 greetings and welcome to the introduction to astronomy. In this lecture we are going to talk about the interior of the Sun and some of the theory that goes along with it and observations that are made to help us to confirm the theory as we work through a scientific method to be able to understand what is going on. And one of the difficulties is that we do not see the center of the Sun directly. We cannot see the interior so we have to make theories and models as to how it works and then use observations of the things that we can see to be able to try to confirm or find out if we need to modify those models. So let's get started here and what we have is first of all how can we determine what the interior of the Sun is like and we can use theoretical models. Now far more than we go into into in the class these are examples of the equations of stellar structure. If we use these we can actually make a model that will allow us to determine things like temperature, pressure, mass and luminosity at every point within the Sun based on various other numbers based on the distance R being the distance from the center of the Sun. So you could do this you could then calculate based on what we believe the interior conditions to be you could put these into a computer set up a model based on various parameters based on the temperatures and the densities at the center of the Sun and then you could use this to solve as you work your way from the interior of the Sun to the surface and find out what the surface should be. So if your models work then you should be able to predict what the surface of the Sun is like and once you do this then of course you can make modifications. This is the whole idea of the scientific method. You can model modify your parameters and then say perhaps that you need to adjust the temperature or the pressure to adjust your model to fix what you might see in on the surface of the Sun and then of course you repeat the process so it's a continual process to be able to explain what is going on here. Now of course this is far more than we go into in this class but it is important to see that there are ways to be able to calculate these these parameters at every position within the star. So let's look a little more detail about what we know about the Sun and we do know that it is composed of a plasma which means that all of the atoms are completely ionized when you get inside inside it all of the atoms have electrons that have been removed and in fact when you get down to the core every atom is completely ionized there would be no electrons bound to atoms so essentially the entire Sun behaves like a very hot gas it doesn't matter the fact that the densities at the center of the Sun are much denser than anything we can imagine here on Earth there is still no solid or liquid component to the Sun we also know that the Sun has been stable for billions of years and that is using what we call hydrostatic equilibrium it is in a complete set of balance we know this because the temperatures on the Sun could not have changed drastically otherwise if it would have gotten too hot or too cold all life on Earth would have been wiped out so we know that the Sun must have had a relatively stable temperature over the last few billion years now what hydrostatic equilibrium says is that gravity which is pulling everything down is balanced by the pressure pushing outward remember that there are nuclear reactions going on in this core that is producing a tremendous amount of pressure and if not balanced by something it would blow the Sun apart so if there were no force pulling down here then the Sun would blow itself apart and we would not be able to be obviously we'd all be gone so we know that the Sun has to balance itself perfectly the amount of gravity pushing down generates enough pressure in the core to keep the right level of nuclear reactions to balance that gravity and keeps the Sun in what we call hydrostatic equilibrium so it's completely balanced and will remain that way for 10 billion years we also know that as I've said the surface temperature of the Sun remains constant it would not change we would know that because we wouldn't be here there would be no life on Earth because even minor changes in the temperature of the Sun would change significantly the amount of energy being received here on Earth and if it was changing by hundreds or even a thousand degrees it would become hot enough to vaporize all of the Earth's oceans or cold enough to freeze everything solid and life would no longer exist we also know that the Sun transfers energy by convection in the convective zone here and then by radiation in the radiative zone here so there are two different ways that it transfers energy but the only production of energy is in the core that is the only place the nuclear reactions are going on the rest of the Sun is essentially a transport mechanism to move that energy being produced here out to the exterior of the Sun where it can escape out into space now with what we know we can then make a model of the interior of the Sun and what we can see is that we can say that the energy is generated at the core it moves outward so first by radiation so it's generated here that's the energy generation then it moves outward through the radiative zone and then finally by the convective zone you get convective cells that move the energy up towards the surface and then it reaches the photosphere to eventually be able to escape out into space so it finally will reach the photosphere and that's when it is able to escape and that's when we actually see it we don't see what is going on in these inner layers we have to be able to infer that based on our model so any model of the interior does have to match what we see on the photosphere if we make a model that goes in and calculates that we should be seeing twice as much energy or twice the temperature from the surface of the Sun we know it is wrong we have to be able to use the model to match what we are actually seeing so let's look a little bit at some of the observations that we get and what we have is pulsations in the Sun is one example and this is what we call the science of helioseismology and that is a study of pulsations in the Sun seismology on earth was earthquakes the Sun is not solid so we don't get sun sunquakes but it does have pulsation going on within it so it does actually change and we see in various parts we can see blue shift so we can get a blue shift here and in other regions we get a red shift just meaning that