1 00:00:01,100 --> 00:00:10,620 Of like. 2 00:00:10,620 --> 00:00:14,900 Right. I will pick up right where James ended. 3 00:00:14,900 --> 00:00:19,340 No, we're not tried to connect the types of explosions, he explained to you to what we see. 4 00:00:19,340 --> 00:00:28,510 So-called supernova explosions and the later part I will tell you the role supernovae play in the universe, including for ourselves. 5 00:00:28,510 --> 00:00:33,750 Now, first from observation point. What is a supernova? 6 00:00:33,750 --> 00:00:39,070 Now the name says Nova. That's a new star. 7 00:00:39,070 --> 00:00:50,270 Supernova means a bright new star. So the original definition is it's a bright new star that all of a sudden appeared in the sky. 8 00:00:50,270 --> 00:01:00,600 Now, as you know, from James's talk, that's of course, a complete misnomer supernovae not connected with the birth of stars, but the death of stars. 9 00:01:00,600 --> 00:01:09,010 Now they do come in quite a variety of flavours. And let me just show you some. 10 00:01:09,010 --> 00:01:15,270 The first one. Well, is before and after image. 11 00:01:15,270 --> 00:01:19,020 That right, is the before and this the after image, so all of a sudden, 12 00:01:19,020 --> 00:01:23,790 if you look in the sky, you have this new object that looks like a bright star. 13 00:01:23,790 --> 00:01:32,490 This is a particularly important supernova event. It was the last one you could see with the naked eye from our from the Earth. 14 00:01:32,490 --> 00:01:39,070 It occurred in 1987 and therefore called supernova 1987A. 15 00:01:39,070 --> 00:01:44,380 It's a very important event, and it was the first observable supernova for hundreds of years. 16 00:01:44,380 --> 00:01:49,390 But more importantly, you could actually see the neutrinos, as James already alluded to, 17 00:01:49,390 --> 00:01:56,080 that were generated in the collapse of what was a massive star. 18 00:01:56,080 --> 00:02:03,220 Now, this is the before image, and you see this arrow is actually pointed to a particular star. 19 00:02:03,220 --> 00:02:11,040 So in this case, this was actually the first case where we could identify the star that actually exploded. 20 00:02:11,040 --> 00:02:15,960 It was a big surprise because that stuff looked like a middle aged star. 21 00:02:15,960 --> 00:02:23,470 It didn't look like a star at the end of its evolution, and that was a puzzle for a long time. 22 00:02:23,470 --> 00:02:30,280 But not an event like this in our galaxy, because roughly every hundred years or so. 23 00:02:30,280 --> 00:02:37,780 But the hundred billion galaxies in the universe. So there are about seven supernovae occurring somewhere in the universe. 24 00:02:37,780 --> 00:02:43,420 So most of our information on supernovae really comes from supernovae in other galaxies. 25 00:02:43,420 --> 00:02:55,270 And this is an example. You see this galaxy, this fire type galaxy almost edge on and you see this apparent new star that it's a supernova. 26 00:02:55,270 --> 00:03:04,840 While the previous one was connected with the explosion of a massive star, this was connected with one of these nuclear explosions James talked about. 27 00:03:04,840 --> 00:03:09,850 The other types of explosions, and I want to discuss those. 28 00:03:09,850 --> 00:03:14,320 This is a picture of a much more distant event. 29 00:03:14,320 --> 00:03:22,090 So you see a galaxy filled in this blow up version, so you can you see a galaxy and you see this bright spot? 30 00:03:22,090 --> 00:03:29,930 Again, it's a supernova event, but this supernova event was associated with what's called a gamma ray burst. 31 00:03:29,930 --> 00:03:35,060 Another come back to that later, though, some of the most energetic explosions we know in the universe. 32 00:03:35,060 --> 00:03:39,350 So let me now pick up where James essentially ended, right? 33 00:03:39,350 --> 00:03:47,360 He told you really. The evolution of stars is different to different and different channels. 34 00:03:47,360 --> 00:03:54,350 Basically, low mass stars like the Sun later become rejoined and eject burn hydrogen and helium, 35 00:03:54,350 --> 00:03:59,450 but ultimately eject the envelopes and become a white wolf box and white dwarf. 36 00:03:59,450 --> 00:04:03,500 The more massive stars that are more massive than eight times the mass of the Sun 37 00:04:03,500 --> 00:04:10,580 that continue to burn after carbon oxygen all the way burned to form iron cores. 38 00:04:10,580 --> 00:04:16,400 And these are the ones that explode in a supernova. It's the core that collapses two neutron stars. 39 00:04:16,400 --> 00:04:23,720 This is the image of the Crab Nebula, the supernova that occurred in 10:54, 40 00:04:23,720 --> 00:04:31,430 and it leaves a compact remnant either a neutron star 10 kilometres enrages rages or a black hole. 41 00:04:31,430 --> 00:04:35,660 Now, both of these channels can ultimately produce explosive events. 42 00:04:35,660 --> 00:04:39,290 This in the formation of these objects, in the case of the white dwarfs, 43 00:04:39,290 --> 00:04:44,960 if later on, they will excrete and reach the Chinese e-commerce, how that happens? 44 00:04:44,960 --> 00:04:52,550 I get back to later the two basic mechanisms core collapse and some nuclear explosions. 45 00:04:52,550 --> 00:05:00,960 Just a quick recap of what James already told you. The collapse happens at the end of life of a massive star. 46 00:05:00,960 --> 00:05:08,040 When you have an iron core and you've run out for the nuclear fuel, the core collapses to form a neutron star. 47 00:05:08,040 --> 00:05:16,350 And in that phase, it released a lot of gravitational energy. All of the gravitational binding energy that ends up in the neutron star. 48 00:05:16,350 --> 00:05:24,240 And it's about three times 10 to 46 to. Which is much more than what you observe in the ultimate supernova. 49 00:05:24,240 --> 00:05:29,470 And the reason for that is that most of that energy comes out in the form of nutrients. 50 00:05:29,470 --> 00:05:35,200 A small fraction of these neutrinos somehow posited in this in falling star and 51 00:05:35,200 --> 00:05:41,870 reverse this collapse into an explosion because the supernova is an explosion. 52 00:05:41,870 --> 00:05:50,570 This explosion occurs in oaks and white walls when you ignite carbon under general conditions and his James said, that's a powder keg. 53 00:05:50,570 --> 00:05:59,750 It's a it's like a nuclear nuclear bomb. It burns, disrupts the star and you don't expect any remnant. 54 00:05:59,750 --> 00:06:03,650 In this case, the energy source is actually nuclear energy. 