in any serious science - we first need to study the evidence for correlations and connections. There is no point in seeking some elaborate explanation for a correlation which simply isn't there! If the kangaroo population varies in a way which is unrelated to the number of sunspots, there is no need to struggle for an explanation. So, analysis of the available data is the important first step. Unfortunately, in understanding the interplay between solar activity and life on Earth, we are limited by the fact that we do not have reliable records dating back very far. (How many sunspots were there in 200 BC, for example? We have absolutely no idea.) As a result, many of the questions are impossible to address at all: there is simply not enough information. To take an extreme example of the limitations imposed by scanty data, consider the following. Suppose you tell me that you had a spell of very poor health eleven years ago, that your health subsequently improved, but that you are now once again on the wane. Should I blame sunspots, which also came and went over the same eleven-year period? Clearly such a conclusion is premature, given the limited data! We would need to study many cycles, and many people, before even daring to propose such a hypothesis. A single anecdotal example could be a mere coincidence, and proves nothing. Moreover, even if many of your friends and neighbours manifested the same behaviour, you would still have to consider a myriad of other possible explanations (such as changes in the air quality in your community as changing municipal governments are more or less serious about cracking down on polluters, for example).The Sunspot Connection: Some (Real?) Effects.Notwithstanding these uncertainties, there is tantalizing evidence that sunspot activity, either directly or indirectly, has at least some effect on global climate. The figure below makes the point. In the late 1600's, there were far fewer sunspots observed than usual, during a time called the Maunder Minimum. This corresponded to a time known as the `Little Ice Age' in Europe, decades during which the climate was much colder and bleaker than nowadays. For instance, the Thames River in London used regularly to freeze over then; it has not done so with any regularity in the three centuries since. There were at the same time extraordinarily severe droughts in the southwestern United States. Of course, the climate is a complex product of many factors and influences, so once again the correlation may be coincidental and illusory. We need much more data before we can be sure. That will take centuries of observation. By the way, there is one other coincidence. Jupiter orbits the sun just about once every 11 years. Could Jupiter be somehow influencing the sun, making its magnetic field get tangled up every eleven years? Could it be, for instance, that there are more sunspots when Jupiter is at the closest point in its orbit, and fewer when it is far away? This does not work: the period of Jupiter is not precisely that of the average sunspot cycle, so we are able to show that they do not correlate correctly over the whole historical record. (It would also fail to explain the Maunder minimum, of course. Jupiter did not briefly leave the Solar System in the 1600's!)Sunspots on Other Stars.It will not surprise you to learn that other stars have spots - after all, I have said before that the sun should not be thought of as special in any particular way - but it is interesting to think about how we deduce this. The problem is that we cannot see the surface features of even the nearest stars. Through the largest telescopes, they merely look like points of light, smeared out by the blurring effects of the Earth's atmosphere. Instead, the presence of sunspots is inferred from the observable consequences of the fact that a sunspot on the face of a star gives off a little less light than the hotter parts. As the star rotates, a sunspot which was on the `far side' (from our point of view) will gradually come around to the front, and the total light which we receive will be slightly diminished. We can watch these small fluctuations in brightness, and relate them to the rate at which the star rotates (as determined from the absorption lines in its spectrum), to deduce not just the presence of a single big spot but indeed work out the complex pattern of many spots on the surface.GONG: Lessons from Seismology and Music.As we learned earlier, the most abundant information we have about the sun is that which is provided by the sunlight streaming away from the surface. Unfortunately, as I noted, that light has only slowly percolated its way to the surface, and although its abundance and nature tells us a lot about the deep interior a long time ago, it says very little about its present condition. (Remember my analogy to the guest at the crowded snack table at the cocktail party.) What we really want is something which gives us a deep look at the interior of the sun. The neutrinos solve part of the problem, providing a glimpse of the present `state of the furnace,' the rate at which nuclear reactions are going on. But there is another tool which tells us about the actual structure of the sun - the pressure and density of its material at various parts in its interior. It is analogous to the seismic studies we carry out on the Earth, and in fact this branch of astronomy