 Greetings and welcome to the Introduction to Astronomy. In this video we are going to talk about the evolution of higher mass stars. Previous videos have looked at the evolution of stars like our Sun and there is quite a difference in what happens to a high mass star as compared to a low mass star because the high mass star is better able to produce heavier elements. It will get higher temperatures and therefore heavier elements produced in its core. So while our Sun was stopped at carbon production in its core, higher mass stars will be able to go beyond that. So let's take a look a little bit at what we can see here. So let's look at the evolution of a more massive star and this should look relatively similar to what we had seen for the lower mass stars that we had built up except that we had only built up carbon in the core. In this case, when something is more massive than the Sun, the temperatures will continue to rise and that means eventually we will get to the point where carbon is able to fuse together and we will build up multiple layers and as we see here we go from hydrogen, the envelope and we get hydrogen and helium fusion and then we can get helium fusion and then carbon and oxygen, magnesium, neon and oxygen, silicon and sulfur and finally up to iron. So iron builds up in the core and this is the key point that happens there because that is the limit as to what we can do as we will see later on. Iron is the last element that can be produced through nuclear fusion and give you any energy. To try to fuse anything heavier than that actually takes energy away from the star and so the build up of iron in the core becomes a very important stage in the life of a star. So let's look at an example and what we have is the star Ada Carinae and this is an example of a very massive star right at the limit of 100 solar masses about as massive as a star can possibly be. It is ejecting material out into space and that material is enriched in heavier elements. So this is going through all of those stages right now and building up heavier elements in its core. There is extensive mass loss going on and while the fusion the core continues to fuse elements eventually this will become unstable produce iron in its core and become a supernova. So this is one that is likely to become a supernova in the relatively near future at least astronomically speaking in that meaning that it could happen in tens of thousands of years or even 100,000 years. It is just when it builds up that iron core that it starts to become completely unstable but this is one star that astronomers look at as a possibility to be the next nearby supernova. So let's look a little bit at how we actually build up the elements by this process. So as we saw here we build up to iron in the core and this is where a lot of the heavier elements come from. Elements normally stars produce a fuse hydrogen into helium. These heavier stars are actually fusing the iron limits fusing the higher limited stars. So iron is that limit. You cannot go any higher than iron when you are fusing materials because no energy will be released. The core eventually becomes unstable, implodes collapses down and then rebounds back outward as a supernova explosion. So what this means is okay we've produced elements up to hydrogen, up to iron, how do we get heavier than iron because iron is element number 26 in the periodic table and we know that there are 92 naturally occurring elements. So where do the rest of them come from? Well it is believed that some of them could be created during the explosion itself. So during the mass of energy of the explosion, heavier elements could be fused together. It's also believed that sometimes in the collision of neutron stars, heavier elements could be created. So there are some other ways to build up those heavier elements. And when we look at how we build those, what we see, we can look at a graph like this which shows the average binding energy of each element. So hydrogen is down here, it has no binding energy because there is nothing for it to be bound to. Helium is then up here and then we have carbon and oxygen, some of the more common elements in the universe. And this is, the iron is the limit because no energy is released because of this binding energy. As long as you are working your way up this area, you can gain energy by nuclear fusion. So fusing hydrogen into helium gains you energy, helium into carbon, carbon into oxygen, and so on up until you reach iron. However, if you try to fuse iron you start going on the downward slope of this and you are unable to get any energy out of this. You can also work the other direction. So this side is fusion. This side works nuclear fusion, which is how energy is produced in a nuclear reactor. We fusing fusing separate uranium into lighter elements and that also gains us energy. Note how it is actually moving upward if you go this way on the graph. So you can get energy going this direction, you can get energy going this direction, but when you end up with iron you actually get a problem because you can't go either direction and gain energy. And this is where things like supernovae will come from. So let's take a look at some of these heavy element abundances. What do we have? Well, we have what we call two different populations of stars. We have population 2 and population 1 stars. Now population 1 stars are the oldest stars, are the older stars. So these are stars that are present in globular clusters and they have very low heavy metal abundances. Heavy elements, metals, again anything other than hydrogen and helium are much less, one one tenth to one one hundredth what we see in the sun. We also see stars that have heavier element abundances that are similar to the sun and those are the population 2 stars. What this allows is where can we form earth-like planets? When we want to wonder where other planets like the earth formed, they could not have formed from the earliest generations of stars. The metal abundances are very low and if there was not enough carbon, iron, silicon and carbon to be able to form planets like the earth then we would not have been able to form earth-like planets from those very earliest generations of stars. However, the later ones, the population 2 stars would have then been able to form these abundances, would have been able to form planets much like our own earth. So let's take a look a little bit about what happens as we approach the end of the life of a star much more massive than the sun. What we find is that each stage takes a shorter and shorter period of time. So the time becomes less each time, the temperatures get higher, which means that nuclear reaction rates are going to go faster and the fuel will last a shorter time. So we have stages that might last millions of years, then hundreds of thousands of years, then tens of thousands of years and you work your way down to things like a thousand years for one source of fuel and then a hundred and then just ten and then even less than one year. And when you build up iron in the core, that's when you've gotten to this stage. Once it builds up iron in its core, it is going to implode, essentially all of the iron will get so hot that the iron will disintegrate and tear itself apart and the whole losing its pressure and then the star will implode around it and then rebound back out. And that is a very short timeframe, in fact when we get down to here, when we produce that iron we can be talking a day or less for the amount of time that it might take that star to go through that stage of its life. So let's finish up as we do with our summary here and what we found is that massive stars are better able to reach higher temperatures, higher pressures because there is more material pushing down or on them from above and that allows the temperature to then increase so you get a higher temperature down here in the core and therefore you confuse heavier elements and in fact they confuse up to creating iron in their core. Eventually if you form an iron core the star becomes unstable and what that means is that it will end its life as a supernova the star will actually explode. This can never happen for a star like our sun. Our sun will never get close to forming iron in its core. It takes a star many times the mass of our sun and it's only the very largest and most massive stars that are able to undergo this type of supernova explosion. So that concludes our lecture on the evolution of high mass stars. 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.