all right now that we discussed a number of uh properties of our sun and we talked a bit about opacity we're going to turn our attention toward the solar interior and most of this of course is theoretical nobody's visited the sun center or dipped their toes into the sun to figure out what's happening below the visible surface but based upon our understanding of opacity and based upon our understanding of energy transport which we're about to be introduced to in this lecture we're pretty sure that a number of things are definitely true and the more fuzzy things i'll be sure to let you know which ones are well you know to be determined still but all of our modeling of the solar interior has to be mathematical based upon the properties and the laws of physics and it has to be consistent in the end with our observations that's always you know how you know that your theory is on the right track as if your theory coincides or you know lends credence to which you can observe so always remember that about uh mathematical models and theoretical models and leave some room for changing them modifying them in the future but in this simple diagram on the slide here we've got uh you know it looks like a balloon it's the sun big ball of gas and it shows a gravitational pressure in blue arrows pushing in toward the center of our sun and it shows pressure outward and red arrows that are basically counteracting or contradicting the blue arrows and as long as those red arrows and those blue arrows are of the same magnitude the same amount then your sun is balanced and the container the balloon if you will does not expand or contract it stays at about the same size so that's just basic physics we know that the pressure inward and the pressure out would have to be you know of equal amount but in opposite directions to balance out the size or the shape the volume of your sphere so in equilibrium it says you know a happy sun the inward gravitational force is balanced by the outward pressure all right good now down below here we've got a couple of graphics a and b you don't know what those are yet but doppler shifts you know we can stare at the sun all the time and especially these days with earth orbiting satellites so we don't have the problem of distortion and turbulence from our atmosphere and we have spacecraft that are devoted to staring at the sun 24 7 every moment and yes they're uh you know near earth so they're only looking at one side of the sun all the time but the sun does rotate and spin so we eventually get to see all sides of our sun but it's very easy for us to detect doppler shifting of light now you know and this pattern here on the left in frame a shows a sun with sort of a checkerboard pattern to it and the red is for red shifted so those are parts of the sun that are getting further and further away from us and then there's the the blue checkers if you will and they're the ones that are blue shifted are approaching us so the sun is vibrating and uh unsteady all the time churning if you will turbulent and uh based upon that doppler shifting of the solar exterior we can actually deduce quite a bit about the solar interior this topic of sort of exploring the sun's interior based upon theory yet with some observations to back it up is called helioseismology that's a mouthful helioseismology helios of course is the greek word for the sun and seismology you're all familiar with on earth living an earthquake country right we have seismometers that are studying the effects of the vibration of our earth's crust all the time detecting earthquakes and trying to triangulate as to where they're occurring as well [Music] but when you use this term for the sun you know there's no little size seismographs or seismology stations up on the sun we have not sent brave astronauts to the sun to implant such things it's rather just this theoretical you know delving into the sun's interior based upon the properties of physics so what we can tell from looking at the surface is that the sun is literally ringing like a bell you know when you hit a belt and you you view it at uh high speed uh imagery you see this the the vibration of the bell and we see the sun vibrating like this all the time but it does it uh not erratically but with very well understood resonant frequency like a bell being rung and the reason the bell does it so absolutely is because it's a solid and our sun is definitely not a solid but parts of it act like even more of a super solid than solids on earth and parts of it act much more like you know the gases of our atmosphere so as you get deeper and deeper into the sun though the density is going up up up up up and it does operate more and more like a solid now over on the right side you see these waves that's what those are waves that are bouncing and refracting and deflecting out these different layers of our sun much like on earth where we have you know seismology stations set up on opposite sides of the world we can kind of probe the earth's interior knowing that there is a two levels or two um layers of core that there is a substantial um mantle and there is a a you know a crust on top of that we know that based upon how earthquake waves s and p waves penetrate through the earth and make it to the other side of the earth to register with different seismology stations so that's what's showing you on the right there is if there were seismology stations on the sun but based upon analyzing the uh the frequencies of vibration on the solar surface we're quite certain the sun has these same sort of layers it does have a super dense core it does have substantial part of its interior which is kind of all homogeneous they're all very similar to it's it's to each other we're going to call that the radiation zone then the outer 20 25 there is going to be called the convection zone and we're going to discuss that uh very shortly here and then when you finally make it to the visible surface we're going to call that the photosphere where you know light then finally easily escapes from our sun but just chalk it up to the fact that by observing the sun and by observing the vibrational structure of its uh exterior we can actually make some pretty decent assertions about the solar interior as well so i love these uh graphs i'm kind of big on graphs you probably have noticed that but i love these graphs over here because they show a cutaway model of our sun we're going to talk about all these layers here and when we're talking about the solar interiors the uh well the layers below the photosphere but the core is where the sun generates all of its energy by a nuclear fusion so got the core here and the yellow zone the radiation zone is where uh energy is transported primarily by uh photons just bouncing off each other and banging into other things and eventually working their way out of the sun convection what comes to mind when i say the word convection convection oven yeah boiling yeah boiling water is moving energy via convection so that's a good way to think about it that's the boiling layer of our sun the convection zone right above the convection zone is where you're going to call the photosphere or the visible layer of our sun now straight below it in these cutaway sorts of views this is density first and what this shows is that the density of the sun's true core is astronomically huge yeah this is uh 1 000 kilograms per cubic meter times the factor on the left so you know this is a hundred thousand uh times the density of water in the center of the sun which is off the charts you know granite has you know four or five times the density of water so it is much more dense than steel in the middle of our sun but look how rapidly that drops off when you get out to just the sun's radiation zone then that density is dropped off by a factor of you know 50 down to a much lower level we call that definitely exponential yes as you're dropping down there about a time you're out to the sun's convection zone look at how low the density is now compared to how it was in the true center so density drops off very rapidly as you work your way out of the sun to the visible surface by the time we get to the visible surface the density is only about that of our atmosphere and you know that's not it's not much so boy it really has to drop off quickly below it similar you know graph is for temperature and that just shows again that the highest temperature of our sun is definitely the true core approaching almost 16 million kelvins and that'll definitely get you a nuclear fusion to happen naturally at that temperature and notice the drop-off is not quite as exponential as it was for density it drops off that's for sure [Music] but uh just not with the same sort of slope or the same sort of steepness yes the density did so it's still going to drop off quickly and by the time we get out to the surface which is only you know 5800 kelvins then my goodness you went from 16 million down to about 6 000 kelvins by the time you make it all the way out of the solar interior so you know that's just a nice little image they have on the back of our heads i think these graphs over here showing how the density and the temperature drop off as you work your way out of the sun okay all right we'll turn our attention to the convection zone next you have the outermost layer of the solar interior and how energy works its way out through that area