55 00:06:03,650 --> 00:06:10,810 The energy that comes out in both cases is comparable, and that has to do with the fact that the structures of these objects, 56 00:06:10,810 --> 00:06:15,830 the iron core in the civil war, are actually quite similar. 57 00:06:15,830 --> 00:06:19,280 Now, someone asked before about the history. 58 00:06:19,280 --> 00:06:28,190 Now the basic idea that neutron stars neutron stars form at the end of the life of massive stars goes back to Oppenheimer. 59 00:06:28,190 --> 00:06:31,100 You already speculated that in the 30s and in fact, 60 00:06:31,100 --> 00:06:36,800 the idea that this could be connected with the supernova was proposed by Fritz Vicki in the thirties. 61 00:06:36,800 --> 00:06:43,820 So only a few years after the discovery of the neutron, this picture had developed. 62 00:06:43,820 --> 00:06:48,380 It took, of course, many decades before we actually started to understand it. 63 00:06:48,380 --> 00:06:53,720 Now would we have been able to predict it without observation? It was another question. 64 00:06:53,720 --> 00:07:01,400 And the answer is no. People are simulating these core collapse calculations. 65 00:07:01,400 --> 00:07:08,850 And until I would say five years ago on the computer, they didn't explode. 66 00:07:08,850 --> 00:07:13,000 All right. They just collapsed. 67 00:07:13,000 --> 00:07:21,720 So if you had to trust computer simulations until a few years ago, we would not have predicted the existence of these core collapse. 68 00:07:21,720 --> 00:07:30,960 So observations really have been key for actually understanding it before guiding the research, the ideas of the thumb, nuclear explosions. 69 00:07:30,960 --> 00:07:36,770 And I think I agree with James this really required first the understanding of nuclear fusion in stars. 70 00:07:36,770 --> 00:07:43,370 It's the first time I would say would be that this was proposed would be the late 60s, early 70s. 71 00:07:43,370 --> 00:07:49,550 But I can't exactly pinpointed anyway, all of this. 72 00:07:49,550 --> 00:07:55,560 All of these explosions occur on a timescale of a second fraction of a second or a few seconds. 73 00:07:55,560 --> 00:07:59,360 So everything James talked about is over after a few seconds. 74 00:07:59,360 --> 00:08:04,040 Now, at that time, you don't see anything, at least not in the actual electromagnetic radiation, 75 00:08:04,040 --> 00:08:08,120 because this happens inside hidden deep inside an envelope. 76 00:08:08,120 --> 00:08:16,390 Either the outer parts of the white wolf or if you have a massive star, it's inside the envelope of the massive star. 77 00:08:16,390 --> 00:08:21,250 But what the energy you you put in there, the 10 to 44 joules, 78 00:08:21,250 --> 00:08:28,690 what they typically do is they drive a shock and that shock passes through the remaining envelope and does two things. 79 00:08:28,690 --> 00:08:33,700 First, it accelerates all the material and makes it explode. 80 00:08:33,700 --> 00:08:45,220 But it also deposits a lot of energy behind the shock. And so the material behind the shock is is quite hot and that that energy in this 81 00:08:45,220 --> 00:08:49,210 shock heated region is actually providing an important source for the energy, 82 00:08:49,210 --> 00:08:54,150 for the light you see later on. Right in the centre. 83 00:08:54,150 --> 00:09:00,900 You also have radioactive heating because in the nuclear in the explosion, 84 00:09:00,900 --> 00:09:07,980 you have nuclear reactions that produce all the radioactive elements and the most important of which is nickel 56, 85 00:09:07,980 --> 00:09:11,670 nickel 56 produced and subsequent decays. 86 00:09:11,670 --> 00:09:17,830 And that's also important energy source for the later observable supernova. 87 00:09:17,830 --> 00:09:23,110 Now, the shock front passes through the embodiment of your half. 88 00:09:23,110 --> 00:09:28,900 And the first time you see any light from this is when the shock front reaches the surface. 89 00:09:28,900 --> 00:09:36,470 Typically produces X Ray Flash and the time between the action. 90 00:09:36,470 --> 00:09:41,870 Cool explosion and the shock outbreak breakout. Depends on the type of star. 91 00:09:41,870 --> 00:09:51,060 If you have a white dwarf or a compact star, it's a few seconds. But if you have a big star like a red supergiant, it actually takes a whole day. 92 00:09:51,060 --> 00:09:58,500 All right, so if you have a supernova, a red supergiant, our galaxy explode, you will see the neutrinos immediately. 93 00:09:58,500 --> 00:10:03,500 But the first light you will see a day later. 94 00:10:03,500 --> 00:10:15,070 But once the shock has passed and now everything's expanding, the ejector start to cool and radiate, and that's what you see. 95 00:10:15,070 --> 00:10:20,620 As material becomes cooler, it becomes neutral and transparent. 96 00:10:20,620 --> 00:10:25,750 And so you can see deeper and deeper into the ejector up to some point. 97 00:10:25,750 --> 00:10:31,370 A photo sphere like photos, we have the Sun where most of the photons escaped from. 98 00:10:31,370 --> 00:10:36,770 Within this, you still have this hot region that tries to pull in right the centre. 99 00:10:36,770 --> 00:10:45,360 You have a radioactive energy source that copiously produces X-rays and gamma rays that then diffuse outwards. 100 00:10:45,360 --> 00:10:55,110 This first process last takes hours, two days. This takes days to a few, perhaps a few years after after that. 101 00:10:55,110 --> 00:11:04,420 Most of the material is transparent and then you go into a different phase going to the supernova remnant phase, which I'll talk about later. 102 00:11:04,420 --> 00:11:18,010 Now, to understand this physics of this is relatively straightforward, not going to get quite that complicated. 103 00:11:18,010 --> 00:11:21,520 What I'm showing you here are two light codes. 104 00:11:21,520 --> 00:11:27,340 This is for type two supernova the core collapse, and this is for type one, a supernova which is similar to explosion. 105 00:11:27,340 --> 00:11:32,920 What you see here is a function of time. Schematically, here is luminosity. 106 00:11:32,920 --> 00:11:41,230 It's a logarithmic luminosity scale and the indicated indicator of the maximum luminosity of a core collapse supernovas about 10 to 36 watts. 107 00:11:41,230 --> 00:11:44,890 Now that doesn't mean much, but that's in solar luminosity. 108 00:11:44,890 --> 00:11:50,350 That's about 10 billion times the luminosity of the Sun. So it's their voluminous. 109 00:11:50,350 --> 00:11:57,890 And that's why, for a brief moment, these supernovae actually can outshine whole galaxies. 