1 00:00:00,820 --> 00:00:07,498 The tension must be mounting, we've been, I've been leaving our star poised with 2 00:00:07,498 --> 00:00:14,477 the core collapsing and we haven't asked. what happens then and so core collapse is 3 00:00:14,477 --> 00:00:20,584 going to be the next topic obviously. And so what happens when the center of a 4 00:00:20,584 --> 00:00:23,090 star collapses. You have this 5 00:00:23,090 --> 00:00:28,783 Solar mass, few solar masses of, solid iron ru, rich core, collapsing and, it 6 00:00:28,783 --> 00:00:34,176 stats with a sort of, typicle stellar core size, size of the earth and it 7 00:00:34,176 --> 00:00:40,394 collapses down it turns out dramatically, in about a tenth of a second it collapses 8 00:00:40,394 --> 00:00:46,537 to a size of a few kilometers and we'll talk about the remnant, in a later clip. 9 00:00:46,537 --> 00:00:52,726 But, this gravitational collapse heats the, core to billions of Kelvin and the, 10 00:00:52,726 --> 00:00:57,835 ambient thermal radiation that's going on there is now gamma rays and this is 11 00:00:57,835 --> 00:01:02,878 terrible because as we discussed this needs to photodiscintigration of heavy 12 00:01:02,878 --> 00:01:07,529 nuclei this breaks up all of the nucleotides that the star has been 13 00:01:07,529 --> 00:01:11,680 diligently collecting. Working its way up from hydrogen to 14 00:01:11,680 --> 00:01:17,477 helium to carbon to neon, ever so slowly. Are now completely eliminated, almost 15 00:01:17,477 --> 00:01:22,822 completely eliminated by the hot radiation soup in the stellar core. 16 00:01:22,822 --> 00:01:28,620 So whatever has not been ejected so far, is pretty much back to protons, 17 00:01:28,620 --> 00:01:33,372 At the same time as the core collapses the out layers of the star find 18 00:01:33,372 --> 00:01:38,727 themselves unsupported and at some point there's not what's called a homologous 19 00:01:38,727 --> 00:01:41,884 collapse so the star collapsing As a system. 20 00:01:41,884 --> 00:01:47,045 In some sense, the outer layers of the star are essentially in gravitational 21 00:01:47,045 --> 00:01:50,236 free fall. They lose contact, they can't respond 22 00:01:50,236 --> 00:01:55,193 faster than the speed of sound. They don't know in time that the core has 23 00:01:55,193 --> 00:01:58,452 collapsed. And kind of like our slinky, the outer 24 00:01:58,452 --> 00:02:03,273 layers find themselves suspended in space with nothing holding them up. 25 00:02:03,273 --> 00:02:08,705 And they fall in gravitational free fall, achieving speeds as high as fifteen or 26 00:02:08,705 --> 00:02:13,050 twenty% of the speed of light. So we're being dicey with ignoring 27 00:02:13,050 --> 00:02:18,301 relativity in this situation. In the core as it collapses, as we said 28 00:02:18,301 --> 00:02:23,181 electron degeneracy has completely been left behind. 29 00:02:23,181 --> 00:02:27,135 Electron degeneracy will not support this core. 30 00:02:27,135 --> 00:02:31,223 In fact the Pressures are so intense that electrons 31 00:02:31,223 --> 00:02:34,825 essentially disappear. They are forced into inverse beta 32 00:02:34,825 --> 00:02:39,907 processes, so that protons and electrons combine, leaving the star, the core, that 33 00:02:39,907 --> 00:02:44,346 is essentially all neutrons. This is not to say there are not protons 34 00:02:44,346 --> 00:02:48,527 and electrons left, but their concentrations are severely reduced. 35 00:02:48,527 --> 00:02:52,644 You have a very, very neutron rich soup, with the extreme density. 36 00:02:52,644 --> 00:02:57,920 Essentially the core has collapsed to the density of an atomic nucleus and beyond. 