 Greetings and welcome to the Introduction to Astronomy. In this lecture, we are going to look at the deaths of high mass stars, which end their lives as supernovae. So we'll look at what supernovae occur, and how that process may go about happening. So let's remind ourselves of what happens in a high mass star. In a high mass star, fusion continues until iron builds up in the core. That is the end of fusion. No energy can be gained by fusing iron. So we build up something like this. Here is the supergiant star, tremendous in size, and zooming into just a little tiny dot of a core there, we would have all these different layers. Hydrogen, which would be most of the layer going outward and would consist of most of the star. We'd have a helium layer, where helium had been fused from the hydrogen. We'd have carbon and neon and oxygen and silicon and sulfur, and then finally iron down there at the core. Now once you build up that iron, you're done, because it takes energy to fuse iron. So what happens? The core continues to compress, and we have material raining down on the core from fusion in the shells above it, so that iron core gets more and more massive, it gets more compressed. Eventually the mass increases so much that it exceeds the value that the electron degeneracy is able to hold. Those electrons become pushed into the nucleus of the star. If you push an electron into the nucleus of a star, it will merge with a proton and become a neutron. So now we will have a new form of pressure that will hold it up. It will be similar to what we had with the electrons, where no two electrons could exist in the same state at the same time, but now we will have the same thing with neutrons. The new pressure of the neutrons, and them not being able to exist in simultaneous states, will then keep the core from collapsing. It will provide a pressure. However, we've gotten rid of all the space between the atoms now, so this object will be more the size of a city than the size of a planet like a white dwarf. Now the supernova explosion itself, this really happens very fast. It quickly collapses from something the size of Earth to something the size of a city. Now we have degeneracy pressure of neutrons to support the core and keep it from further collapse, and it becomes what we call a neutron star. Once it does, it becomes incredibly stiff and solid, and this material is still raining down in on it, that material will then strike the neutron core and rebound, and we believe a shockwave will then move out through the star and disrupt the outer layers and expel them out into space. Now exactly how this works is a good question, because in many models the shockwave kind of fizzles out before it actually disrupts the star. So we still have work to do on these models, but this is the thought as to how it would go about happening. And this is an example of what we call a type 2 supernova explosion. We will look at type 1 supernovae later on. So what is the ultimate fate of a star? Well it really depends on the mass, the mass of the object, depending on this object it forms. If it's a very small object, less than one one hundredth the mass of the sun, we call it a planet. Between one one hundredth and eight one hundredths we have a brown dwarf star, not quite enough to fuse. Now the other ones, these stars between point zero eight and eight solar masses will all become white dwarf stars. Now note that the very small mass ones it's all helium, the intermediate mass ones are carbon and oxygen, that's where our sun will be, and the higher mass ones will add neon and magnesium. For the higher mass stars between ten and forty solar masses that's where we start to get a supernova. This will leave a neutron star behind, this will leave a black hole behind, and we will of course talk about black holes later on. So depending on the mass that we have, it tells us what the end state of that star may be. Now what is the effect of a supernova? Well supernovae are good at creating heavier elements. In fact that's where many of the heavy elements occur. And many of the elements in our bodies are created in supernovae and are then expelled back into the interstellar medium in that explosion. So that enriches the heavy elements that then help form the next generation of stars. However supernovae are a tremendous amount of energy. A supernova emits more energy there than the sun would do in its lifetime. So it is a tremendous amount of energy that is being produced. Now how dangerous could this be? Well within 50 light years of a supernova to occur within 50 light years of Earth, it could wipe out life on Earth. That's how much energy, even at that distance, 50 light years away, it could wipe out life. And within 100 light years we could have mass extinctions due to the increased radiation levels. Fortunately there are no stars that could go supernova this close to Earth. But that doesn't mean that a supernova in the past did not actually lead to some of the extinctions that have happened over Earth's history. Now what about supernovae? What do we see when we see a supernova? Well, let's look at what we see left behind. We see supernova remnant, and here are some historical supernovae. There is the supernova of 1006. And this is recorded in multiple locations. And we can see the remnant visible expanding outward for now over a thousand years in the constellation Lupus. So we see the material continues to expand outward in space. So this is what is going to eventually be enriching the interstellar medium that material is enriched. It doesn't mean it's not mostly hydrogen and helium still, but it is enriched in heavier elements which will eventually become seed material for future stars. We also have one of the more famous supernovae is the Crab Nebula. That's the famous remnant. This was seen to explode in 1054 on Earth. And nearly a thousand years ago was seen to explode and has been expanding outward for all that time. At the center there is a compact rotating neutron star. Now we'll look at those in more detail later. But that is the remnant core of the star that existed at this location before the supernova exploded. But astronomers did note this, and it was recorded that at this location in that year that a new star appeared that was so bright that it was visible during the day. And that's how bright a supernova can be. And these are even things that we're talking that are still quite a distance away. These are nowhere near close enough to be dangerous to life on Earth. Now we've also had other supernovae more recently in the late 15 and early 1600s. We had Tycho supernova, Kepler supernova, where they each had observed a supernova. This was the last one observed in our galaxy was in 1604. No supernova has been observed in our galaxy since the telescope was invented. So that's just a few years before the telescope was invented. We have not had a supernova visible in our galaxy. That doesn't mean there haven't been supernovae, but there has not been one in our galaxy to be able to study since the telescope was invented. And that makes it difficult because you want to be able to study it as close as you can where it's still safe. So you want to be able to have had a star that was well studied and well known where you had a lot of data going back to be able to understand what happened and how it became a supernova so we can improve our models. In all but one case, we do not know what the star looked like before the star exploded in the last few hundred years. That one case is the supernova known as 1987A. Now this is the naming convention for supernovae. It starts off with the year and then the letters through the alphabet and then they go to lower case letters and double up the letters depending on how many you find. But that supernova 1987A means the first supernova discovered in the year 1987. This is the nearest one to have occurred since the invention of the telescope. So it was discovered in February of 1987 in the Large Magellanic Cloud. Now that is a satellite galaxy of our own Milky Way. It is the first time the progenitor star, the previous star, had been seen and catalogued and studied. Now this is interesting because the progenitor star was a blue supergiant. These are not the ones that we expected to supernovae. We expected it would be a red supergiant. So again we're already starting to learn something else from our models but here we see that material starting to expand outward. Here it's only been expanding out for a few decades but later on eventually it'll form a nice supernova remnant much as we see in the other ones today. When we continue monitoring we see lots of supernovae in other galaxies but again we have not seen one nearby since the advent of a telescope. 1987A is the most important one because it gives us some extra information about the supernova. So let's go ahead and finish up with our summary and what we've looked at is that electron degeneracy cannot support the core of a high mass star that is more than 1.4 solar masses. A neutron star will be formed in that collapse while the outer layers are expelled in a massive explosion that we call a supernova. A number of supernovae have occurred in the last millennium but none in our own galaxy since the invention of the telescope. So that concludes this lecture on supernovae. 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.