 greetings and welcome to the introduction to astronomy. In this lesson we are going to talk about the Sun and more specifically the structure of the Sun, so what the different parts are of the Sun that we see. So let's get started and look at a picture of the Sun here and here we see an example of the Sun and some of the basic properties. Now what you'll note with some of these properties is that the Sun is very big. It has 2 times 10 to the 33rd grams of material in it. So we refer to that as one solar mass and can use that to compare it to other stars. Its surface temperature is 5800 Kelvin, much hotter than anything else you are familiar with. The luminosity of the Sun at 3.8 times 10 to the 26 watts again we refer to this as one solar luminosity which allows us to compare it to other stars. And we do the same thing with the radius a very large number of kilometers but we can call that one solar radius. Now in terms of density the density is actually less than that of the earth. The density is only 1.4 grams per cubic centimeter comparable to the densities of the giant planets. However it is significantly larger in terms of mass and size. And it has a rotational period at the equator of a little under 25 days. So the Sun actually undergoes what we call differential rotation meaning that it rotates faster at the equator and slower as you get towards higher and higher latitudes. So unlike the earth where every part rotates the same speed when you look at the Sun the equator will rotate faster and the other parts you get further away from the equator will rotate slower and slower. So what is the Sun made up of? Let's take a look here and we find that the Sun how can we determine what the Sun is made up of? And we can do that by studying its spectrum. And we find that the majority of this is hydrogen and helium. Now depending on whether you look by the total number of atoms or the mass either way you get a large proportion of it being hydrogen and helium. By number of atoms it's 99.9 percent if you add these two together. So 99.9 percent of the atoms in the Sun are hydrogen or helium. That means one in a thousand is going to be something other than either hydrogen or helium. If we look at the total mass and we add those together we get 98.2 percent. So 98.2 percent of the mass of the Sun is hydrogen or helium and the rest is broken down among other elements. Now we also know that one element was actually discovered in the Sun before it was seen here on earth. When we started taking the spectrum of the Sun we found that there were elemental lines that could not be identified and that element was then named helium after helios for the Sun. So it was found in the solar spectrum before we found it here on earth. Now to really determine this the person who first determined the abundance of this was Cecilia Pane-Gaposhkin and she determined the abundances when working on her PhD thesis and it was an amazing shocking discovery and in fact she had to put in her thesis specifically that these could not possibly be correct. There was no way we could understand how could the Sun be made of something that is so different than the earth. So even back in 1925 this was not immediately accepted but it turns out that her calculations were correct and that the abundances that the Sun is made up almost completely of just these two elements. Now let's look a little further into the Sun. Let's look inside the Sun and see what we can find. The interior of the Sun is divided into three parts. We have the core which is the central portion of the Sun and you can see that way down here in the central portion that is where the energy is being generated. So that is the region of energy production and that occurs by nuclear fusion fusing of lighter elements into heavier elements. A little further out we get the the radiative zone so that's the little bit outer section out here little further out from the core and that is where energy is transported outward by radiation photons traveling being absorbed and then re-emitted and then finally we end up at the outer layers towards the convective zone which is the outermost layer of the interior of the Sun and in this region energy is transported as you might expect by convection. So here by convection in the radiative zone by radiation. Now if you go a little further out those are parts of the Sun we can't see what can we actually see. Well what we see of the Sun is what is called the photosphere. So the photosphere or the sphere of light is the visible portion of the Sun that we see. No energy is generated here and that's very important to remember even though it looks like the Sun is something burning no energy is being generated in the Sun the energy is only being generated in the core. Essentially the rest of the Sun is a transport mechanism to get the energy from the core where it's being generated to the photosphere where it is being radiated away. So this is what we see is the visible surface of the Sun. However it is not a solid surface it's not something even considering the massive temperatures that you could ever land on. It just will continue to get denser and denser as you move deeper into the Sun. When we look at the surface of the Sun we do see what we call granulation. We can see that if we zoom in a little bit here if we look at a small portion of the Sun we see what is called granule. So all of these little chunks here of material which are the result of convective currents. So material down below inside the Sun if we have the photosphere up here material down below is welling up hot material rising that will give a brighter area here so this will look bright and then the cooler material will go back down so we actually set up convective currents that will continue in the Sun. So