37 00:02:57,920 --> 00:03:03,058 Does this stop the core what is left there that will be for the next video in 38 00:03:03,058 --> 00:03:08,065 the mean time what happens in the atmosphere well within about a quarter of 39 00:03:08,065 --> 00:03:13,203 a section the core is converted into neutrons and it achieved a super nuclear 40 00:03:13,203 --> 00:03:17,420 density notice ten to the seventeen kilo's per meter squared 41 00:03:17,420 --> 00:03:21,775 The core's at this point so dense that even light cannot escape. 42 00:03:21,775 --> 00:03:27,152 On the other hand, many of the neutrinos formed in the inverse beta decay carry 43 00:03:27,152 --> 00:03:31,644 off most of the energy of the gravitational decay of the core and 44 00:03:31,644 --> 00:03:36,068 there's a lot of energy. This is only a few solar masses but they 45 00:03:36,068 --> 00:03:41,649 are at very small R squares, the collapse is very intense and the power emitted in 46 00:03:41,649 --> 00:03:44,681 neutrini. for the ten seconds following the 47 00:03:44,681 --> 00:03:48,940 collapse, exceeds the combined luminosities of all the stars in the 48 00:03:48,940 --> 00:03:51,865 universe. So, it's presumably, a good thing that 49 00:03:51,865 --> 00:03:56,632 neutrini mostly did not interact strongly, this is an extreme luminosity. 50 00:03:56,632 --> 00:04:01,781 the density as we will discuss, is so high, that even the neutrino departure 51 00:04:01,781 --> 00:04:04,832 takes time. There's a neutrino sphere for awhile, 52 00:04:04,832 --> 00:04:09,790 where even neutrinos are trapped. But they escape first, carrying off the 53 00:04:09,790 --> 00:04:14,925 majority of the energy of the collapse. Now, when the core achieves nuclear 54 00:04:14,925 --> 00:04:20,168 density or super nuclear density, the core collapse is I'm giving away the 55 00:04:20,168 --> 00:04:24,389 punch line, does stop. And when the core collapse stops, you now 56 00:04:24,389 --> 00:04:29,496 have the atmosphere, or the envelope, falling down at relativistic speeds on 57 00:04:29,496 --> 00:04:34,058 top of this, now rigid, core. And this generates the mother of all 58 00:04:34,058 --> 00:04:35,080 shockwaves. Now. 59 00:04:35,080 --> 00:04:40,511 If everything I said were correct supernovi would look very different than 60 00:04:40,511 --> 00:04:43,506 they do. And in fact the details of how the 61 00:04:43,506 --> 00:04:49,704 explosion works out are still a very much a topic of simulation and debate and, and 62 00:04:49,704 --> 00:04:53,174 research. An important role is played in the fact 63 00:04:53,174 --> 00:04:58,179 that the shock wave is highly turbulent. If you really just compress, imagine 64 00:04:58,179 --> 00:05:02,592 modelling an atmosphere that's compressing and then suddenly stops. 65 00:05:02,592 --> 00:05:07,794 the atmosphere absorbs the shock wave instead of, as we observed being blown 66 00:05:07,794 --> 00:05:10,560 away. What in practice happens is, that the 67 00:05:10,560 --> 00:05:15,697 shock wave manages to propagate all way up to the surface of the star blowing 68 00:05:15,697 --> 00:05:21,230 away the atmosphere, blowing away 96% of the mass of the 25 solar mass star and. 69 00:05:21,230 --> 00:05:25,500 The, the, in the, in the atmosphere of this, 70 00:05:25,500 --> 00:05:31,050 Shock wave propagating, matter is compressed and heated so much that it is 71 00:05:31,050 --> 00:05:37,125 here that massive stars reproduce fusion, producing nucleotides all the way up to 72 00:05:37,125 --> 00:05:41,250 iron and beyond. So all of the uranium and the gold and 73 00:05:41,250 --> 00:05:46,800 the silver, the heavy elements are produced primarily, as we talked about, S 74 00:05:46,800 --> 00:05:52,800 process nucleus synthesis in heavy stars and then injection into the atmosphere 75 00:05:52,800 --> 00:05:58,050 before photo disintegration, but the, another important source of heavy 76 00:05:58,050 --> 00:06:01,658 elements is. The shock wave in a supernova. 