 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to continue talking about stellar evolution, and this time specifically look at the higher mass stars and what happens to them. So let's take a look at these, and if you recall, we looked at what happens to these even more massive stars. So what if we have stars more massive than the Sun by a good amount? Well, temperatures can continue to rise in the core of the star. And that means even heavier elements will be able to fuse. Now we looked at an image like this four stars like our Sun, but here we have even more layers from hydrogen envelope and we have helium fusion going on in the green. We have carbon and oxygen fusion going on in the purple section, magnesium neon and oxygen in the pink, silicon and sulfur in the darker blue, and then iron ash in the central core. So multiple layers continue to build up in the star until you reach iron. Once you get iron in the core we are going to find out that this is the end of what the star is able to do. So fusing hydrogen into helium gets you energy, helium into carbon, all the way up to iron, and that is the limit of where you can get iron from nuclear fusion. Now let's look at an example of a star that is doing something just like this, and that is the star known as Ada Karinae. It is a 100 solar mass star and it's been ejecting shells of material. Those shells are enriched in heavier elements, probably that were fused in the interior of the sign. It has extensive mass loss, it is losing a lot of mass, and the core fusion continues. This star will eventually become unstable and will explode as a supernova. But will that be next week or will that be a hundred or a thousand years from now? Is a very good question. We simply do not know when that will happen because we cannot get that look at the interior of the star without being able to see exactly where it sits. We know that once it builds up that iron core, it does not have much time. So let's look at building these elements. What do we do? Well, heavier elements continue to be fused, iron is the limit. There is no energy released from fusing iron. In fact, it takes energy to fuse iron, meaning that the amount of energy is lost and the core starts to cool off. So when you get the core hot enough, it starts to fuse iron, it loses energy, that sucks energy out of the core, and the core implodes, collapsing downward and then rebounding outward and exploding as a supernova. Now when we look at this chart here, this shows how much energy you can get out of fusing. So fusing hydrogen into helium, helium is way up here, the helium-4, gives you a lot of energy. Helium into carbon gives you a lot less, carbon to oxygen even less, and it slowly creeps up until you get to iron, but iron is at the peak. So after that, it starts to take energy and more and more energy by fusing these elements. So iron 56 is as high as you can possibly get where, within a star, by fusion. To get any other heavier element, we need a lot of energy to produce these because they cannot be produced by nuclear fusion in a star. So where do these heavier elements come from? We obviously have heavier elements that make up our Earth. And here we show a chart that demonstrates where most of these came from. So fusion within the Big Bang, counted for all of the hydrogen that exists, most of the helium, and a little bit of the lithium, and that's about it. That's what the Big Bang did. Cosmic rays actually form all the beryllium and boron that exist in the world, are formed from cosmic ray strikes on material. Now we can see the other stars that exploding stars, which are the blue and the green here, those are exploding massive stars, which is what we're talking about now, and exploding white dwarfs, which we'll talk about later, form a lot of the elements from oxygen up through krypton or so. So a lot of these middle elements, things like the iron and the nickel, aluminum, sulfur, very common elements that are present, are formed in these supernova explosions. Now the low mass stars can actually form some of these other elements, what we see in the yellow as they are dying, they can form a little bit in parts of these, but a lot of the material is formed by these merging neutron stars. So as we look at the heaviest elements, they either come from reactions in the outer layers of a low mass star or merging neutron stars that occur, and that's where a lot of the elements, a lot of this gold and silver, for example, come from, these much higher elements. So a lot of these elements are created during the explosion of one type or another. Now what do we know about these abundances? Let's take a look at abundances in stars. We have several different types. We have open clusters we've talked about, have what we call population one stars. That is a young population and they have heavy metal abundances comparable to our sun. The globular clusters or population two stars have metal abundances which are much less, one tenth to one one hundredth of what our sun has, so they have far fewer metals. This allows Earth-like planets to form from the later generations of stars. The very earliest generations of stars would not have had things like iron, silicon, or carbon that make up our Earth. They would not have been present when the earliest generations of stars formed. What happens as we get to the end of the life of a star? Well we find that each stage takes less and less time. We get higher and higher temperatures in the core. So that means the nuclear reaction rates are going faster and the fuel lasts a much shorter time. So hydrogen fuses, gets you a lot of energy and the reaction rates are relatively slow. Each helium to carbon reaction gives you a lot less energy and so you need a lot more of them, higher temperatures, a lot more reactions. It takes a lot more time to do that. So it takes a lot more reactions to give you this amount of energy needed to support the star against gravity. Quickly that star will then reach its limit. We'll build up iron in the core, there will be an implosion, and a supernova. And we will look at that process in a little more detail in coming lectures. So let's go ahead and finish up with our summary. And what we've looked at is massive stars are better able to reach higher temperatures in their core and fuse heavier elements. In fact they can fuse things up to iron. And those very massive stars will end their lives as a supernova. So that concludes this lecture on 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.