1 00:00:01,300 --> 00:00:05,802 I can't resist discussing what is probably the most 2 00:00:05,802 --> 00:00:11,421 famous supernova of the recent times. many of you are too young to remember 3 00:00:11,421 --> 00:00:14,934 this. But I was a graduate student in 1987 and 4 00:00:14,934 --> 00:00:20,925 the story of the supernova is part of my education, so I'll tell you about it. 5 00:00:20,925 --> 00:00:28,309 the story starts about 168,000 years ago when a blue super giant, a type B super 6 00:00:28,309 --> 00:00:34,357 giant collapsed in a large magellanic clowd, the fact that the collapsing star 7 00:00:34,357 --> 00:00:39,070 managed to make it back from red giant to blue giant was 8 00:00:39,070 --> 00:00:45,373 surprising to theorists. But because this supernova occured so close to the solar 9 00:00:45,373 --> 00:00:50,499 system and the nearby large magellanic cloud, in fact, the progenitor star being 10 00:00:50,499 --> 00:00:56,145 a giant star was very luminous and was known and had been observed and this is I 11 00:00:56,145 --> 00:01:01,271 think the only case where we have observations of a star before and then of 12 00:01:01,271 --> 00:01:05,943 the supernova, remember, stellar evolution is by and large a slow process. 13 00:01:05,943 --> 00:01:10,810 This is one of the few cases where we really get to see a before and after 14 00:01:10,810 --> 00:01:16,488 image of the same star and you can study it and this forced some modifications to 15 00:01:16,488 --> 00:01:20,020 theory. the light from the supernova reached us 16 00:01:20,020 --> 00:01:25,560 in 1987 and it was the first supernova observed in 1987, so it was 1987A. And 17 00:01:25,560 --> 00:01:30,835 the remnants of the supernova are still under careful observation, we're still 18 00:01:30,835 --> 00:01:35,231 seeing the development. the sequence of Hubble images starting 19 00:01:35,231 --> 00:01:40,303 from 1994 and ending in 2006 shows you a ring-like structure, we'll just talk 20 00:01:40,303 --> 00:01:45,444 about it, around the supernova. the center is the remnant and then this 21 00:01:45,444 --> 00:01:50,989 ring which is progressively becoming more and more glowing and developing this 22 00:01:50,989 --> 00:01:55,988 blob-like structure is an exciting thing to, to, to pay attention to. 23 00:01:55,988 --> 00:02:01,681 even more famous, you can see it in the left, you can see an image of the 24 00:02:01,681 --> 00:02:07,218 surroundings of the explosion, and then a close-up on the right-hand side, which 25 00:02:07,218 --> 00:02:11,496 shows you the ring that I mentioned before, as well as two larger rings. 26 00:02:11,496 --> 00:02:16,357 I think this was referred to as the three-ring circus when it was first 27 00:02:16,357 --> 00:02:19,662 observed. And it took a lot of work by theorists to 28 00:02:19,662 --> 00:02:25,366 figure out what it is that we are seeing. the model that describes it is contained 29 00:02:25,366 --> 00:02:28,218 here. This was a large blue supergiant star. 30 00:02:28,218 --> 00:02:32,690 It had been experiencing mass loss. There was around it, this envelope, 31 00:02:32,690 --> 00:02:37,958 that is what this blue, haze there is attempting to describe in 32 00:02:37,958 --> 00:02:44,016 this rendition is the emission, remember, we talked about this the past 33 00:02:44,016 --> 00:02:50,623 emissions of the biploar emission of the star through mass loss prior to the 34 00:02:50,623 --> 00:02:55,600 supernova explosion. And what we're seeing where these two 35 00:02:55,600 --> 00:03:02,894 extreme rings on the left and right are at a distance of 20 light years from the 36 00:03:02,894 --> 00:03:06,045 supernova. These are glowing where the height, 37 00:03:06,045 --> 00:03:12,367 where the light from the supernova is hitting the ejecta from the previous mass 38 00:03:12,367 --> 00:03:15,676 loss. similarly, the glowing ring surrounding, 39 00:03:15,676 --> 00:03:21,167 the sort of waist of the hourglass, is an area where ejecta from the supernova, 40 00:03:21,167 --> 