77 00:06:01,658 --> 00:06:04,785 And. The atmosphere is blown away, it takes a 78 00:06:04,785 --> 00:06:10,248 while for light to be able to escape but as the atmosphere expands and cools a 79 00:06:10,248 --> 00:06:14,479 little bit, the, opasidy is reduced, and then light can 80 00:06:14,479 --> 00:06:18,207 finally escape. It takes a few hours and the luminosity 81 00:06:18,207 --> 00:06:23,698 in light, which is, as we'll see, factor of almost a hundred down from the 82 00:06:23,698 --> 00:06:26,817 luminosity of nutrini is still significant. 83 00:06:26,817 --> 00:06:32,036 It can be 30 million solar luminosity. So this again is the luminosity of the 84 00:06:32,036 --> 00:06:36,985 entire galaxy on the scale of the luminosity of the galaxy coming out of 85 00:06:36,985 --> 00:06:40,103 the star. The total energy released is some 86 00:06:40,103 --> 00:06:45,656 humongous number ten to the 47 joules, in In light and more in neutrini, and the 87 00:06:45,656 --> 00:06:48,769 phenomenon is what is called a type two supernova. 88 00:06:48,769 --> 00:06:53,813 So, we've now seen type 1A supernovae and type two supernovae, they're qualitavely 89 00:06:53,813 --> 00:06:56,490 different. Remember, a type 1A supernova was, 90 00:06:56,490 --> 00:07:01,471 essentially, a, a nuclear explosion, the explosive fusion of a white dwarf made of 91 00:07:01,471 --> 00:07:04,460 carbon. type two supernova is a core collapse 92 00:07:04,460 --> 00:07:07,262 supernova. The energy source is gravitational 93 00:07:07,262 --> 00:07:10,126 energy. Essentially, all of these ten to the 47 94 00:07:10,126 --> 00:07:14,174 jewels are Kevin Helmhold's energy from the collapse of the core. 95 00:07:14,174 --> 00:07:16,808 And so. Two very different mechanisms two 96 00:07:16,808 --> 00:07:21,721 different fundamental forces driving these two types of extremely energetic 97 00:07:21,721 --> 00:07:26,958 explosions and can we see these things well with the luminosity of 30,000,000 98 00:07:26,958 --> 00:07:32,065 suns one would hope so the most famous one as mentioned in the quote at the 99 00:07:32,065 --> 00:07:36,286 beginning of The, the, the title slide of this video 100 00:07:36,286 --> 00:07:41,864 is the famous supernova of 1054 AD we have records for this from Japanese 101 00:07:41,864 --> 00:07:47,658 sources, Arabic sources nate even Native American sources possibly, no mention in 102 00:07:47,658 --> 00:07:53,163 European sources that I know of and the very famous Sung dynasty scholar 103 00:07:53,163 --> 00:07:57,072 describing the. Guests star and the remnant of this super 104 00:07:57,072 --> 00:08:02,583 nova is still visible its the famous crab nebula in Taurus m1 the first item on a 105 00:08:02,583 --> 00:08:08,362 Messier's list of non comet objects this is a beautiful recent hubble image the 106 00:08:08,362 --> 00:08:14,007 filimentary structure and all kind of properties of this nebula might come up 107 00:08:14,007 --> 00:08:17,935 in a future clip. One should note that while supernova are 108 00:08:17,935 --> 00:08:23,030 extremely explosive and energetic processes, it is a bit surprising that 109 00:08:23,030 --> 00:08:28,396 the supernova remnant is so luminous a thousand years after the light from the 110 00:08:28,396 --> 00:08:31,250 supernova reached Earth. when it was 111 00:08:31,250 --> 00:08:36,848 When the, the 1054 supernova exploded it's reasonably nearby the star was 112 00:08:36,848 --> 00:08:42,259 visible for about a week in daytime. So this was a very