WEBVTT 1 00:00:00.000 --> 00:00:09.320 Good morning. I'd like us to begin today by acknowledging that we're gathered on the traditional country of the Kaurna people of the Adelaide Plains 2 00:00:09.320 --> 00:00:18.100 and to recognize and respect their cultural heritage, the beliefs, and their relationship with the land. 3 00:00:19.000 --> 00:00:26.420 I know that we acknowledge that they are of continuing importance to the Kaurna people that are living today 4 00:00:26.420 --> 00:00:30.080 and their elders whom we respect past and present. 5 00:00:31.460 --> 00:00:43.620 So before we get into the meat of this morning, you've been reminded to please turn your mobile phones on to silent or turn them off, 6 00:00:43.840 --> 00:00:51.320 but we do want to encourage you to tweet if you're so inclined at Ideas Adelaide. 7 00:00:51.400 --> 00:00:55.920 And I understand that the hashtag is pound sign AF. 8 00:00:56.420 --> 00:00:59.840 F-O-I, Adelaide Festival of Ideas. 9 00:01:01.920 --> 00:01:08.680 Now, I know our speaker today fairly well, so I know that you're in for a treat. 10 00:01:09.840 --> 00:01:15.700 It's been quite a month, I would say, for astronomy in general. 11 00:01:15.740 --> 00:01:19.920 You may have read that the neutrino may travel faster than the speed of light. 12 00:01:20.120 --> 00:01:24.200 I think the jury's still out on that one, but that's something to watch 13 00:01:24.200 --> 00:01:26.220 because it would certainly change. 14 00:01:28.680 --> 00:01:30.400 It's something that supports the whole issue of biodiversity and biodiversity needs. 15 00:01:30.700 --> 00:01:32.000 If you look at the 16 00:01:32.800 --> 00:01:33.320 ! 17 00:01:33.320 --> 00:01:36.000 If you look at the! 18 00:01:36.000 --> 00:01:37.260 If you look at the! 19 00:01:37.260 --> 00:01:38.060 If you look at the! 20 00:01:38.060 --> 00:01:40.820 If you look at the! 21 00:01:40.820 --> 00:01:40.840 If you look at the! 22 00:01:40.840 --> 00:01:40.880 If you look at the! 23 00:01:40.880 --> 00:01:41.060 If you look at the! 24 00:01:41.060 --> 00:01:42.520 If you look at the! 25 00:01:42.520 --> 00:01:43.520 If you look at the! 26 00:01:43.520 --> 00:01:45.700 If you look at the! 27 00:01:45.700 --> 00:01:46.440 If you look at the! 28 00:01:46.440 --> 00:01:47.420 If you look at the! 29 00:01:47.420 --> 00:01:50.000 If you look at the! 30 00:01:50.000 --> 00:01:52.140 If you look at the! 31 00:01:53.220 --> 00:01:53.920 If you look at the! 32 00:01:53.920 --> 00:01:58.380 his discovery with his team members that the universe is not only expanding, that we knew, 33 00:01:58.520 --> 00:02:04.360 but is accelerating at an ever-rapidly increasing rate. 34 00:02:05.220 --> 00:02:07.220 And so thank you very much. 35 00:02:07.400 --> 00:02:14.420 I think, Brian Schmidt, we should celebrate these things ever so much 36 00:02:14.420 --> 00:02:18.580 as we celebrate sports victories for Australia, don't you think? 37 00:02:19.340 --> 00:02:19.980 Absolutely. 38 00:02:19.980 --> 00:02:31.240 So it's been an exciting month, and Brian, who is an expert in magnetism in space, 39 00:02:31.580 --> 00:02:36.620 in how stars die, and gigantic explosions in the cosmos, 40 00:02:36.740 --> 00:02:43.300 which in fact Brian Schmidt used cosmic explosions for his Nobel Prize winning result, 41 00:02:43.540 --> 00:02:47.380 and all of these things are things that Professor Brian Gainsler works on. 42 00:02:48.440 --> 00:02:49.960 He has also... 43 00:02:49.980 --> 00:02:53.580 He's authored over 450 refereed papers and conference proceedings. 44 00:02:54.300 --> 00:02:57.700 He was a former Young Australian of the Year. 45 00:02:58.080 --> 00:03:02.700 He's been on the faculty of Harvard, but we're very pleased that he's come back to Australia. 46 00:03:03.380 --> 00:03:05.040 He's returned to the University of Sydney. 47 00:03:05.360 --> 00:03:08.440 He's now an Australian Laureate Fellow there, 48 00:03:08.640 --> 00:03:13.500 where he's Director of the Center of Excellence for All-Sky Astrophysics, 49 00:03:13.680 --> 00:03:18.600 and most recently has won Australia's Posse Medal in Physics, 50 00:03:18.840 --> 00:03:19.840 which is given... 