the convective cells are very large and you have an inset here of North America to be able to compare that to. So even a large state something like Texas or Alaska would be something that would be comparable in size to one of these sunspots. But they are just the natural result of the convection of the Sun with that convection zone going down below all these different convective currents bringing energy from deeper in the Sun up to the surface releasing it in the brighter areas that we see. So the brighter spots that we see in all of these are where the energy is being released and in the cooler areas are where it is flowing back down. Now if we move a little further out from the photosphere there is a little more to the Sun than just what we see and we have above that the chromosphere, the chromosphere or the sphere of color because it is a very distinct red due to the emission of hydrogen. So we look at that here this is the inner layer of the solar atmosphere. It is fainter than the photosphere but hotter at reaching temperatures of 10,000 kelvins. We say we call that the sphere of color because of the red color of hydrogen emission and we see it here. We are looking at this not as an actual picture of the chromosphere. What we are looking at is the emission of the hydrogen line. So we are specifically looking just at the emission of the hydrogen line here and that gives us this very distinct coloring and why we call that the sphere of color but we can also see many of the different areas that are active regions that appear very bright in this section. So the brighter areas here are actually regions of activity on the solar surface. Now if we move out a little bit further we will see what we call the transition region. This has a temperature of about 10,000 degrees. Once you get out the past the chromosphere the temperature rises sharply and that is what we call the transition region between the chromosphere and the next layer that we want to look at which is the corona of the sun. Now the corona is the outer layer of the solar atmosphere and is faint compared to the rest of the sun but can be seen during a solar eclipse. So when the moon blocks out the sun then we're blocking out the very bright photosphere and the fainter areas such as the chromosphere and the corona can be seen. Now it has a very high temperature. The temperature in that transition zone rises up to 1 to 2 million Kelvin so it gets very very hot up in the corona very high temperatures. It's not quite hot enough for nuclear reactions but even more importantly even if it did have the temperatures for nuclear reactions the particle density is far too low to be able to sustain nuclear reactions. So even if this were 15 million degrees there would not be enough particles there that they could ever fuse together. Now the corona is the outermost atmosphere but we can go out beyond that and talk a little bit about the solar wind. The solar wind is material escaping outward through what we call coronal holes and we can see holes in the corona where the magnetic field lines kind of stream off into space and the material is not trapped to the sun. So material can stream out of those coronal holes at the rate of about one and a half million tons of material every single second. So the sun is constantly losing material doesn't matter it can afford to lose that much mass every single second and its mass will essentially be unchanged over 10 billion years because of how massive it is. Now this solar wind and the streams of particles are what cause the aurora here on the earth. So we can see the aurora here's an image of one and what we see is that the aurora is this greenish glow that we get in the sky and that is caused by solar wind particles striking the earth's atmosphere. Now they don't strike it directly if we have the earth here the earth is surrounded by a magnetic field which looks like a little bar magnet field that we get around it and when those particles strike they don't cross the magnetic field lines but instead they follow along it and end up coming in and striking the earth close to either the north or the south magnetic poles and that's why we always see the aurora very far north or south because those are the places where the magnetic field comes into the atmosphere and we can actually have the particles from the sun striking the earth's atmosphere and just like hydrogen in the sun glows distinctly red oxygen atoms in the upper earth's atmosphere will have a distinct green color and we can see that very distinctly here there are some other colors that you can sometimes see green is by far the most prominent although you can sometimes get reds or violets up even higher in the atmosphere. So this is one of the examples of how what's going on the sun directly impacts what we see here on the earth so let's finish up with our summary and what we've looked at this time is that the sun is divided into many layers and we looked at a number of those but it has no solid surface there is no place on it to ever land so even the high temperatures but there is no solid surface to it just like Jupiter just like the outer planets there is nothing down below there that is solid the sun is completely gaseous we cannot directly view the inner layers that we've talked about we have to use indirect methods to be able to study them we can see the photosphere that is what we see is the visible surface and some of those outer layers are visible during an eclipse when the photosphere is blocked and then we looked at how particles from the sun and the solar wind can interact with the earth's atmosphere giving us the auroral displays that we see here on earth so that concludes our lecture on the structure of the sun 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