110 00:11:57,890 --> 00:12:05,210 And the right hand side, you will see it on this it more or less the same thing, except that this uses a magnitude scale. 111 00:12:05,210 --> 00:12:08,750 You don't need to know what that is. Important thing. It's a logarithmic luminosity scale. 112 00:12:08,750 --> 00:12:15,260 The power output on algorithmic scale, more negative means more luminous. 113 00:12:15,260 --> 00:12:20,390 The peak luminosities in these two types are comparable the type 1A. 114 00:12:20,390 --> 00:12:26,240 The nuclear explosions tend to be a bit more luminous, but they're comparable. 115 00:12:26,240 --> 00:12:32,130 Now, how do you understand? The difference between the two. 116 00:12:32,130 --> 00:12:42,240 Now, the duration of the supernova phase is supernova phase in both cases is ultimately determined by the diffusion time, 117 00:12:42,240 --> 00:12:50,070 how long it takes all the energy, all the photons inside the ejecta to escape at later times. 118 00:12:50,070 --> 00:12:54,300 You see this straight line is this zero growth in scale? 119 00:12:54,300 --> 00:12:56,580 That means it's an exponential. 120 00:12:56,580 --> 00:13:07,900 You also see it here, the beginning of it, and we'll continue at this long exponential tail exponential energy input that suggests radioactivity. 121 00:13:07,900 --> 00:13:17,190 And that's what it is. This is caused by just by the cooling, by the by the decay of 1956, first to Cawood and then to 1856. 122 00:13:17,190 --> 00:13:24,150 We know this very well. You know the half life of the decay for nickel 56 and also for cobalt 56. 123 00:13:24,150 --> 00:13:33,090 And we know how much energy is released in the decays. And so that's very useful because by just measuring the luminosity in this late phase, 124 00:13:33,090 --> 00:13:39,030 we can immediately measure how much mass of this radioactive material was produced. 125 00:13:39,030 --> 00:13:43,470 It's a very simple calculation, high school calculation you can do. 126 00:13:43,470 --> 00:13:51,470 And so for. And this tells us really how much of the nickel 56 was actually produced. 127 00:13:51,470 --> 00:13:54,320 You know, core collapse supernova on the left, 128 00:13:54,320 --> 00:14:02,240 the typical mass is about point one solar mass or even less in contrast for the thermonuclear explosions. 129 00:14:02,240 --> 00:14:07,130 It's typically point seven solar masses, so it's much more neatly produced there. 130 00:14:07,130 --> 00:14:10,370 And since the Nicolas Otamendi case to Iron 56, 131 00:14:10,370 --> 00:14:20,720 this is why we believe actually why we know that these thermonuclear explosions are the dominant producers of iron in the universe. 132 00:14:20,720 --> 00:14:26,240 Now to understand the length of the light curve. 133 00:14:26,240 --> 00:14:31,400 When I said it's a diffusion process, and again, it's actually a simple calculation can do. 134 00:14:31,400 --> 00:14:40,010 It's a random walk process, but the photons scatter back and forwards and to escape from the photo sphere. 135 00:14:40,010 --> 00:14:45,860 Now this is just probably false in this simple random walk in one dimension. 136 00:14:45,860 --> 00:14:54,650 If you have a random walk in each step, you move by something else and you want to know where after any steps you are, 137 00:14:54,650 --> 00:15:00,260 if the probability going forward and backwards is equal to the average expectation is zero. 138 00:15:00,260 --> 00:15:08,270 You don't go anywhere. But what changes over time is the envelope of this path. 139 00:15:08,270 --> 00:15:14,860 In fact, if you calculate what's the R squared after in steps, that's end times L script. 140 00:15:14,860 --> 00:15:21,180 So the root mean square distance after in steps is this gridlock end times that. 141 00:15:21,180 --> 00:15:28,140 If at any point you reach the photosphere from which the photons then escaped freely to infinity. 142 00:15:28,140 --> 00:15:33,650 Then this process has ended. And so this tells you how many steps you need. 143 00:15:33,650 --> 00:15:41,010 So if you want to go to a Radius R and each step is L, you know you need R overall squared steps. 144 00:15:41,010 --> 00:15:44,370 It's very common resolve, which I'm sure you've all seen before. 145 00:15:44,370 --> 00:15:51,110 So we can then define the diffusion time just as the number of steps times the time of each step. 146 00:15:51,110 --> 00:15:58,690 I just just l sees the speed of light. And so you get our script over and see. 147 00:15:58,690 --> 00:16:02,560 So all we need to know is what is the length of each step, 148 00:16:02,560 --> 00:16:10,420 which is the mean free path for the photons and that rewrite in terms of one of Chaparro where Kappos called the opacity. 149 00:16:10,420 --> 00:16:16,270 But what it is, it's just a cross-section per unit mass and rose the mass density. 150 00:16:16,270 --> 00:16:23,880 This this absorption per unit mass across the community is something which we know just from basic atomic physics. 151 00:16:23,880 --> 00:16:32,360 Of course, all of this occurs not in a static star. It occurs in an object that's readily expanding. 152 00:16:32,360 --> 00:16:35,120 So we need to take that into account. 153 00:16:35,120 --> 00:16:46,410 But we can estimate the velocity of the ejector bus just taking the energy and calculating the average velocity, which is the scourge of tree over. 154 00:16:46,410 --> 00:16:55,540 And if you put all of that in and it's really very simple physics, very simple mathematics, you can estimate what the diffusion time is. 155 00:16:55,540 --> 00:17:03,610 It ultimately depends on the mass of the ejecta. This absorption coefficient and the energy. 156 00:17:03,610 --> 00:17:14,080 If you put in typical values for massive star, the 10 to 44 jewels and a typical mass of 10 solar masses, that gives you about 150 days. 157 00:17:14,080 --> 00:17:22,170 And that's very typical, very comparable to what you see. If, on the other hand, you have a much more much smaller mass. 158 00:17:22,170 --> 00:17:30,160 Just one solar mass, you get some much shorter time to get more like 20 days again, which is what you see here. 159 00:17:30,160 --> 00:17:32,200 So it's very simple, very simple process. 160 00:17:32,200 --> 00:17:42,640 I mean, I've oversimplified a little bit because the absorption properties in the UK is a bit different, but I think we get the idea. 161 00:17:42,640 --> 00:17:53,590 I want to make this point a bit further to show you here is just a bunch of supernova light curves for assuming four core collapse supernovae, 162 00:17:53,590 --> 00:17:57,810 assuming that in all of these you have the same type of explosion. 