51 00:03:19.980 --> 00:03:25.220 to a scientist who's done outstanding work in physics and is under the age of 40. 52 00:03:26.460 --> 00:03:27.800 Now I just attended... 53 00:03:27.800 --> 00:03:31.240 I don't know if any of you got to the immortality session yesterday. 54 00:03:32.020 --> 00:03:32.500 Yeah. 55 00:03:32.700 --> 00:03:34.340 That was an interesting one, wasn't it? 56 00:03:35.100 --> 00:03:39.040 And I'm just imagining what things might be in store for Brian. 57 00:03:39.400 --> 00:03:40.060 You know. 58 00:03:40.400 --> 00:03:42.900 If this is what he's done before the age of 40. 59 00:03:43.340 --> 00:03:46.120 And we can expect... 60 00:03:46.120 --> 00:03:49.900 We were told that it may be that humans can live as long as 1,000. 61 00:03:49.900 --> 00:03:53.120 1,000 years old by the end of this decade. 62 00:03:53.760 --> 00:03:54.180 So... 63 00:03:54.860 --> 00:03:59.240 But we're just going to hear something about the first 40 years of Brian's work. 64 00:03:59.380 --> 00:04:06.040 And in particular, why the cosmos might actually get you in the end. 65 00:04:06.120 --> 00:04:08.420 So just in case you need something else to worry about, 66 00:04:10.440 --> 00:04:12.280 I will now invite... 67 00:04:12.280 --> 00:04:15.580 Put your hands together to welcome Professor Brian Gainsler 68 00:04:15.580 --> 00:04:18.620 to tell us why the cosmos may get you in the end. 69 00:04:19.920 --> 00:04:21.380 Well, good morning. Can you all hear me okay? 70 00:04:21.560 --> 00:04:23.520 Thanks so much to Penny for her introduction. 71 00:04:23.680 --> 00:04:26.820 And I'd like to thank both the Adelaide Festival of Ideas 72 00:04:26.820 --> 00:04:30.100 and the Royal Institution for inviting me to come. 73 00:04:30.180 --> 00:04:31.700 It was a real pleasure to accept the invitation 74 00:04:31.700 --> 00:04:33.820 and to tell you about astronomy today. 75 00:04:34.200 --> 00:04:37.380 Now the theme of the festival 76 00:04:37.380 --> 00:04:40.600 is supposed to be something along the lines of dealing with change. 77 00:04:40.900 --> 00:04:44.200 And I thought, well, does it really matter how we deal with change 78 00:04:44.200 --> 00:04:46.660 given that we're all going to be destroyed 79 00:04:46.660 --> 00:04:49.720 by various cataclysmic events in the universe anyway? 80 00:04:49.720 --> 00:04:53.460 So the rest of the sessions this stimulating weekend 81 00:04:53.460 --> 00:04:57.340 will challenge you and worry you and excite you about the future. 82 00:04:57.440 --> 00:05:00.120 And I'm just saying that whether you feel elated or a bit depressed 83 00:05:00.120 --> 00:05:02.280 by everything going on on this planet, it doesn't really matter. 84 00:05:02.620 --> 00:05:04.920 Because the cosmos will get you in the end. 85 00:05:05.360 --> 00:05:09.140 Now let's start off with the night sky. 86 00:05:09.380 --> 00:05:12.260 Now it's sort of become a byword over centuries 87 00:05:12.260 --> 00:05:15.340 that the sky is the one constant thing. 88 00:05:15.540 --> 00:05:18.900 Whatever turmoil or change or disaster is happening here, 89 00:05:18.900 --> 00:05:21.020 at least you can rely on the stars. 90 00:05:21.240 --> 00:05:23.320 There's this famous quote from Shakespeare 91 00:05:23.320 --> 00:05:27.740 that you would hope that you're as constant as the northern star. 92 00:05:28.260 --> 00:05:31.840 And this is very much embedded in all of human history. 93 00:05:33.080 --> 00:05:36.260 In fact, the assumption that stars don't change 94 00:05:36.260 --> 00:05:37.680 is written in the names of the stars. 95 00:05:38.180 --> 00:05:41.520 Names of stars like Sirius, Betelgeuse, 96 00:05:41.580 --> 00:05:44.380 constellations like Orion, Virgo, 97 00:05:44.540 --> 00:05:46.360 these aren't English words. 98 00:05:46.560 --> 00:05:48.460 These are words that are Egyptian, 99 00:05:48.900 --> 00:05:50.020 Greek and Babylonian. 