 Greetings and welcome to the Introduction to Astronomy. In this lecture, we are going to discuss the end state of the more massive stars. And that is a supernova explosion. And then we will look later at what is left behind after that supernova explosion. But we're going to look first at the supernova itself. And let's go ahead and get started with these. So what is happening with these higher mass stars? And as you may recall, we have that when we look at nuclear fusion, the fusion can continue until iron is built up in the core. Once we get to iron, we cannot gain any energy. So no more energy can be gained once we get to iron. It actually takes energy to fuse iron together. And as the mass increases then in the central portion, the electrons are pushed into the nucleus of the atom forming neutrons. So essentially, the central portion will become a great big ball of neutrons down here where we see iron. That will eventually under enough pressure and enough mass. Remember that we had electron degeneracy pressure and that could hold up a white dwarf star. However, if we went over that 1.4 solar mass limit, then that electron degeneracy pressure was no longer enough to be able to hold it up under the immense pressure of all of that material pushing on it. And at that point, the electrons are pushed into the nucleus. The electron, which is negative, combines with the proton, which has a positive charge, to form a neutron. And essentially this becomes a big ball of neutrons and now instead of degenerate electron pressure, we have degenerate neutron pressure. The neutrons are packed together as close as they can possibly be and that will keep the core from collapsing further. So let's look a little bit about how the supernova will occur. So what happens in the supernova is that the core quickly collapses. This is extremely fast, not just from an astronomical standpoint, but from any standpoint. So essentially you've compressed it down to the size of the Earth, remember that was the electron degeneracy pressure holding it up, to something about 20 kilometers in size. So 20 kilometers being about 12 miles in size. So we're talking about going from the size of a... the Earth, planet size, to the size of a city. At this point, the degeneracy pressure of the neutrons will go ahead and support the core and we form what is called a neutron star. So a white dwarf will form when the electron degeneracy pressure holds up the star. A neutron star will form when the degeneracy pressure of the neutrons is supporting it. And the rebound of the material as material falls into that newly formed neutron star, it will bounce and rebound off and that will create a shock wave that moves out through the star. This will disrupt the outer layers and expel them out into space. Now the exact method of this is not completely understood and is something that astronomers still strive to understand what is going on how this bounce works to be able to form a supernova. And this is what we call a type 2 supernova. Yes there is a type 1 that we will look at as well which is very important for determining distances, but a type 2 is what happens at the end of a very massive star. So there are several different things that can happen at the end of a star's life. And that depends on what its mass is. So when something forms, we can go and look at everything now. The final state of any object depends on its mass. That mass is what controls what it is going to become. A very low mass object will become a planet and here we see compared to the mass of the sun, anything about one one hundredth or less will become a planet. A lower mass, but higher than the planet, will become a brown dwarf star. That is going to be anything in the range of 0.01 to 0.008 solar masses. So anything outside of that will be anything outside of that in that range will become a brown dwarf star. The next big range is where the white dwarfs occur and the white dwarfs are forming in this range. And there are different types but essentially any star with an initial mass of 0.08 meaning it was actually a star that was able to undergo hydrogen fusion up to about 10 solar masses will eventually become a white dwarf star. Now depending on the mass, some cases they'll stop at helium, others like our sun will get up to carbon and oxygen, others will get up to neon and magnesium depending on the mass of the star but they will all stop as a white dwarf star. Stars in the range of 10 to 40 solar masses will undergo a supernova explosion that leaves behind the neutron star that we have discussed. So those high mass stars will leave us either a neutron star or a black hole. And the black hole is generally only those stars that are over 40 times the mass of the sun when they form. So they are very rare to occur. Remember that the higher the mass, the fewer stars we get. So there's lots of these low mass stars here. Very few of the high mass stars present here. So what are some of the effects of a supernova explosion? And we'll take a look at that here. First of all it does create the heavy elements. So heavier elements, some of the objects that are created, gold, silver, lead, uranium are examples of objects that can be created in a supernova explosion. Not only does it create them, but it also enriches the interstellar medium. So we now have more of these heavy elements to form the next generation of stars that is now enriched in the interstellar medium. So the first generation of stars would have only had hydrogen and helium. Later generations of stars would have had things like carbon and oxygen and silicon. And as supernovae continue to enrich that, we get more things like gold and silver and lead and uranium to make up those as well. Now a supernova could actually be devastating if it were to occur close to the Earth. In fact a supernova calculation showed that a supernova within about 50 light years would wipe out all life on the planet. Within about 100 light years it could still cause extinctions, great extinctions due to the high radiation levels. However we don't need to worry about that because fortunately there are no stars that could go supernova