163 00:17:57,810 --> 00:18:04,620 You have an energy of 10 to 15, 10 to 44 Joule that's deposited deep inside a massive star. 164 00:18:04,620 --> 00:18:12,650 The top two actually are two observational light curves. This times in a second, but. 165 00:18:12,650 --> 00:18:18,110 Never mind, this is a supernova in 1969. 166 00:18:18,110 --> 00:18:23,120 Because you had this long faced with the light curve is almost flat. 167 00:18:23,120 --> 00:18:27,170 Now this in the later years of this exponential you. 168 00:18:27,170 --> 00:18:34,780 This was the star that exploded here was a big red supergiant, probably a thousand times the radius of the sun. 169 00:18:34,780 --> 00:18:40,150 Also shown here is the light curve of the supernova 1987A, and you see it early time. 170 00:18:40,150 --> 00:18:42,850 It's actually quite a bit fainter. 171 00:18:42,850 --> 00:18:51,610 Now the reason for that is that the star that exploded here was not a big supergiant was a compact star, but 40 times the radius of the Sun. 172 00:18:51,610 --> 00:18:57,040 And a lot of this energy in the early days had to actually go into expanding it. 173 00:18:57,040 --> 00:19:06,340 And that essentially made it a much fainter event at early times, at late times when the heating is just due to radioactivity. 174 00:19:06,340 --> 00:19:13,610 It just depends on how much nickel was produced. To make this point further, in the bottom panel, 175 00:19:13,610 --> 00:19:21,260 you see some theoretical ideas which you generated and what we just did here is we took a massive star of turf solar masses, 176 00:19:21,260 --> 00:19:26,350 a big big red supergiant and took off different amounts of mass. 177 00:19:26,350 --> 00:19:32,050 We didn't take any mask off, then this is the light curve you get. 178 00:19:32,050 --> 00:19:34,840 And you see, again, you have this long. 179 00:19:34,840 --> 00:19:46,690 More or less constant portion that's called a plateau, and observationally you would classify this as a type two p where the P stands for plateau. 180 00:19:46,690 --> 00:19:51,010 But it's basically just a red supergiant with a massive envelope. 181 00:19:51,010 --> 00:19:57,610 If you reduce the mass from eight point eight to five to the plateau, becomes shorter, 182 00:19:57,610 --> 00:20:04,370 ultimately disappears, and then the light curve looks much more like a linear decline. 183 00:20:04,370 --> 00:20:10,670 And observation that's classified as a to L L stands for linear. 184 00:20:10,670 --> 00:20:15,970 As you reduce the even more well, you get a very short peek. 185 00:20:15,970 --> 00:20:22,480 And after a few days, tens of days, you don't see any when you see something different. 186 00:20:22,480 --> 00:20:31,520 These are called type to be and I'm not going to explain why. But if you take of all of the hydrogen, then there are different type. 187 00:20:31,520 --> 00:20:38,610 He would call them a one B. And if you take off also healing, you get one see. 188 00:20:38,610 --> 00:20:44,640 But the point I want to make here is just by taking a particular star and taking off different amounts of mass, 189 00:20:44,640 --> 00:20:52,470 you can generate a whole series of different light curves and therefore different observations supernova types. 190 00:20:52,470 --> 00:21:01,920 But it's all for the same type of explosion in the centre. In fact, if you have a massive sign, take of all the hydrogen. 191 00:21:01,920 --> 00:21:08,730 It will look very similar to this one, which I said is a separate explosion. 192 00:21:08,730 --> 00:21:11,970 So how can you distinguish that now for that, 193 00:21:11,970 --> 00:21:19,920 you have to do some more forensic analysis and you have to look at what is the what are the ejecta actually made of? 194 00:21:19,920 --> 00:21:24,600 And that leads us to the topic of supernova classification, 195 00:21:24,600 --> 00:21:32,790 what you do is you look at spectra and long before the different types of supernovae were understood. 196 00:21:32,790 --> 00:21:39,240 Observers noticed that some supernovae have hydrogen in the spectrum and others do not. 197 00:21:39,240 --> 00:21:44,160 And that's where the basic classification comes from type 1A supernovae, 198 00:21:44,160 --> 00:21:50,830 those with which don't show hydrogen the spectrum type to show hydrogen in the spectrum. 199 00:21:50,830 --> 00:21:56,810 Now, for a long time, people thought that these two observation classes could actually be connected. 200 00:21:56,810 --> 00:22:02,850 The two explosion mechanisms James talked about the thermal explosions being in white water 201 00:22:02,850 --> 00:22:09,400 because there's no hydrogen perhaps being type one and the core collapse being type two. 202 00:22:09,400 --> 00:22:13,180 We now know that this is no longer. It's not that simple. 203 00:22:13,180 --> 00:22:20,680 You can have probably both types of explosions in both types of envelopes, but you can look further in it. 204 00:22:20,680 --> 00:22:26,950 And what I'm showing you here different spectra of supernovae near maximum light. 205 00:22:26,950 --> 00:22:30,340 So there's just a flux is a function of wavelength. 206 00:22:30,340 --> 00:22:40,750 And if you look at the top one, which has various atomic lines identified, there's a strong line of silicon. 207 00:22:40,750 --> 00:22:49,840 Now, silicon is one of the elements that's copiously produced in a thermonuclear explosion, so it's a telltale sign of some nuclear explosion. 208 00:22:49,840 --> 00:22:56,270 And therefore, this tells us that this was not a core collapse, but a similar explosion. 209 00:22:56,270 --> 00:23:01,250 The next one has this line labelled air elephants a hydrogen line. 210 00:23:01,250 --> 00:23:12,230 It has a funny shape, has an absorption component and emission component, but that's line shape you get when you have spherical gas expanding. 211 00:23:12,230 --> 00:23:16,910 It gives us this profile. So it's a very obvious thing. 212 00:23:16,910 --> 00:23:24,280 So when you see that you immediately know this was the explosion of the hydrogen rich star was the core collapse. 213 00:23:24,280 --> 00:23:30,130 Of course, we then take off the hydrogen. You don't see that, but we also don't see the silicon. 214 00:23:30,130 --> 00:23:37,920 Then you get two objects that somehow lost the hydrogen and the weather. 215 00:23:37,920 --> 00:23:41,560 If they have some helium, they're called one BS. They also don't join helium. 