100 00:05:50.740 --> 00:05:53.680 The stars that the Greeks and the Egyptians wrote about 101 00:05:53.680 --> 00:05:55.940 are exactly the same stars that we see today. 102 00:05:56.940 --> 00:05:59.500 The ancient Greeks and Romans saw a hunter 103 00:05:59.500 --> 00:06:02.800 and with a bit of imagination we can still see a hunter today 104 00:06:02.800 --> 00:06:03.980 in the constellation of Orion. 105 00:06:04.160 --> 00:06:06.580 The stars have not changed in thousands of years. 106 00:06:07.480 --> 00:06:10.180 If you look at the stars that the great navigators 107 00:06:10.180 --> 00:06:13.140 of the age of exploration used to sail around the world, 108 00:06:13.260 --> 00:06:16.400 well, those are exactly the same stars that we refer to today 109 00:06:16.400 --> 00:06:18.760 and they still have the same navigational power. 110 00:06:19.520 --> 00:06:21.500 Aboriginal astronomy has a history going back 111 00:06:21.500 --> 00:06:22.580 tens of thousands of years 112 00:06:22.580 --> 00:06:25.960 and indeed the same stars, the same patterns, 113 00:06:26.140 --> 00:06:28.860 the giant emu in the sky, the guru guru bird, 114 00:06:29.120 --> 00:06:31.180 the river going across the sky, 115 00:06:31.380 --> 00:06:34.180 these Aboriginal stories are also exactly the same patterns 116 00:06:34.180 --> 00:06:35.440 as we see today. 117 00:06:35.680 --> 00:06:38.180 So you might assume, quite reasonably, 118 00:06:38.340 --> 00:06:41.460 that given that someone who wrote a story thousands of years ago 119 00:06:41.460 --> 00:06:43.640 was talking about the same pattern of stars that we see today, 120 00:06:43.820 --> 00:06:46.420 you might assume quite reasonably that the stars are indeed 121 00:06:46.420 --> 00:06:47.980 constant and unchanging. 122 00:06:48.900 --> 00:06:50.220 But if you look a little bit more carefully, 123 00:06:50.280 --> 00:06:52.560 you realise that that is certainly not the case. 124 00:06:53.220 --> 00:06:55.120 So the first thing that we now realise 125 00:06:55.120 --> 00:06:57.660 that people might have thought was impossible 126 00:06:57.660 --> 00:07:00.320 until the era upon astronomy is that stars die. 127 00:07:00.540 --> 00:07:01.720 They do not live forever. 128 00:07:02.480 --> 00:07:05.560 Now, later on I'm going to talk about what happens when the sun dies, 129 00:07:05.840 --> 00:07:09.300 but let's first start talking about the very biggest stars, 130 00:07:09.540 --> 00:07:13.780 stars that are 8, 10, 20, 100 times bigger than the sun. 131 00:07:14.400 --> 00:07:16.880 Now, these stars are quite rare, these heavyweights, 132 00:07:16.880 --> 00:07:19.220 but when they die and they run out of fuel, 133 00:07:19.420 --> 00:07:22.220 you know about it because they blow themselves to bits 134 00:07:22.220 --> 00:07:25.220 in a massive detonation which we call a supernova. 135 00:07:25.680 --> 00:07:26.940 And we actually see this happen. 136 00:07:27.600 --> 00:07:31.700 Certainly one of the most famous supernovae of the 20th century 137 00:07:31.700 --> 00:07:36.600 happened in the constellation of Dorado on the 23rd of February, 1987. 138 00:07:36.900 --> 00:07:38.820 And if you were looking up at Dorado at that moment, 139 00:07:38.900 --> 00:07:40.880 you'd just see a typical picture of the sky. 140 00:07:41.020 --> 00:07:42.420 But all of a sudden, quite abruptly, 141 00:07:43.700 --> 00:07:45.060 a star exploded. 142 00:07:45.400 --> 00:07:46.800 Now, we didn't hear that sound. 143 00:07:47.580 --> 00:07:49.460 So allow me a bit of artistic license. 144 00:07:49.740 --> 00:07:51.380 But this is the aftershot. 145 00:07:51.400 --> 00:07:53.100 This star blew itself to bits. 146 00:07:54.140 --> 00:07:56.500 So this is quite a catastrophic explosion. 147 00:07:56.960 --> 00:08:01.500 This particular event was 170,000 light years away. 148 00:08:02.040 --> 00:08:04.340 So that means that even though we saw it in 1987, 149 00:08:04.740 --> 00:08:08.600 it actually happened in about the year 168,000 BC. 150 00:08:09.160 --> 00:08:10.840 And for all of human history, 151 00:08:10.960 --> 00:08:14.680 that light had been traveling towards the Earth at the speed of light. 