that are this close to the Earth. The nearest stars would be many hundreds of light years away so while there could be some more minor effects they will definitely not be as devastating as here as if we happen to be really close to a star that went supernova. Now when do we see supernovae? We've seen a number of these in the past. So let's take a look at some of these historical supernovae, ones that have been recorded in the past and the first is the supernova of 1006. So this occurred over a thousand years ago and we see the remnant now. This was recorded by multiple sources and is visible today. This remnant is visible in the constellation of Lupus. So we are able to see the remnant. We had records from described locations where people had noted the location of the supernova, what stars it was close to and we can point our telescopes to that today and we see this remnant. Now the original star would have been somewhere at the center and it would have imploded and then it exploded outward and is pushing all of this material out into space to become part of the interstellar medium. So this is where that material that has been enriched by the supernova explosion will hundreds of thousands of years from now or millions of years from now probably become parts of other stars. In a way this is where we all come from. That material in our bodies was likely a lot of it was expelled out and in a supernova at some point in the distant past. So this may have occurred at some point in the past has occurred at some point in the past and is where a lot of the heavy elements in your body come from. Now another supernova is the supernova of 1054. Now another supernova is the supernova of 1054 and that is known as the crab nebula and is located in the constellation of Taurus. So if we look in the constellation of Taurus with a telescope we can actually see this object there and this supernova was seen to occur on Earth in 1054 and again the locations have been recorded and we can point our telescopes there today and find out that yes there are still there is the remnant still visible. At the center of this is a neutron star so we can actually see the remnant of this as a neutron star it is still present there today and will remain and that is the central portion of this star that exploded and the outer layers like in the supernova of 1006 are still expanding back out into space to become parts of seed material for other generations of stars. Now there are many others that have occurred in fact the last couple that have occurred here are the supernova of 1572 and 1604 sometimes known as Tycho supernova and Kepler supernova and hopefully you recognize these names some of our studies of the history of astronomy and Tycho was the great observer he happened to observe a supernova that occurred and Kepler did the calculations to figure out the orbits of the planets and he observed a supernova as well but what we do note is that no supernova has been observed in our galaxy since the invention of the telescope Kepler supernova in 1604 was the last supernova to be seen in our galaxy however we do see supernovae in lots of other galaxies we can still study them but nothing up close obviously as we've already said we don't want one too close but it would be very interesting for astronomers to be able to study an object that went supernova that was much closer where we had had a chance to really see it up close before it became a supernova now the one closest example of this is the supernova known as 1987A supernovae are named by the year at which they were discovered and then letters A, B, C and so on going through the alphabet to determine when it was discovered in the year so in this case this was the first supernova discovered in 1987 and it was actually found in February of 1987 in the large Magellanic cloud so this was bright enough to be easily visible from the Earth, the Magellanic cloud is easily visible this was visible with the naked eye this was very important because it was the first time that the progenitor star, the star was seen before the supernova exploded images taken of this region previously showed the star and we saw the star there and what was confusing about this to a lot of astronomers is that the star was a blue supergiant not the type that was expected to form a supernova astronomers had thought previously to this that it would be the red supergiants that would form a supernova at the end of their lives a blue supergiant was not expected to be this so it is still causing us to really better understand what is going on and how supernovae work but it is still striving to understand now we can see here in the image this is an image taken by Hubble Space Telescope of this and we can see where the material is still beginning to expand now this is much more recent than the other two supernovae which had occurred nearly a thousand years ago and this one is still expanding outwards so eventually we will be able to see a nice supernova remnant here but after only a few decades it is still just beginning to expand outward into space but we can see that today and astronomers will continue to map that over the coming decades to really be able to understand this supernova so let's finish up as we do with our summary here and in terms of supernovae what we've looked at first of all is that electron degeneracy cannot support the core of a high mass star if the star was originally more than about 10 solar masses it will not be able to be held and if the core remnant is more than 1.4 solar masses in those cases a neutron star will be formed in the collapse as electrons are forced into the nucleus of the atom with protons and forming neutrons the outer layers will be expelled out in a massive explosion that we call a supernova so that is an extreme explosion and one of the most energetic events that we know of in the universe we've seen a number of these and we looked at a few examples of these that have occurred over the last thousand years but none have occurred in our own galaxy since the telescope had been invented so that concludes our 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