216 00:23:41,560 --> 00:23:55,320 They're called onesies and. Now, why do stars lose their envelopes, and this has a lot to do with. 217 00:23:55,320 --> 00:24:06,200 To binding to actions, most stars in the sky are not single objects, single isolated objects, most massive stars actually have a companion star. 218 00:24:06,200 --> 00:24:14,360 And what that means you have two massive stars orbiting each other, basically replacing the Earth by another star. 219 00:24:14,360 --> 00:24:21,040 Now, if the stars are very far away, that doesn't matter. They will just evolve in like isolating stars. 220 00:24:21,040 --> 00:24:25,770 However. Interestingly, four massive stars. 221 00:24:25,770 --> 00:24:34,080 Up to about 70 percent of all massive stars are so close that there will be a direct interaction between the two stars. 222 00:24:34,080 --> 00:24:40,710 And what that means is that one of the star one of the star grows because it evolves and wants to become bigger. 223 00:24:40,710 --> 00:24:48,830 It can reach a point where the gravitational pull from the other star can pull off material from it. 224 00:24:48,830 --> 00:24:57,020 And in that situation, you then get into a phase where mass is transferred from one star to the other, 225 00:24:57,020 --> 00:25:05,800 so one star loses and one star, it creates metal. Now that will change the structure of the star, but in particular of the envelope. 226 00:25:05,800 --> 00:25:15,540 And as I just showed you earlier, it's the envelope properties that largely determine what a supernova looks like. 227 00:25:15,540 --> 00:25:22,320 Want to go a bit further? What typically happens if you have this in this mess into occurs in a stable way? 228 00:25:22,320 --> 00:25:26,460 The star that loses massive its hydrogen initially it. 229 00:25:26,460 --> 00:25:32,760 This process will only stop when it has actually transfers most of the hydrogen. 230 00:25:32,760 --> 00:25:39,540 So that way you are then just left with the core of the star, which, if it's an evolved star, would just be a helium core. 231 00:25:39,540 --> 00:25:49,070 So then you have helium star. And when it goes further and explodes, it produces one of these core collapse supernovae that have helium. 232 00:25:49,070 --> 00:25:57,550 If you were to lose the helium, then that might happen by mass transfer, you get another type one C. 233 00:25:57,550 --> 00:26:02,900 Now, it's not only the mass losing star, you change is also big treating star. 234 00:26:02,900 --> 00:26:11,210 If the accreting sides are relatively unevolved style early in its evolution, then it will just behave like a more massive star. 235 00:26:11,210 --> 00:26:20,660 We say this to other be rejuvenated. However, if the star is already somewhat evolved and has already finished its hydrogen burning phase, 236 00:26:20,660 --> 00:26:26,960 the further evolution is quite different and it will not become a red supergiant. 237 00:26:26,960 --> 00:26:39,070 The red zone is the normal expected endpoint of a massive star, but this one will just become a blue supergiant like was observed for supernova 1987A. 238 00:26:39,070 --> 00:26:41,320 So this is not the whole story, 239 00:26:41,320 --> 00:26:49,810 because this massive interface may not be stable if the mess to have faced the mess up to rate is so high that the companion star cannot agree it. 240 00:26:49,810 --> 00:26:54,850 Then what happens is that the expanding stack essentially engulfs the companion star. 241 00:26:54,850 --> 00:27:00,490 And you get into a situation where the one star encodes the companion star. 242 00:27:00,490 --> 00:27:06,700 And if the course of both stars orbiting in in a joint envelope. 243 00:27:06,700 --> 00:27:13,060 Different things can happen, but one of the most dramatic outcomes of this is that the two stars merge completely. 244 00:27:13,060 --> 00:27:18,310 So in the end point then, is that you don't have two stars anymore, but the single one. 245 00:27:18,310 --> 00:27:24,380 Now, this might sound like an extreme thing to you, but it actually isn't. 246 00:27:24,380 --> 00:27:29,300 I would estimate that between 10 and 20 percent of all massive stars do this. 247 00:27:29,300 --> 00:27:33,980 So when you look at a star in the sky, you can't be sure single star in the sky. 248 00:27:33,980 --> 00:27:37,860 You can't be sure that it always was single. 249 00:27:37,860 --> 00:27:51,450 Now, there's a lot to be said about this, but I want to return to supernova and in some way because I believe that this is an example of such a mood. 250 00:27:51,450 --> 00:27:56,880 I already told you that the neutrinos were detected from this event. 251 00:27:56,880 --> 00:27:59,910 This was the event in the satellite galaxy. 252 00:27:59,910 --> 00:28:12,450 18 neutrinos, but it doesn't sound like much, but in actual fact, it was enough to estimate the mass in the explosion. 253 00:28:12,450 --> 00:28:20,340 The total energy estimate in this neutrino flux was about four times 10 to the 46 tool, which is right exactly. 254 00:28:20,340 --> 00:28:26,310 The rest mass energy of a neutron star at the Smithsonian is the binding edge of a neutron star, 255 00:28:26,310 --> 00:28:32,700 so it really confirmed that neutron stuff was formed in this explosion. 256 00:28:32,700 --> 00:28:39,450 Now, that was a big success, but that was the only early success. 257 00:28:39,450 --> 00:28:47,670 I already told you before that the star that exploded was not supposed to, but a few years later some of the anomalies. 258 00:28:47,670 --> 00:28:54,350 But few years later, the Hubble Space Telescope produced this picture. 259 00:28:54,350 --> 00:29:01,760 Probably most of you have seen this. What is it? Well, it shows the nebula around the event. 260 00:29:01,760 --> 00:29:08,350 It shows these three overlapping rings. These rings have nothing to do with the supernova. 261 00:29:08,350 --> 00:29:13,690 The supernova is just what you see at the centre. This little blob there, that's actually the supernova. 262 00:29:13,690 --> 00:29:20,130 These are the supernova ejecta expanding into more expanded. 263 00:29:20,130 --> 00:29:28,190 This is all material that was ejected about 20000 years before the explosion. 264 00:29:28,190 --> 00:29:33,950 You cannot see it because the supernova, the shock breakout, it's ionised meteor. 265 00:29:33,950 --> 00:29:41,520 And now you see the fluorescent meteor in these rings. 266 00:29:41,520 --> 00:29:57,050 Why does the state do that? Actually, before I tell you, can you see the geometry of these rings in three dimensions? 267 00:29:57,050 --> 00:30:03,410 And have been some debates of it, but I'll tell you what it is actually really three rings, 268 00:30:03,410 --> 00:30:17,690 three rings in space the centre ring, the supernova is at the centre of the centre in. 269 00:30:17,690 --> 00:30:22,400 The other rinks are displaced from that centre ring plane above and below. 