00:03:26,235 matter from the supernova, traveling much more slowly is now crashing in. 41 00:03:26,235 --> 00:03:31,796 The remnants of the star's atmosphere is now, are crashing in to the previously 42 00:03:31,796 --> 00:03:37,428 existing gas from previous mass loss and that's heating and compressing them, 43 00:03:37,428 --> 00:03:42,285 and that is the glow that we are seeing. And as I said, all of this is under a 44 00:03:42,285 --> 00:03:48,103 study but this was not even, but all of this for all its excitement, was not the 45 00:03:48,103 --> 00:03:51,306 most exciting aspect to me of supernova 1987A. 46 00:03:51,306 --> 00:03:55,300 Yes, we were back to those Neutrinos. 47 00:03:55,300 --> 00:04:00,840 So, what transpired is that neutrino detectors, although they were not 48 00:04:00,840 --> 00:04:05,803 designed to be neutrino detectors were active at the time, they were looking 49 00:04:05,803 --> 00:04:09,200 actually for signals of proton decay and found none. 50 00:04:09,200 --> 00:04:14,033 But the people who were looking for proton decay had large tanks of water 51 00:04:14,033 --> 00:04:19,062 that they were observing for small flickers of light which we now know to be 52 00:04:19,062 --> 00:04:24,091 a very good way to detect neutrinos and it transpired in the weeks after the 53 00:04:24,091 --> 00:04:28,728 detection of the light from supernova 1987A is that a burst of neutrinos, a 54 00:04:28,728 --> 00:04:32,850 total of 20 or 21, in three detectors worldwide 55 00:04:32,850 --> 00:04:37,096 were detected at about the time the supernova exploded. 56 00:04:37,096 --> 00:04:41,177 And you can again remember that remembering that most neutrinos go 57 00:04:41,177 --> 00:04:46,432 through most detectors completely without interacting the fact that we saw 20 of 58 00:04:46,432 --> 00:04:50,883 them, you can figure out how many actually have to go through the tank in 59 00:04:50,883 --> 00:04:55,767 order for 20 to be detected. You multiply that by the 4 pi r squared 60 00:04:55,767 --> 00:04:58,830 for 160,000 light years away which is where the 61 00:04:58,830 --> 00:05:03,557 neutrino exploded and you realize that this means the supernova admitted about 62 00:05:03,557 --> 00:05:09,014 ten to the 58 neutrini which since we know their average energy would have 63 00:05:09,014 --> 00:05:13,973 carried ten to the 46 joules, this was a subluminal supernova. 64 00:05:13,973 --> 00:05:19,702 it's a little it was a little bit less than luminous than a typical type 65 00:05:19,702 --> 00:05:23,585 two supernova. The reason for this as we now understand 66 00:05:23,585 --> 00:05:27,937 having seen the progenitor, is because this was a blue giant star. 67 00:05:27,937 --> 00:05:31,620 It had gone horizontal branch. It had shrunk the ejecta. 68 00:05:31,620 --> 00:05:35,986 had to escape a deeper gravitational well, they were more tightly bound. 69 00:05:35,986 --> 00:05:40,906 This was not a star, typically when we modeled supernovi, we were talking about 70 00:05:40,906 --> 00:05:45,518 red super giants, very bloated, most of the atmosphere is at large distances. 71 00:05:45,518 --> 00:05:50,377 In this case, all of the atmosphere would have had to have been blown away from 72 00:05:50,377 --> 00:05:55,310 very close relatively to the star's core and this means that less energy was 73 00:05:55,310 --> 00:05:59,750 available to escape in the supernova. And then, the fun thing about these 74 00:05:59,750 --> 00:06:03,042 neutrinos is when you track exactly when they were detected. 75 00:06:03,042 --> 00:06:06,718 They were detected about three hours before the light was detected. 