152 00:08:14.800 --> 00:08:16.580 And only in February of 1987, 153 00:08:16.880 --> 00:08:19.260 did it suddenly reach the Earth, and we saw it. 154 00:08:19.660 --> 00:08:22.500 So this particular explosion was quite important to me 155 00:08:23.500 --> 00:08:25.320 because you could see it with your naked eye. 156 00:08:25.700 --> 00:08:29.040 This was the first time anyone had seen a supernova with their naked eye 157 00:08:29.040 --> 00:08:30.640 since the year 1604. 158 00:08:31.460 --> 00:08:32.280 Would you believe it? 159 00:08:32.300 --> 00:08:33.740 A telescope was invented in 1608. 160 00:08:34.100 --> 00:08:35.820 We've been waiting hundreds and hundreds of years. 161 00:08:35.980 --> 00:08:38.200 This was the first time we had a bright naked eye supernova. 162 00:08:38.460 --> 00:08:39.760 So growing up in Sydney, 163 00:08:39.940 --> 00:08:42.600 I could walk out into my backyard in suburban Sydney, 164 00:08:42.740 --> 00:08:45.060 and suburban Sydney is not a great place to do stargazing, 165 00:08:45.060 --> 00:08:48.760 and I could see with my naked eye a star that was not there the night before. 166 00:08:49.020 --> 00:08:51.340 So that actually had quite a pivotal influence on me. 167 00:08:51.380 --> 00:08:53.600 I think I'd already decided that I wanted to be an astronomer, 168 00:08:53.720 --> 00:08:55.600 but being able to see something, 169 00:08:56.160 --> 00:08:58.540 an exploding star 170,000 light years away, 170 00:08:58.600 --> 00:09:00.040 was certainly an inspirational moment. 171 00:09:01.060 --> 00:09:03.000 So this is what it looks like with your naked eye 172 00:09:03.000 --> 00:09:05.260 from 170,000 light years away. 173 00:09:06.100 --> 00:09:08.160 Well, any further than that, we need telescopes. 174 00:09:08.220 --> 00:09:10.500 But just to show you how bright these things are, 175 00:09:10.560 --> 00:09:11.840 this is a much more distant supernova. 176 00:09:12.060 --> 00:09:14.760 This is the supernova that we saw in March of 1994. 177 00:09:15.060 --> 00:09:18.300 On the edge of a galaxy in the constellation of Virgo. 178 00:09:18.640 --> 00:09:21.160 And it's about 50 million light years away. 179 00:09:21.440 --> 00:09:24.020 So even though, again, we saw this in 1994, 180 00:09:24.700 --> 00:09:27.180 it actually happened 50 million BC. 181 00:09:27.440 --> 00:09:30.760 And the light was travelling towards us for all of that time. 182 00:09:31.020 --> 00:09:33.260 And what you can see here with that supernova in the lower left 183 00:09:33.260 --> 00:09:34.800 is that after the explosion, 184 00:09:35.020 --> 00:09:39.780 this supernova is brighter than every other star in the galaxy put together. 185 00:09:40.240 --> 00:09:43.340 So to try and understand how incredible that is, 186 00:09:43.560 --> 00:09:45.040 there are about 400 stars in the galaxy. 187 00:09:45.060 --> 00:09:47.760 400 billion stars in a typical galaxy like this one. 188 00:09:47.840 --> 00:09:48.680 400 billion. 189 00:09:48.960 --> 00:09:51.840 And this one star exploding is brighter, 190 00:09:51.900 --> 00:09:53.280 at least for a few days, 191 00:09:53.500 --> 00:09:56.320 than every other single star in that galaxy put together. 192 00:09:57.420 --> 00:09:58.780 Now, the one I showed you previously, 193 00:09:58.960 --> 00:10:00.340 the one that I saw as a teenager, 194 00:10:00.560 --> 00:10:01.880 that was not actually in our galaxy. 195 00:10:02.040 --> 00:10:05.020 170,000 light years away puts it in the next galaxy to us, 196 00:10:05.140 --> 00:10:06.420 the Large Magellanic Cloud. 197 00:10:06.700 --> 00:10:09.300 And here is another galaxy that's 50 million light years away. 198 00:10:09.720 --> 00:10:13.120 So you might ask, well, what would happen if a supernova happened in our own galaxy? 199 00:10:14.020 --> 00:10:17.020 Well, if it happened close enough, we would certainly know about it. 200 00:10:17.060 --> 00:10:18.620 We wouldn't just be able to see it with your naked eye. 201 00:10:18.800 --> 00:10:20.920 You'd be able to see it in broad daylight. 