270 00:30:22,400 --> 00:30:31,190 And when you're observing it from this direction. This is what you see. 271 00:30:31,190 --> 00:30:36,090 Very. OK, now the picture, 272 00:30:36,090 --> 00:30:45,960 which we have developed over the years and quite confident of now is that what happened 20000 years ago was actually the merger of two stars. 273 00:30:45,960 --> 00:30:50,910 And we're not going to in details, but the two stars merged that releases a lot of energy, 274 00:30:50,910 --> 00:30:57,690 which ejects part of the envelope of the star, and the star then wants to become a blue supergiant. 275 00:30:57,690 --> 00:31:03,690 And there's a strong wind that sweeps up any of the structures that ejected in the merger event. 276 00:31:03,690 --> 00:31:12,060 No, this is just the cartoon. These are some calculations, and this is the final product which we predict. 277 00:31:12,060 --> 00:31:19,170 And so this shows the structures you see at the time of the explosion around this time the centre. 278 00:31:19,170 --> 00:31:23,310 This, by the way, is about a light year in size. 279 00:31:23,310 --> 00:31:31,680 And you see the centre ring and you see these bipolar structures, but they're at the end, you have these ring like features. 280 00:31:31,680 --> 00:31:37,330 And when you calculate what that would look like in age alpha, this is what you see. 281 00:31:37,330 --> 00:31:40,930 And, well, we agree this looks very similar to the exhibition. 282 00:31:40,930 --> 00:31:49,910 Of course, we fine tuned it, but. Interesting point. 283 00:31:49,910 --> 00:31:57,890 I mean, this is an old picture. The supernova has now expanded for for more than 30 years. 284 00:31:57,890 --> 00:32:01,880 In fact, the supernova ejecta have now reached the inner ring, 285 00:32:01,880 --> 00:32:08,120 and the inner ring is we now see how the inner ring is being destroyed by the supernova. 286 00:32:08,120 --> 00:32:13,400 And it actually has become bright again and has some interesting structures. 287 00:32:13,400 --> 00:32:20,740 But but ultimately, all of this will be destroyed by the supernova itself. 288 00:32:20,740 --> 00:32:27,520 So this sort of gives you an idea about the importance of binaries for core collapse supernovae, but of course, 289 00:32:27,520 --> 00:32:36,490 for the some nuclear explosions, the even more important this is something James already briefly alluded to. 290 00:32:36,490 --> 00:32:42,800 Now you need to ask why does a lonely CosmoQuest and white dwarf grow to reach the Chinese e-commerce? 291 00:32:42,800 --> 00:32:51,770 For that, it needs at least at least one companion, and the two basic ideas are that you either have a star binary system again, 292 00:32:51,770 --> 00:32:59,270 whether you have a white dwarf accreting from the companion star and once it reaches agenda, Zika comes, it explodes or gets close to it. 293 00:32:59,270 --> 00:33:10,280 It explodes. No, wait, Wolf, systems in binaries are very common, except that white dwarfs don't like to treat. 294 00:33:10,280 --> 00:33:17,530 And so it's not quite clear how that actually works, but it seems like you. 295 00:33:17,530 --> 00:33:23,710 Plausible think the alternative already mentioned by James as well is that rather than having this process, 296 00:33:23,710 --> 00:33:31,120 you have actually two white dwarfs, and if they're close enough, gravitational radiation will bring them closer till they collide. 297 00:33:31,120 --> 00:33:39,580 And this can then, just like a hammer, detonate, detonate and produce its own accord, rather than many variations of this. 298 00:33:39,580 --> 00:33:51,070 These two basic pictures we just organised the conference discussing just that for a whole week and had a rather controversial discussions. 299 00:33:51,070 --> 00:34:02,890 And I can tell you we are not any wiser afterwards. But I mean, these are just want to tell you these are sort of the basic idea. 300 00:34:02,890 --> 00:34:11,500 I mentioned gamma ray bursts. Cameras are really big puzzle for many decades now. 301 00:34:11,500 --> 00:34:16,870 What are they? Well, there are flashes of gamma rays that come from somewhere in the sky. 302 00:34:16,870 --> 00:34:20,710 This is just a random sample of of these events. 303 00:34:20,710 --> 00:34:27,460 You can't read the scale, but I tell you the typical scales varies from milliseconds to thousands of seconds. 304 00:34:27,460 --> 00:34:30,010 So really like a flash? 305 00:34:30,010 --> 00:34:39,460 Now we actually discovered by a US spy satellite that was checking whether the Russians were doing illegal nuclear explosion in the atmosphere. 306 00:34:39,460 --> 00:34:47,860 But rather than finding gamma rays from the Russians, they found these funny gamma rays from all over the sky. 307 00:34:47,860 --> 00:34:57,460 Now they realised they didn't have to keep it secret in 1973. And then this has remained one of the big puzzles for for several decades, actually. 308 00:34:57,460 --> 00:35:05,400 Certainly in the 70s, there were more theories than there were actually gamma ray bursts. 309 00:35:05,400 --> 00:35:15,230 Many more, but the general idea, at least in the early days, was this has probably something to do with neutron stars in our galaxy. 310 00:35:15,230 --> 00:35:22,270 Like having a comet or asteroid falling onto a neutron star and that should produce a flash of cameras. 311 00:35:22,270 --> 00:35:33,610 Now we know now that that's not the case when the 90s was a satellite that that called Betsy that discovered almost 3000 gamma ray bursts. 312 00:35:33,610 --> 00:35:40,990 And what you see here is the distribution in the plane of the sky, and you see it's pretty random. 313 00:35:40,990 --> 00:35:48,010 This coordinate system, the galaxy is along the equator, the galactic centre is right in the centre. 314 00:35:48,010 --> 00:35:57,580 So what you would expect if they are in our galaxy is that you see a strong concentration towards the galaxy. 315 00:35:57,580 --> 00:36:04,000 This didn't solve the problem because they could just be in the outer parts and the galactic halo. 316 00:36:04,000 --> 00:36:07,750 But then in the late 90s. 317 00:36:07,750 --> 00:36:16,660 And was able not just to see the cameras that you can't localise very well, you could also see optical counterparts associated with this. 318 00:36:16,660 --> 00:36:25,690 And once you have an optical counterpart, you can also look at the spectrum, and those spectra showed red shifted absorption lines. 