is called helioseismology. Occasional earthquakes cause disturbances to pass through the body of the Earth, and the way and rate at which these shock waves travel tells us a great deal about the deep interior of our own planet. (See the section on planetary structure in the Physics 015 notes to remind yourself about how this works.) Can we apply similar techniques to the Sun? Does it undergo the stellar equivalent of earthquakes, or will we need to set it jiggling in some fashion? How will we measure the vibrations? Of course, we cannot apply some machine to the edge of the sun to make it `jiggle', but there is no need. The sun is already doing this on its own. In a sense, it is `ringing' or oscillating like a giant bell. Needless to say, the oscillations are small in size (or else we would actually see the surface of the sun jiggling about) and quite quick. In class, I drew an analogy to a bellmaker's experience. If you had the requisite expertise, you could strike a bell and tell, simply from the purity of its tone, whether it was cracked, or whether its sound could be improved by the judicious smoothing or reshaping of the bell. In like fashion, the way the sun vibrates tells us a great deal about its interior structure. When objects like a bell are struck, they vibrate in many ways at once (the so-called `modes' of vibration). This is perhaps more easily visualised with something like a kettle-drum. Think of the drumhead made of stretched skin, which is gripped all around the sides by the metal ring at the top of the drum itself. You can easily imagine one mode of vibration, as follows: When the drumhead is struck by the drumstick, it is pressed down in the middle (and momentarily takes up a shape like a shallow bowl), then pops back out (like an shallow hill). It repeats this back and forth motion very quickly after it has been struck, many times before the vibration finally dies away. It may not be so obvious that there are other 'modes' of vibration. (They were explored in particular by a man named Chladni.) For instance, only the very central parts of the drumhead may dip downwards, while an outer ring of drumhead material goes up. This deformation would then quickly reverse, so that the central parts pop up while the outer parts go down. There are any number of these modes of vibration of various degrees of complexity, and many modes can be in action at once. Indeed, the richness of the sound of many musical instruments is directly related to the presence of these modes! Something which vibrates very simply, like a tuning fork, generally has a musically uninteresting `pure tone' sound. The highest notes on a piano are rather like this: the more complex modes in the vibration of the piano string are of too high frequency to be heard by our ears, and the notes played sound `tinny' and metallic rather than rich and full. Here I will not go into what sets the sun jiggling. (It has to do with the large convective motions within it. It is not caused by comets hitting it, for example, which would make a good analogy to striking a bell!) Take it from me that it does jiggle, and that the motions can be measured by observing the detailed motions of the material at the sun's surface. The relevant observations are undertaken as part of a project called GONG (Global Oscillations Network Group) which is monitoring this behaviour in efforts to improve our understanding of the interior of the sun. On page 505 of your text, you will find a figure which shows some of the many modes of vibration. The GONG network includes a telescope at the South Pole - a location which allows uninterrupted solar observations during the southern hemisphere's summer season, when the sun is above the horizon for literally months at a time. GONG has allowed us to determine the interior structure of the sun to high precision, almost right into the very core. As noted in an earlier lecture, the material within the sun has a very simple nature: it is all ionised gas. This makes the interpretation of the GONG results remarkably straightforward. Combined with what we have learned from the neutrino observations, is safe to say that we have an astonishingly good understanding of the interior of the sun and the nuclear reactions that are taking place in its core. Previous chapter:Next chapter0: Physics 016: The Course Notes, spring 2005. 1: The Properties of the Sun: 2: What Is The Sun Doing? 3: An Introduction to Thermonuclear Fusion. 4: Probing the Deep Interior of the Sun. 5: The Sun in More Detail. 6: An Introduction to the Stars. 7: Stars and Their Distances: 8: The HR Diagram: 9: Questions Arising from the HR Diagram: 10: The Importance of Binary Stars: 11: Implications from Stellar Masses: 12: Late in the Life of the Sun: 13: The Importance of Star Clusters in Understanding Stellar Evolution: 14: The Chandrasekhar Limit: 15: Supernovae: The Deaths of Massive Stars, 16: Pulsars: 17: Novae: 18: An Introduction to Black Holes: 19: Gravity as Geometry: 20: Finishing Off Black Holes: 21: Star Formation: 22: Dust in the Interstellar Medium: 23: Gas in the ISM: 24: The Size and Shape of Our Galaxy: 25: The Discovery of External Galaxies: 26: Galaxies of All Kinds: 27: The Expanding Universe: 28: Quasars and Active Galaxies: 29: The Hot Big Bang: 30: The Geometry of the Universe: 31: Closing Thoughts: Part 1:Part 2:Part 3: |
(Monday, 20 April, 2026.)