the Sun is pulsating and various parts of it are moving in or out and that allows us to kind of see into the Sun just as earthquakes allow us to see into the interior of the earth so we can get an idea of what is going on further inside by using these variations that we can see on the on the exterior of the Sun so we can make models of the densities and the pressures at various points inside the Sun to be able to match and those variations that we see depend on the interior structure so based on our model of the interior structure we can then adjust that to fit the variations that are actually seen on the surface so it can allow for confirmation of the models as per the scientific method or maybe not quite right and we need to make modifications to them now one other way we can use to study the interior is the solar neutrinos neutrinos are produced in the very first step of the proton-proton chain and if you recall that was when we had two protons coming together and they formed deuterium which had a proton and a neutron and one of the other things that came off was a positron which is an electron with a positive charge and a neutrino so when the neutrino comes out that is something that travels right through these are what we call weakly interacting particles meaning that they interact not through the strong nuclear force as protons and electrons do but through the weak nuclear force and that means they don't interact with ordinary matter so they don't interact through gravity the electromagnetic force the strong nuclear force they just travel straight through everything and in fact they can travel through the entire sun without interacting with anything that means that they leave the sun immediately so this is our look into the core this is a way to look into the core of the sun these neutrinos only take eight minutes to reach the earth they can get there directly in eight minutes because they don't get stopped the energy being produced at the sun right now could take hundreds of thousands of years to get to the surface so if the sun had turned off we would know it very quickly by looking at the neutrinos if we have a way to detect them and but the the energy generation could take hundreds of thousands of years now when we look at this how do we detect something that doesn't interact with stuff well they do interact rarely and if we look one in a billion billion neutrinos will actually interact with the chlorine atom and what it does is it converts this to an argon it converts this into argon which is radioactive and then gives off a flash of light that can be detected this is our direct look into the interior of the sun what is the sun doing right now even if we can only detect one in a billion billion neutrinos or even less we can still get a look and use those probabilities to figure out what is going on in the interior of the sun right now and this leads us to what we have called the solar neutrino problem why is it a problem well we made these detectors essentially large tanks of cleaning fluid that had lots of chlorine they were done in deep mines as you can see here having a lot of surface a lot of earth up above them which shields from cosmic rays which could also interact with the chlorine giving us false detections and then the problem was that we are only detecting one third of the predicted number of neutrinos so that was a problem that we were not detecting the proper number of neutrinos and that is a difficulty with any model if you don't detect what you're doing then you have to go make some modifications so what could we do what could the possible solutions to the solar neutrino problem be well we can look at a couple examples here could the solar models be wrong were the solar models wrong so were they wrong could cooler temperatures would mean lower reaction rates or fewer neutrinos the problem is how do we reconcile all of this with the other observations which seems to fit the models that we were using so if we used cooler temperatures and that would throw off all of our other models and and while we might fit the neutrino observation we would not fit the other observations so could the interior of the sun be cooler than we otherwise expect the other could be did we not understand the neutrino what we find is that with a very tiny amount of mass it was determined that the neutrino could oscillate between different flavors and in fact there were three of these the experiment was set up only to detect one of these flavors the other ones would not be detected by this method and therefore that would mean if we did not then we would be detecting one third because by the time they got to the earth of every hundred neutrinos that were being sent from the sun only 33 of them would still be the same form the others would have converted to other forms which would not be detected so if they could not be detected by this method then detecting one third was correct but we had to find out did the neutrino have a little bit of mass and what the solution was then what we find is that in 1998 we saw the first evidence of neutrino oscillation that neutrinos could oscillate from one form to another so this was found here that was the first evidence here we found clear evidence just a couple of years later knowing that this could happen and what it meant is this new neutrino had a little bit of mass and the neutrinos can oscillate between these three flavors so during the trip from the sun to the earth two-thirds of the neutrinos that had produced had changed into another type that could not be detected so in all what this solar neutrino problem told us was it confirmed models of the solar interior but it also taught us something new about neutrinos and their oscillation that neutrinos could oscillate between these different flavors so let's finish up with our summary and what we see is that first of all we can use models to understand the interior of the sun we cannot see it directly and we can use observations that are made conditions on the surface and things like oscillations in the sun to be able to help refine these models to make them more accurate one example that we looked at was the solar neutrino problem that resolution of this problem confirmed the solar model and changed our understanding of the neutrino learning that neutrinos could oscillate so that concludes our lecture on the solar interior theory and observation we'll be back again next time for another topic in astronomy so until then have a great day everyone and i will see you in class