319 00:36:25,690 --> 00:36:31,480 And what that meant is that these gamma ray bursts must have occurred behind some galaxy. 320 00:36:31,480 --> 00:36:39,310 You have a spectrum, you have some galaxy than the light. The the some of the light will be absorbed by the galaxy. 321 00:36:39,310 --> 00:36:46,110 And you can see an absorption line. But because the galaxy is distant, it is red shifted. 322 00:36:46,110 --> 00:36:52,980 And you will see a regifted line so that we solve that puzzle, at least the location. 323 00:36:52,980 --> 00:37:00,170 It means autrement that these explosions were some of the most energetic explosions we know. 324 00:37:00,170 --> 00:37:03,300 You sometimes call them now hypernova. 325 00:37:03,300 --> 00:37:12,540 And because to indicate that a more energetic than normal supernovae, the typical energy of these explosions actually 10 to 45 joules. 326 00:37:12,540 --> 00:37:21,670 So at least a factor of 10 more than this metric tons of 44. So that immediately tells you what's going on here must be something different now. 327 00:37:21,670 --> 00:37:32,480 We don't fully understand it, but I'm giving you here one picture, though to illustrate it. 328 00:37:32,480 --> 00:37:40,820 But we now think is happening is that you have an explosion of a massive star. But but that the core is actually rapidly rotating. 329 00:37:40,820 --> 00:37:46,520 And if you have rapid rotation, then you can just collapse to the centre because the aluminium is conserved. 330 00:37:46,520 --> 00:37:48,590 And so at some point you get in the collapse, 331 00:37:48,590 --> 00:37:55,250 you get centrifugal support and then rather than collapse into the centre, you form some disk like structure. 332 00:37:55,250 --> 00:38:07,540 And then you create from this desk to the centre and somehow in that process and the here for good reasons, somehow you produce a relativistic jet. 333 00:38:07,540 --> 00:38:11,440 And that's probably done by some may need to hide and process. 334 00:38:11,440 --> 00:38:19,570 But these voters, it's important because if you observe the event along these set axes, you get a gamma ray burst. 335 00:38:19,570 --> 00:38:25,330 But the energy that's released in this process is of the order of the rotation energy of this inner part, 336 00:38:25,330 --> 00:38:35,200 and that is of the order of 10 to 45 Joule and that is probably what sets the energy scale. 337 00:38:35,200 --> 00:38:41,710 I'm very vague because if more debates on this than about the to explosions, 338 00:38:41,710 --> 00:38:48,500 but these events are relatively rare only happens to about 1000 core collapse supernovae. 339 00:38:48,500 --> 00:38:56,750 But the camera bills themselves because the cameras actually beamed into very narrow beams can be seen to the very distant universe, 340 00:38:56,750 --> 00:39:02,180 become one of the most distant objects seen so far was a gamma ray burst that was seen in 341 00:39:02,180 --> 00:39:08,930 2011 that occurred at redshift of 9.4 when the universe was only 400 million years old. 342 00:39:08,930 --> 00:39:16,560 In fact, in principle, one can see gamma ray bursts and even larger distances. 343 00:39:16,560 --> 00:39:21,440 Which? Actually, that's not the whole story. 344 00:39:21,440 --> 00:39:25,430 What I would just describe more applies to what's called long duration cannabis. 345 00:39:25,430 --> 00:39:30,750 We know that two types of cannabis that last tens or hundreds of seconds. 346 00:39:30,750 --> 00:39:35,740 Another type that only occurs takes place over a second or less. 347 00:39:35,740 --> 00:39:43,710 These, we think, are caused by having the merger of two neutron stars now. 348 00:39:43,710 --> 00:39:50,430 We actually have fairly good evidence for this now, because as I'm sure you all have heard two years ago, 349 00:39:50,430 --> 00:39:54,630 Legault discovered gravitational waves from the mergers of two neutron stars, 350 00:39:54,630 --> 00:40:00,930 and that merger obsessed merger was associated with one of these short duration benefits. 351 00:40:00,930 --> 00:40:09,900 So we have some confidence that this is probably the right picture, but I will come back to that briefly in a few minutes. 352 00:40:09,900 --> 00:40:18,680 OK, so that gives us some idea of the different types of supernovae, how we analyse them once the supernovae. 353 00:40:18,680 --> 00:40:23,420 Become transparent. The gas expands and you get some beautiful remnants. 354 00:40:23,420 --> 00:40:28,220 But ultimately, all of that material will mix with the interstellar medium. 355 00:40:28,220 --> 00:40:29,870 And that has several consequences. 356 00:40:29,870 --> 00:40:38,630 First, it provides an energy input to the interstellar medium and that can affect star formation can stop far off formation. 357 00:40:38,630 --> 00:40:41,760 Sometimes it may even initiate star formation. 358 00:40:41,760 --> 00:40:49,380 But more importantly, for the last thing, I want to talk about, it mixes chemical elements with the interstellar medium, 359 00:40:49,380 --> 00:40:55,110 all the elements that were produced before in stars and supernovae. 360 00:40:55,110 --> 00:40:59,040 And this is a recurrent cycle that just sort of illustrated here. 361 00:40:59,040 --> 00:41:10,470 So you see if gas, the gas you form, stars, stars produce heavy elements or and supernovae, and then you mix that again with the interstellar medium. 362 00:41:10,470 --> 00:41:18,210 And this process has continued, and it's taken many place many times over the age of the universe. 363 00:41:18,210 --> 00:41:22,740 So supernovae are not only a means of producing heavy elements, 364 00:41:22,740 --> 00:41:31,050 but they are also the main agent of enriching the interstellar medium in producing the next generation of stars. 365 00:41:31,050 --> 00:41:36,420 Now, the core collapse supernovae in the nuclear explosions have slight differences. 366 00:41:36,420 --> 00:41:45,450 In the case of the core collapse supernovae you eject, in particular, the material was produced in the star before lots of oxygen, lots of magnesium. 367 00:41:45,450 --> 00:41:55,490 Even though we have an iron core, most of the iron cores actually collapse into the neutron star, and it's locked up so they don't freeze much on. 368 00:41:55,490 --> 00:42:04,800 These produce. Or you said the main producer of iron, and since the timescale for these two different processes are different here, 369 00:42:04,800 --> 00:42:10,170 it's determined by the lifetime of a massive star, which is order 10 to seven years. 370 00:42:10,170 --> 00:42:15,270 Yes, depends on the evolution of evolution, which is more 10 to nine years. 