 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to begin talking about our Sun and specifically this time we will talk about the structure. So some introductory material about our Sun and then what the structure of our Sun is like. And this will lead us into being able to talk about other stars later on. So what do we know about the Sun? Well we have its mass, tremendous mass, 2 times 10 to the 33rd grams. That's a 2 followed by 33 zeros. Now because it's such a large number we tend to write these things for the Sun and for other stars relative to the Sun. So we call this mass one solar mass, also written like this. Now we do the same thing for the luminosity. The luminosity of the Sun is one solar luminosity. The radius of the Sun is one solar radius. And while that doesn't tell us anything specifically about the Sun, we can use those numbers if we really want to look at that, it does allow us to compare and say that another star is 2 solar masses. It's twice as massive as our Sun. Now the surface temperature is almost 6000 Kelvin and that is very hot although we'll see much hotter temperatures when we get to the interior. The density is about the density of the Jovian planets, the larger planets in the solar system and a little bit denser than water. It has an extremely dense core but very low density atmosphere. And its rotation period is a little less than 25 days. Note that's at the equator. The Sun is not a solid body and does not rotate like one. So the rotational period at the poles would be about 4 or 5 days longer than this. Now what is the Sun made up of? Well, the Sun and other stars are primarily made up of hydrogen and helium. And we see that here. Hydrogen and helium make up 99.9% of the total atoms of the Sun and 98.1% of the total mass of the Sun. So vast majority of the Sun is hydrogen and helium. And we find that elsewhere that is the same for all other stars. Stars are almost completely made of hydrogen and helium. The other materials, the things that we talked about in the solar system are much smaller percentages, very rare. And in fact this was determined in 1925 by Cecilia Payne-Gaposhkin who was determining abundances. And this was an amazing discovery and not actually believed to be correct at the time. At that point it was figured that stars would be much like Earth in terms of their composition. So finding out that they were so different was an extremely shocking discovery and it took a little while before that became accepted and understood that yes it was true that the Sun was made up of something different than Earth. Now the interior of the Sun and we can look at that in a cutaway has several different parts. It has a core which is the region of energy production. The surface of the Sun that we see is not producing any energy. The only region that produces energy is the core. The rest of it is a transport mechanism to transport energy out. Now if you recall the three methods of energy transfer are conduction, convection and radiation. Well conduction is not very important in stars except in maybe some very extreme dense stars. But things like our Sun, energy is transferred either by radiation and it does that in the inner region just past the core and that is where energy is transferred by photons. So photons travel through that. However they don't travel through quickly because they are constantly being absorbed and re-emitted so it can take them a long time to work their way through that radiative zone. Then there is the convective zone closer to the surface where there are convective currents. Material is heated up, rises to the top, cools off and then goes back down. So convective currents will occur within the convective zone here where material rises up to the top and then convects back down. Now when we get out to the outer layers what can we see for the Sun? We have the photosphere. The photosphere is the visible part that we actually see and it radiates away that energy that was generated. Again there is no energy being generated in the photosphere. It's not burning in any sense. It is just glowing because of its extremely high temperature. It's also not a solid surface. There is no solid surface to the Sun just as there was not to the Jovian planets. It only gets denser and denser as you get deeper into the Sun. Now when we look at the Sun and look a little closer in at the surface we see granulation. Granulation is one of those things that we see and this is the result of the convective currents that we talked about. Those convective currents bringing energy to the surface and that material bubbles up as the lighter colored areas where it's a little bit warmer and then that cools off and the darker colored areas sink down. So that is some of the evidence of those convective cells we can actually see it on the Sun's surface and we can get an idea of the scale with the map of North America here and we can see how large these are compared to different parts of North America. Now we can still go up above the Sun also has an atmosphere so the inner layer of the atmosphere is called the chromosphere. Chromosphere is fainter than the photosphere but hotter and it is sometimes called the sphere of color because it is the red color from hydrogen emission. At this 10,000 degrees hydrogen emission is at its peak most intense so well there's some hydrogen emission at the surface 6,000 degrees isn't enough to excite the hydrogen as well. When you get to 10,000 degrees very good at exciting that hydrogen gas and causing it to glow. Now beyond that we see the transition zone so here we saw the photosphere here and then we warmed up a little bit more into the chromosphere and then we have this transition region where temperatures just spike up till we get to the outer atmosphere of the Sun. Temperature rises sharply from thousands of degrees to a million degrees or more. So we see that in the corona here and we can take a look at the corona because that's something we can see during an eclipse. So this is the outer layer of the solar atmosphere which we can see during solar eclipses and I should say that's a total solar eclipse not a partial solar eclipse you have to completely block out the surface of the Sun. It has an extremely high temperature of 1 to 2 million Kelvin. However there is there are no nuclear reactions there 1 to 2 million can support some nuclear reactions but not most of the regular hydrogen one but the density is far too low for any nuclear reactions to occur so even though you had high temperatures even if this was 10 or 15 million degrees like the center of our Sun there is not enough density of particles you have to have enough particles squush together to get them to fuse together. Now out beyond the corona what else is there? Well we have the solar wind that is material leaving the Sun so through corona holes material is able to escape from the Sun and the Sun is losing 1.5 million tons of material every single second and over its 10 billion year lifetime remember what its mass was it will not even notice that amount of material being lost. What this does do is cause the aurora here on Earth the solar wind particles will strike the atmosphere and when they do that they will cause the oxygen atoms in the upper Earth's atmosphere to glow and the oxygen gives a distinct green glow much as hydrogen gave a distinct red glow so we get that greenish glow in the atmosphere now these are often called the northern or southern lights because they are visible at very far northern or southerly latitudes that's because the particles are funneled along the magnetic field and then strike Earth's atmosphere when those magnetic field lines enter the atmosphere which would be near the north and south magnetic poles so we tend to see these very far north and south although large storms can push them visible a little bit further down toward in the Earth so let's go ahead and finish up this with our summary and what we've looked at is that we looked at many layers of the Sun but there is no solid surface anywhere there we cannot directly view those inner layers we have to use various methods to interpret what they must be like the outer layers are visible during an eclipse when we block out the photosphere so when that is blocked then we can see those fainter outer layers such as the chromosphere and corona particles from the Sun can interact with Earth's atmosphere causing the aurora that we saw so that concludes this 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