202 00:10:21.180 --> 00:10:23.800 It'd be so bright at night that you could read by it, 203 00:10:23.860 --> 00:10:25.760 and it would actually cast a shadow. 204 00:10:26.280 --> 00:10:28.040 Now, you might think that sounds a bit fantastic, 205 00:10:28.100 --> 00:10:28.980 and I'm just guessing, 206 00:10:29.100 --> 00:10:31.300 but I'm not, because this has actually happened, 207 00:10:31.400 --> 00:10:33.760 and people have seen supernovae in daylight, 208 00:10:33.900 --> 00:10:36.320 and people have been able to read by them at night. 209 00:10:36.760 --> 00:10:39.180 Now, you're probably thinking, I don't remember that happening. 210 00:10:39.360 --> 00:10:42.640 And that's because, as I said, the last one in our Milky Way happened in the year 1604, 211 00:10:42.640 --> 00:10:45.320 but if you go back through historical writings, 212 00:10:45.620 --> 00:10:49.160 you discover that there have been many spectacular supernova explosions 213 00:10:49.160 --> 00:10:51.140 quite near to the Earth in human history. 214 00:10:51.920 --> 00:10:54.200 Unfortunately, astronomy is a very old science, 215 00:10:54.260 --> 00:10:57.640 and people have been keeping astronomical records for thousands of years. 216 00:10:57.960 --> 00:11:00.280 So, for example, there was a very famous supernova 217 00:11:00.280 --> 00:11:03.500 that happened in July of the year 1054, 218 00:11:04.160 --> 00:11:07.240 and here are two records from different parts of the world of it. 219 00:11:07.620 --> 00:11:10.060 The top is a pictograph, a cave painting, 220 00:11:10.120 --> 00:11:12.620 from the Anasazi people in what is now New Mexico. 221 00:11:12.640 --> 00:11:15.240 And they simply recorded it visually. 222 00:11:15.380 --> 00:11:18.200 You can see a picture of a star and a crescent moon 223 00:11:18.200 --> 00:11:22.260 showing the relationship of this explosion to the moon on the day that it happened. 224 00:11:22.840 --> 00:11:27.180 Somewhat more detailed and specific description is written here in ancient Chinese. 225 00:11:27.540 --> 00:11:30.580 The Chinese had a court astronomer or astrologer 226 00:11:30.580 --> 00:11:32.380 who was there to monitor the stars, 227 00:11:32.460 --> 00:11:35.820 to make predictions and to advise the emperor 228 00:11:35.820 --> 00:11:38.800 on what the stars might be telling them about what decisions to make. 229 00:11:38.900 --> 00:11:42.240 And the same event was recorded by the Chinese court astronomer. 230 00:11:42.800 --> 00:11:45.860 So it's really interesting to look, thousands of years ago, 231 00:11:45.880 --> 00:11:47.260 long before we had modern astronomy, 232 00:11:47.440 --> 00:11:49.640 at the sorts of records the Chinese kept. 233 00:11:49.940 --> 00:11:52.840 So I'm now going to show you a translation from the Chinese records 234 00:11:52.840 --> 00:11:55.940 of another supernova that happened, this is only 1,000 years ago, 235 00:11:55.980 --> 00:11:58.380 I'm going to show you one that happened almost 2,000 years ago. 236 00:11:58.740 --> 00:12:01.460 So what the Chinese astronomer wrote 237 00:12:01.460 --> 00:12:04.560 is they wrote that in the Zhongping Rain Period of Emperor Ling, 238 00:12:04.780 --> 00:12:07.960 second year, tenth lunar month, day 60, 239 00:12:08.180 --> 00:12:10.160 a guest star appeared within Nanmen. 240 00:12:10.380 --> 00:12:12.420 Now this is actually incredibly useful information, 241 00:12:12.640 --> 00:12:14.180 and it's pretty much the same information 242 00:12:14.180 --> 00:12:16.500 that a modern astronomer would immediately report 243 00:12:16.500 --> 00:12:17.660 if they saw a supernova. 244 00:12:18.180 --> 00:12:21.680 The first thing that this astronomer tells us is the date. 245 00:12:21.840 --> 00:12:25.220 Now this isn't a date that I or you would typically use, 246 00:12:25.420 --> 00:12:27.820 but if you know your Chinese emperors and your Chinese calendars, 247 00:12:27.840 --> 00:12:30.080 and you can translate this into our modern calendar, 248 00:12:30.260 --> 00:12:33.800 you can obviously work out exactly what day this explosion happened. 