371 00:42:15,270 --> 00:42:25,620 And so you can see that you get in by looking at the ratio of the elements that are produced here to here, particularly the Oxendine ratio. 372 00:42:25,620 --> 00:42:35,240 You can actually deduce something about the different timescales where star formation may have occurred. 373 00:42:35,240 --> 00:42:41,210 Now we have a broad understanding of of how most of this works. 374 00:42:41,210 --> 00:42:49,940 But one of the big puzzles for understanding the elements have been the nutrient-rich elements, things like gold and platinum. 375 00:42:49,940 --> 00:42:56,470 And this is the last point I want to make. There has been really a major breakthrough in this in 2017. 376 00:42:56,470 --> 00:43:01,020 This is detection of the merger of two neutron stars. 377 00:43:01,020 --> 00:43:08,070 All right. How you make gold has been the income stream now since those days. 378 00:43:08,070 --> 00:43:19,230 We know quite a bit more. We know how we formed these stars. It's fusion helium, carbon oxygen, ultimately iron, but fusion doesn't go beyond that. 379 00:43:19,230 --> 00:43:22,590 All right. I mean, it's the most stable nucleus. 380 00:43:22,590 --> 00:43:30,210 So by nuclear fusion, you can't build up elements more heavier than iron unless you have some energy source. 381 00:43:30,210 --> 00:43:40,770 But if you look at the abundance pattern, the solar abundance pattern of elements and when it goes to quite high atomic number, 382 00:43:40,770 --> 00:43:45,540 when some of the light elements like helium and some other elements you can produce in the Big Bang. 383 00:43:45,540 --> 00:43:49,740 But most of this here you produce, either in stars or in supernovae. 384 00:43:49,740 --> 00:43:57,600 But where do you how do you produce these very nutrient rich elements, particular things like platinum here or gold? 385 00:43:57,600 --> 00:44:04,200 This has been a long puzzle. What you need for that is a very high nutrient density. 386 00:44:04,200 --> 00:44:09,050 And the density has to be high enough that you start to build up heavy elements. 387 00:44:09,050 --> 00:44:15,960 You've bombarded iron nucleus with a neutron, but before it decays, you want to add another neutron. 388 00:44:15,960 --> 00:44:20,320 That's the only way of how you can build up a neutron rich elements. 389 00:44:20,320 --> 00:44:28,930 For this, you need very high nutrient densities and that the conditions have to be right for a long time, like about 100 seconds. 390 00:44:28,930 --> 00:44:36,280 Now people have looked for this for many decades and the candidates there for many years were supernova explosions, 391 00:44:36,280 --> 00:44:46,890 but they couldn't really identify the conditions that were right. Now, 20 years ago, the idea that this might be merging neutron stars became popular. 392 00:44:46,890 --> 00:45:01,610 And this, I think, is actually something that has been confirmed by this detection of neutrons of. 393 00:45:01,610 --> 00:45:19,490 Of legal. And what a show you want to show you here is actually a simulation of the merger of two neutron stars. 394 00:45:19,490 --> 00:45:26,330 There's a simulation by Stefan Rosberg. See two neutron stars colliding, dissolving and forming a single object. 395 00:45:26,330 --> 00:45:33,710 But the thing I want you to particularly notice is actually the fluffy stuff around it, right? 396 00:45:33,710 --> 00:45:42,860 This is actually material of these two neutron stars that's flung out dynamically by the tilings, or the aluminium has to go somewhere. 397 00:45:42,860 --> 00:45:47,690 And it is ejected in these tidal arms. 398 00:45:47,690 --> 00:45:58,890 Now this is between, let's say, neutron rich. But as a decompress, it now starts to decay, produces protons and all the elements start to fuse again. 399 00:45:58,890 --> 00:46:07,320 Right. Just like James was saying in the last phase of the collapse, you undo or the nuclear reactions and this phase, you do the opposite. 400 00:46:07,320 --> 00:46:13,100 You build it up again. So within a few seconds, you build up all the elements again. 401 00:46:13,100 --> 00:46:16,900 But all of this is in the nutrient rich environment. 402 00:46:16,900 --> 00:46:25,260 And that's why this is the ideal environment to form elements like this neutron rich elements like like gold. 403 00:46:25,260 --> 00:46:39,640 In fact, on the day when this was announced by labour in China and some Chinese told me that on that day, the price of gold rose by 10 percent. 404 00:46:39,640 --> 00:46:43,860 Mm hmm. Right. So this was the event. 405 00:46:43,860 --> 00:46:49,380 The point was the event was could actually be I mean, you were seeing traditional waves, 406 00:46:49,380 --> 00:46:54,960 but they were able to localise it well enough that then anyone who had a telescope in their backyard 407 00:46:54,960 --> 00:47:00,150 used a telescope to look at this part of the sky to see whether they found an optical counterpart. 408 00:47:00,150 --> 00:47:11,370 And about 10 days, 10 hours after the explosion, after the collapse of the military, so it was discovered by this telescope. 409 00:47:11,370 --> 00:47:19,320 You see this top there outside this galaxy and this is a transition because 20 days before it wasn't there. 410 00:47:19,320 --> 00:47:24,400 So this turns into appeared and then everyone looked at it. 411 00:47:24,400 --> 00:47:38,810 It's produced a paper that's a short paper, but it has about 3600 authors and said everyone who had a telescope pointed and. 412 00:47:38,810 --> 00:47:46,910 I think you do get to come to an end. You could observe the spectrum of this event and how it evolves. 413 00:47:46,910 --> 00:47:53,850 And I'm not going to explain this, but if you analyse it, you can estimate how much mass was ejected. 414 00:47:53,850 --> 00:48:01,730 It was about two or three 2.5 solar masses and that this material must have been extremely neutron rich. 415 00:48:01,730 --> 00:48:08,450 So if you combine that with the fact that you roughly know the rate of these events. 416 00:48:08,450 --> 00:48:17,000 These neutron star mergers are actually more than enough to produce all the gold and or the platinum in the universe. 417 00:48:17,000 --> 00:48:20,930 So I think that was a major breakthrough. 418 00:48:20,930 --> 00:48:29,960 But to end, I actually want to leave you with another thought since I told you all the heavy elements produced even in stars and supernovae. 419 00:48:29,960 --> 00:48:36,140 And so all the material you are made of is really stardust. 420 00:48:36,140 --> 00:48:40,430 So we have been in this tiny supernova at some point. 421 00:48:40,430 --> 00:48:49,174 OK, thank you.