impressive effect, 113 00:08:42,259 --> 00:08:45,762 artifact. There have been Milky Way supernovae 114 00:08:45,762 --> 00:08:48,836 three times since then, or around that time. 115 00:08:48,836 --> 00:08:53,554 In 1006, 1572 and 1604. The estimate is that there's a supernova 116 00:08:53,554 --> 00:08:58,700 in the Milky Way every 300 years. Most of them, as most things are in the 117 00:08:58,700 --> 00:09:02,990 Milky Way, are far away from us and are obscured by the dust. 118 00:09:02,990 --> 00:09:07,836 The disk of the galaxy so we actually observe many more super novie in other 119 00:09:07,836 --> 00:09:12,501 galaxies then we do in our own because we're we can drew them edge on and we 120 00:09:12,501 --> 00:09:17,228 don't have to fight with Dust extinction and another fun example 121 00:09:17,228 --> 00:09:22,426 is this supernova in 2011, M51 the beautiful whirlpool galaxy and the, you 122 00:09:22,426 --> 00:09:27,767 can see a before and after shot here, before on the left, after on the right. 123 00:09:27,767 --> 00:09:32,609 The arrow points to the bright new star that shows up in the galaxy. 124 00:09:32,609 --> 00:09:37,309 That is a type two supernova. The fun thing about this one, as with 125 00:09:37,309 --> 00:09:42,935 many supernovas of the modern era, is that it was discovered in fact by amateur 126 00:09:42,935 --> 00:09:46,346 astronomers. There are a lot of us amateurs; I try to 127 00:09:46,346 --> 00:09:50,884 count myself among the amateur astronomers but I don't perhaps qualify 128 00:09:50,884 --> 00:09:55,996 and there's a lot of us looking at a lot of places that the professionals can't 129 00:09:55,996 --> 00:09:59,511 all be looking. So the more citizen science involvement. 130 00:09:59,511 --> 00:10:04,241 Supernovae are, we now have seen both core collapse and nuclear supernovae. 131 00:10:04,241 --> 00:10:09,098 In general, there's a classification which, like many things in astronomy, was 132 00:10:09,098 --> 00:10:13,380 made before the phenomena were understood and so is somewhat weird. 133 00:10:13,380 --> 00:10:16,640 The spectral classification is that a supernova is. 134 00:10:16,640 --> 00:10:21,505 Classified as a type 1a supernova, we now know that is the explosion of a white 135 00:10:21,505 --> 00:10:25,026 dwarf, if it has strong silicone absorption 136 00:10:25,026 --> 00:10:30,246 lines but no hydrogen or helium. In the spectrum, it's a 1b supernova if 137 00:10:30,246 --> 00:10:33,530 it has weak hydrogen lines and strong helium. 138 00:10:33,530 --> 00:10:37,132 Lines in the spectrum. It's a 1C if there are weak silicon lines 139 00:10:37,132 --> 00:10:40,566 but no hydrogen or helium. And, finally, it's type two, the core 140 00:10:40,566 --> 00:10:45,238 collapse we've been discussing right now, if it has strong hydrogen lines in the 141 00:10:45,238 --> 00:10:47,915 spectrum and. It turns out we know what type 1a 142 00:10:47,915 --> 00:10:52,262 supernovae are those are the nuclear explosion of a white dwarf it turns out 143 00:10:52,262 --> 00:10:54,837 that type two and what are called one and 1c. 144 00:10:54,837 --> 00:10:59,356 One because there are no strong hydrogen lines in fact our core collapse 145 00:10:59,356 --> 00:11:03,704 supernovae resulting from stars that, to some extent or other, have lost their 146 00:11:03,704 --> 00:11:07,136 atmosphere through the mass loss mechanisms we've discussed. 147 00:11:07,136 --> 00:11:10,340 So the hydrogen and helium lines are absent or weakened. 148 00:11:10,340 --> 00:11:15,486 Simply because by the time the core collapses most of the hydrogen and helium 149 00:11:15,486 --> 00:11:19,680 envelope has been lost and is not present to absorb the radiation.