249 00:12:33.960 --> 00:12:35.720 It says a guest star, a star suddenly appeared, 250 00:12:35.900 --> 00:12:37.560 they didn't know it was a supernova explosion, 251 00:12:37.860 --> 00:12:41.140 but they knew there was a star in the sky that was not there the night before. 252 00:12:41.560 --> 00:12:42.620 And they say that it was a supernova, 253 00:12:42.620 --> 00:12:43.740 and that it appeared within Nanmen. 254 00:12:44.080 --> 00:12:45.740 Now Nanmen is the southern gate, 255 00:12:45.940 --> 00:12:47.580 two bright stars next to each other. 256 00:12:48.720 --> 00:12:51.240 At contemporary usage we call those two stars the pointers. 257 00:12:51.700 --> 00:12:53.940 They're near the Southern Cross, Alpha and Beta Centauri, 258 00:12:54.060 --> 00:12:55.580 the same stars we talk about today. 259 00:12:55.860 --> 00:12:56.820 So that's very helpful. 260 00:12:56.980 --> 00:12:58.900 They've told us what day it occurred, 261 00:12:59.120 --> 00:13:00.460 and where in the sky it occurred. 262 00:13:00.720 --> 00:13:04.120 If you go to any email announcing a supernova today, 263 00:13:04.260 --> 00:13:06.620 the very first things it will tell you is what day they found it, 264 00:13:06.640 --> 00:13:07.920 and exactly where in the sky it is, 265 00:13:08.040 --> 00:13:09.780 so we know exactly where this occurred. 266 00:13:10.840 --> 00:13:12.200 What does the astronomer say next? 267 00:13:12.620 --> 00:13:14.400 They say that it was as large as half a mat. 268 00:13:14.680 --> 00:13:18.280 It showed the five colours and was both pleasing and angry. 269 00:13:18.900 --> 00:13:20.300 It gradually became smaller, 270 00:13:20.420 --> 00:13:22.820 and the sixth lunar month of the year after next, it vanished. 271 00:13:23.280 --> 00:13:25.980 Now this might seem a little more poetic than the first paragraph, 272 00:13:26.220 --> 00:13:27.960 but again it's giving us the basic information 273 00:13:27.960 --> 00:13:30.200 that any modern astronomer would report. 274 00:13:30.600 --> 00:13:33.140 It was as large as half a mat, it's telling us how bright it was, 275 00:13:33.360 --> 00:13:35.200 it's telling us what colour it was, 276 00:13:35.380 --> 00:13:38.280 and critically, and again this is a vital standard piece of information 277 00:13:38.280 --> 00:13:40.640 that every modern astronomer reports, 278 00:13:40.920 --> 00:13:42.440 it's telling you how quickly 279 00:13:42.620 --> 00:13:44.820 it faded away before it couldn't be seen anymore. 280 00:13:45.780 --> 00:13:48.920 So you can see that astronomy 2,000 years ago 281 00:13:48.920 --> 00:13:51.380 was quite well aligned with the sorts of measurements we like today. 282 00:13:51.900 --> 00:13:53.760 Of course the role of astronomers was different, 283 00:13:53.800 --> 00:13:56.520 they were there as essentially astrologers to advise people, 284 00:13:56.640 --> 00:13:59.480 and so here's where the ancient account somewhat diverges 285 00:13:59.480 --> 00:14:00.400 from our modern experience. 286 00:14:01.020 --> 00:14:03.000 The court astronomer goes on to write, 287 00:14:03.180 --> 00:14:05.880 the standard interpretations say this means insurrection. 288 00:14:06.380 --> 00:14:08.120 Wu Kuang attacked and killed He Mao, 289 00:14:08.280 --> 00:14:09.500 the general of chariots and cavalry, 290 00:14:09.700 --> 00:14:11.520 and several thousand people were killed. 291 00:14:12.620 --> 00:14:15.040 So fortunately we don't react this way now 292 00:14:15.620 --> 00:14:16.900 when we see a supernova explosion, 293 00:14:17.220 --> 00:14:18.960 but certainly for thousands of years 294 00:14:18.960 --> 00:14:21.180 these are things that people have definitely noticed 295 00:14:21.180 --> 00:14:24.580 and have acted on and tried to understand. 296 00:14:25.660 --> 00:14:28.420 So the bottom line is the sky indeed changes 297 00:14:28.420 --> 00:14:30.000 and we've known this for thousands of years. 