76 00:06:06,718 --> 00:06:10,832 Now, neutrinos, I said, are not quite massless but they are almost massless, 77 00:06:10,832 --> 00:06:15,331 they travel essentially at the speed of light but they certainly travel no faster 78 00:06:15,331 --> 00:06:18,511 than light. incorrect news from a couple of years 79 00:06:18,511 --> 00:06:23,197 ago, notwithstanding. The reason neutrinos arrived prior to the 80 00:06:23,197 --> 00:06:28,799 light is that the density in the shock wave the supernova explosion, the 81 00:06:28,799 --> 00:06:35,000 compression is so extreme that for the first three hours light cannot escape 82 00:06:35,000 --> 00:06:39,020 but neutrinos escape first. So, the neutrino sphere breaks first, 83 00:06:39,020 --> 00:06:42,594 neutrinos has escaped. The ejecta have to thin for three more 84 00:06:42,594 --> 00:06:47,572 hours before light ceases to be trapped and so measuring this time difference, 85 00:06:47,572 --> 00:06:52,933 gave us a very good handle on checking the, our models of supernova and many 86 00:06:52,933 --> 00:06:57,018 things were verified and other things have been corrected since. 87 00:06:57,018 --> 00:07:01,951 this discovery, in sort of inadvertent launched a new field called neutrino 88 00:07:01,951 --> 00:07:05,376 astronomy. There are many new experiments planned to 89 00:07:05,376 --> 00:07:10,315 try to study the sky just as we learned new things when we turned on radio 90 00:07:10,315 --> 00:07:14,068 telescopes and x-ray telescopes and gamma ray telescopes. 91 00:07:14,068 --> 00:07:19,368 it is hoped that observing the universe in this, for essentially the first time 92 00:07:19,368 --> 00:07:23,524 in this non-light radiation, non-electromagnetic radiation will give 93 00:07:23,524 --> 00:07:27,618 us a handle on phenomena that have not previously been observed. 94 00:07:27,618 --> 00:07:32,813 there are many neutrino experiments. One of the most fun ones to think about is 95 00:07:32,813 --> 00:07:37,641 this experiment called ice cube, so to conduct a modern neutrino experiment, you 96 00:07:37,641 --> 00:07:42,459 take a tank a very, very pure water, the water has a lot of protons and electrons 97 00:07:42,459 --> 00:07:46,865 in it and if you happen to have a neutrino passing by, well, mostly nothing 98 00:07:46,865 --> 00:07:51,521 happens, but if it happens to interact with an electron, that electron or the 99 00:07:51,521 --> 00:07:56,111 products will be moving at a high speed through the tank and they will emit a 100 00:07:56,111 --> 00:08:00,582 characteristic, radiate a kind of light called Cherenkov radiation and then you 101 00:08:00,582 --> 00:08:04,818 put in light detectors called photomultipliers to detect the radiation. 102 00:08:04,818 --> 00:08:09,018 So, all you need is a great big tank of very pure water with a lot of 103 00:08:09,018 --> 00:08:13,322 photomultipliers attached to it. and you need to hide this deep 104 00:08:13,322 --> 00:08:18,241 underground because otherwise, cosmic rays and various kinds of contamination 105 00:08:18,241 --> 00:08:23,222 on the surface will, will overwhelm the neutrino signal so you hide it deep 106 00:08:23,222 --> 00:08:28,369 underground where only neutrini can get. And one of the best place to do, places 107 00:08:28,369 --> 00:08:31,414 to do this is actually in Antarctica, near the south pole. 108 00:08:31,414 --> 00:08:36,691 It turns out that polar ice, ice is not particularly transparent as we know it, 109 00:08:36,691 --> 00:08:41,495 but if you compress it, all of the impurities, it turns out are squeezed 110 00:08:41,495 --> 00:08:47,246 out, deep amount hundreds of meters below the surface, polar ice is very, very 111 00:08:47,246 --> 00:08:50,223 transparent. So, you have, ready-made, a deeply 112 00:08:50,223 --> 00:08:53,244 submerged huge collection of very pure water. 113 00:08:53,244 --> 00:08:58,119 And what the ice cube project is doing is lowering a bunch of photomultipliers. 114 00:08:58,119 --> 00:09:02,746 Here's a, a depiction of the grid into Antarctic ice and hooking them up to 115 00:09:02,746 --> 00:09:07,621 computers to try to detect neutrinos. And this is one of the more unusual and 116 00:09:07,621 --> 00:09:11,760 one of the more interesting telescopes that we have in the world. 117 00:09:11,760 --> 00:09:17,055 And since I was there for the actual excitement, I had to share this story 118 00:09:17,055 --> 00:09:17,700 with you.