298 00:14:31.460 --> 00:14:33.960 So supernovae that we can see with an naked eye 299 00:14:33.960 --> 00:14:37.080 and that have this sort of reaction involving thousands of people being killed 300 00:14:37.080 --> 00:14:37.620 are pretty rare. 301 00:14:38.000 --> 00:14:39.800 Here is the Milky Way galaxy. 302 00:14:40.640 --> 00:14:42.280 This is not what we see from Earth, 303 00:14:42.620 --> 00:14:44.700 it's not a photograph or a telescope image, 304 00:14:44.860 --> 00:14:47.720 it's an artist's impression of what the Milky Way would look like 305 00:14:47.720 --> 00:14:49.720 if you could somehow step outside it. 306 00:14:49.780 --> 00:14:50.640 I'm going to use a laser pointer, 307 00:14:50.760 --> 00:14:52.260 so just look at the top screen for a moment, 308 00:14:52.420 --> 00:14:53.520 don't look at those ones. 309 00:14:53.800 --> 00:14:55.100 So here is the centre of the galaxy 310 00:14:55.100 --> 00:14:58.620 and the Sun is out here very much in the suburbs. 311 00:14:58.780 --> 00:15:00.080 We're not even in a spiral arm, 312 00:15:00.140 --> 00:15:01.860 we're sort of halfway between two spiral arms. 313 00:15:02.040 --> 00:15:05.680 About 25,000 light years from the centre of the galaxy, 314 00:15:05.940 --> 00:15:09.300 with the whole galaxy being about 100,000 light years across. 315 00:15:10.800 --> 00:15:12.600 So when a supernova might happen, 316 00:15:12.600 --> 00:15:13.500 it might happen every few centuries, 317 00:15:13.640 --> 00:15:15.460 it typically happens in one of the spiral arms, 318 00:15:15.540 --> 00:15:17.620 it might happen here or here or over here. 319 00:15:19.380 --> 00:15:22.400 But you've got to remember that there's a very different place in our galaxy, 320 00:15:22.640 --> 00:15:24.580 the centre of our galaxy right here. 321 00:15:24.700 --> 00:15:26.880 I'm going to be talking a little bit about the centre of our galaxy 322 00:15:26.880 --> 00:15:28.200 for the next few minutes. 323 00:15:28.520 --> 00:15:30.500 The centre of the galaxy is a very violent, 324 00:15:30.580 --> 00:15:33.260 different place from the rest of the galaxy. 325 00:15:33.760 --> 00:15:35.380 And while a lot of people express disappointment 326 00:15:35.380 --> 00:15:36.680 that we live out in the suburbs, 327 00:15:37.000 --> 00:15:38.460 we should be grateful for that 328 00:15:38.460 --> 00:15:41.040 because the centre of the galaxy is a very violent place, 329 00:15:41.040 --> 00:15:42.840 it's very hostile to life. 330 00:15:43.400 --> 00:15:46.520 In particular, there are a lot of stars in the centre of the galaxy, 331 00:15:46.780 --> 00:15:49.740 and many of them are quite old, and they go supernova. 332 00:15:50.200 --> 00:15:54.420 So while out in the suburbs we might only have a supernova 333 00:15:54.420 --> 00:15:56.280 every few centuries, 334 00:15:56.460 --> 00:15:57.900 or maybe only once every thousand years, 335 00:15:57.960 --> 00:15:58.860 quite close to us, 336 00:15:59.020 --> 00:16:00.980 at the centre of our galaxy there are supernovae 337 00:16:00.980 --> 00:16:02.460 going off far more often. 338 00:16:03.520 --> 00:16:04.840 And just to show you what's going on, 339 00:16:04.860 --> 00:16:06.260 I'm going to take an extreme example of this, 340 00:16:06.380 --> 00:16:08.380 another galaxy that's a little bit like ours, 341 00:16:08.460 --> 00:16:09.640 but with a lot more supernovae. 342 00:16:11.040 --> 00:16:11.720 There's a supernovae going off in the centre 343 00:16:11.720 --> 00:16:13.480 that's essentially like listening to popcorn pop, 344 00:16:13.620 --> 00:16:15.160 they're just going off all the time. 345 00:16:15.340 --> 00:16:16.960 And that has a rather dramatic consequence 346 00:16:16.960 --> 00:16:19.460 on the overall structure of the galaxy. 347 00:16:20.060 --> 00:16:22.020 So this is a galaxy called Messier 82. 348 00:16:22.560 --> 00:16:24.640 You can sort of imagine it's a bit like our galaxy 349 00:16:24.640 --> 00:16:26.300 that I just showed you, but viewed edge-on. 350 00:16:26.540 --> 00:16:30.680 And this galaxy is about 12 million light-years away, 351 00:16:30.720 --> 00:16:32.200 and the constellations of Earth are major. 352 00:16:32.400 --> 00:16:34.460 And so here's the centre of the galaxy up here. 353 00:16:34.700 --> 00:16:37.460 And there are so many supernovae going off all the time, 354 00:16:37.520 --> 00:16:38.380 like popcorn, 355 00:16:39.260 --> 00:16:42.700 that essentially it's blasting the guts of the galaxy out into space. 356 00:16:42.980 --> 00:16:45.680 So you can actually see the galaxy being gradually ripped apart, 357 00:16:45.740 --> 00:16:48.260 it's like it's burst and it's flying outwards to the top and the bottom, 358 00:16:48.320 --> 00:16:51.100 because of all of the supernovae going off. 359 00:16:51.320 --> 00:16:53.500 And so when you add up all these supernovae together, 360 00:16:53.560 --> 00:16:56.580 you get what astronomers call a galactic wind. 361 00:16:56.740 --> 00:16:58.400 There's just a steady outflow of material 362 00:16:58.400 --> 00:17:01.400 being blasted off into space at the centre of the galaxy 363 00:17:01.400 --> 00:17:03.200 because of all these explosions. 364 00:17:03.740 --> 00:17:06.000 You can now see why it's a good thing to be living out here. 365 00:17:06.080 --> 00:17:08.160 You really don't want to be living out in here, 366 00:17:08.380 --> 00:17:09.940 where there's a risk that you and everything around you 367 00:17:09.940 --> 00:17:11.560 might get blasted off into space. 368 00:17:12.600 --> 00:17:15.380 Now the Milky Way is nowhere near as vigorous 369 00:17:15.380 --> 00:17:17.440 and as violent as Messier 82, 370 00:17:17.940 --> 00:17:20.320 but it too has a galactic wind. 371 00:17:20.680 --> 00:17:22.500 Of course I can't show you a picture of it like this, 372 00:17:22.540 --> 00:17:24.740 because we're inside the galaxy and it's much harder to tell, 373 00:17:24.900 --> 00:17:27.760 but at a lower level we too have the same thing going on, 374 00:17:27.840 --> 00:17:30.260 lots of big stars and supernova explosions 375 00:17:30.260 --> 00:17:31.820 at the centre of our galaxy, 376 00:17:32.260 --> 00:17:34.360 blasting material in a big fountain 377 00:17:34.360 --> 00:17:37.860 up above and below the galaxy out into space. 378 00:17:38.460 --> 00:17:40.660 Now that's not the only horrible thing going on 379 00:17:40.660 --> 00:17:42.160 at the centre of the Milky Way. 380 00:17:42.920 --> 00:17:44.460 Let's go back to the galaxy. 381 00:17:45.080 --> 00:17:48.660 We know that right at the very, very centre of the Milky Way, 382 00:17:48.740 --> 00:17:52.980 the absolute dead heart is a giant supermassive black hole. 383 00:17:53.480 --> 00:17:56.100 Now a black hole is a giant collapsed object 384 00:17:56.100 --> 00:17:57.900 whose gravity is so intense 385 00:17:57.900 --> 00:18:01.120 that nothing can escape from it once it gets too close. 386 00:18:01.400 --> 00:18:04.040 Even if you tried to travel at the speed of light, 387 00:18:04.200 --> 00:18:06.120 fast as we think anything can go, 388 00:18:06.780 --> 00:18:08.960 you still could not escape from a black hole. 389 00:18:09.200 --> 00:18:11.080 So that's why black holes are black, 390 00:18:11.200 --> 00:18:13.560 because even if you were unfortunate enough 391 00:18:13.560 --> 00:18:15.100 to be standing in side one with a big searchlight 392 00:18:15.100 --> 00:18:16.360 saying, hello, can anyone see me? 393 00:18:16.420 --> 00:18:17.720 The light just could not get out. 394 00:18:18.320 --> 00:18:19.820 The black hole appears black. 395 00:18:20.040 --> 00:18:22.420 So we know there's a black hole at the very centre of our galaxy. 396 00:18:22.460 --> 00:18:23.440 It's an absolute monster, 397 00:18:23.640 --> 00:18:26.320 weighs many millions of times the mass of the sun, 398 00:18:26.440 --> 00:18:30.980 and everything in the Milky Way is orbiting that central black hole. 399 00:18:31.800 --> 00:18:33.180 Now we're way out here, 400 00:18:33.280 --> 00:18:35.540 and we take 200 million years 401 00:18:35.540 --> 00:18:37.940