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These are the user uploaded subtitles that are being translated: 1 00:00:02,080 --> 00:00:04,640 NARRATOR: Twenty-six thousand light years from Earth... 2 00:00:06,200 --> 00:00:08,720 ..shrouded in cosmic dust and gas... 3 00:00:10,200 --> 00:00:12,640 ..is a mysterious region of space. 4 00:00:13,920 --> 00:00:17,440 The centre of the Milky Way. 5 00:00:17,520 --> 00:00:20,120 The centre of the Milky Way galaxy 6 00:00:20,200 --> 00:00:23,720 is one of the strangest, most exotic, 7 00:00:23,800 --> 00:00:27,680 and violent places in our galaxy. 8 00:00:29,800 --> 00:00:33,600 Gas streaming everywhere, radiation blasting out. 9 00:00:33,680 --> 00:00:35,680 Stars moving willy-nilly. 10 00:00:37,080 --> 00:00:40,240 And at the very heart is the mysterious black hole, 11 00:00:40,320 --> 00:00:42,320 four million times the mass of the sun. 12 00:00:44,120 --> 00:00:49,000 Now we're exploring the centre of the Milky Way like never before. 13 00:00:50,320 --> 00:00:54,160 Uncovering powerful forces that affect us all. 14 00:00:55,680 --> 00:00:58,360 Everything that happens at the centre of the Milky Way galaxy 15 00:00:58,440 --> 00:01:02,040 really is connected to what's going on in the rest of the Milky Way. 16 00:01:02,120 --> 00:01:05,160 Understanding the centre of our galaxy, 17 00:01:05,240 --> 00:01:10,360 unlock secrets of our past, present and future. 18 00:01:27,560 --> 00:01:30,520 March 2019, 19 00:01:30,600 --> 00:01:34,000 we focus the XMM-Newton space telescope 20 00:01:34,080 --> 00:01:38,040 on a region of space around Sagittarius A-star... 21 00:01:40,160 --> 00:01:43,520 ..the supermassive black hole at the heart of our galaxy. 22 00:01:47,680 --> 00:01:52,080 We spot two huge columns of gas blowing an X-ray light. 23 00:01:59,800 --> 00:02:03,640 The column seemed to be coming from Sagittarius A-star. 24 00:02:05,920 --> 00:02:09,480 We see giant fountains of gas extending outward 25 00:02:09,560 --> 00:02:11,560 from the central region, 26 00:02:11,640 --> 00:02:15,040 as though it's like a wind or a giant expulsion event. 27 00:02:17,400 --> 00:02:21,280 The fountains of gas extend 500 light years 28 00:02:21,360 --> 00:02:23,920 above and below the supermassive black hole. 29 00:02:27,720 --> 00:02:31,880 That's over a million times the distance from the sun to Neptune. 30 00:02:34,880 --> 00:02:37,240 It looks like this material is actually leaving 31 00:02:37,320 --> 00:02:39,360 the vicinity of the black hole, 32 00:02:39,440 --> 00:02:42,920 like it's burping out these giant, hot X-ray chimneys. 33 00:02:44,600 --> 00:02:48,480 So why is Sagittarius A-star burping out hot gas? 34 00:02:50,240 --> 00:02:53,000 Typically, around a black hole, you have an accretion disc 35 00:02:53,080 --> 00:02:55,080 funnelling material into the black hole, 36 00:02:55,160 --> 00:02:57,520 but all of it doesn't end up in the black hole. 37 00:02:59,160 --> 00:03:01,760 There is a little bit of gas falling onto it right now, 38 00:03:01,840 --> 00:03:03,840 even as I'm speaking, right? 39 00:03:05,520 --> 00:03:08,280 As gas falls toward the supermassive black hole, 40 00:03:08,360 --> 00:03:12,360 it becomes superheated, it liberates an enormous amount of energy, 41 00:03:12,440 --> 00:03:14,440 and that energy has to go somewhere. 42 00:03:16,320 --> 00:03:19,800 As gas spirals towards the black hole, 43 00:03:19,880 --> 00:03:23,560 some of the material accelerates to near the speed of light. 44 00:03:25,280 --> 00:03:27,600 It blasts out from the accretion disc... 45 00:03:30,760 --> 00:03:33,040 ..creating chimneys of superheated gas 46 00:03:33,120 --> 00:03:38,040 that seem to connect to two of the largest structures in the galaxy, 47 00:03:38,120 --> 00:03:40,640 the Milky Way's Fermi Bubbles. 48 00:03:42,720 --> 00:03:46,320 A few years ago, we noticed that there are these giant bubbles 49 00:03:46,400 --> 00:03:49,280 coming out of the very heart of the Milky Way galaxy. 50 00:03:49,360 --> 00:03:52,960 In each direction, there's a bubble 25,000 light years long. 51 00:03:55,280 --> 00:03:58,240 But the gas-filled bubbles dwarf the chimneys. 52 00:04:00,280 --> 00:04:04,400 Scientists wonder if another more powerful force blew the bubbles. 53 00:04:06,160 --> 00:04:09,040 So what could have created all of the superheated gas 54 00:04:09,120 --> 00:04:11,600 that actually blew these tremendously large bubbles? 55 00:04:12,800 --> 00:04:16,040 Black holes in other galaxies might offer clues. 56 00:04:17,920 --> 00:04:19,640 Black holes at the centres of galaxies 57 00:04:19,720 --> 00:04:21,200 go through different phases, 58 00:04:21,280 --> 00:04:23,800 so they can be either active or they can be calm. 59 00:04:25,560 --> 00:04:27,920 Sometimes black holes at the centres of galaxies 60 00:04:28,000 --> 00:04:30,400 go through an active phase. 61 00:04:30,480 --> 00:04:34,080 And when that happens, the black hole is actively feeding 62 00:04:34,160 --> 00:04:36,520 on material around it, which means it's growing, 63 00:04:36,600 --> 00:04:39,320 and it also gives off huge jets of radiation. 64 00:04:42,520 --> 00:04:46,240 Calm supermassive black holes release a trickle of hot gas. 65 00:04:51,080 --> 00:04:54,760 But when lots of material falls on them, 66 00:04:54,840 --> 00:04:58,840 they can shoot out jets up to millions of light years long. 67 00:05:03,920 --> 00:05:07,640 At the current time, Sagittarius A-star is what we call quiescent. 68 00:05:07,720 --> 00:05:11,040 It's quiet. There's some material swirling around it, 69 00:05:11,120 --> 00:05:13,920 but really, not very much. 70 00:05:14,000 --> 00:05:16,920 But we don't think that's always been the case. 71 00:05:18,840 --> 00:05:21,720 The centres of galaxies are busy places. 72 00:05:21,800 --> 00:05:25,520 There are stars there, there's gas there, there's dust there, 73 00:05:25,600 --> 00:05:28,520 and sometimes, these things fall into that black hole. 74 00:05:32,120 --> 00:05:34,160 Six million years ago, 75 00:05:34,240 --> 00:05:37,560 Sagittarius A-star may have had a feeding frenzy. 76 00:05:40,280 --> 00:05:44,680 Eating too much, and blasting out the remains in huge jets. 77 00:05:49,360 --> 00:05:51,200 Those jets plough through the galaxy, 78 00:05:51,280 --> 00:05:53,080 initially at near the speed of light. 79 00:05:55,560 --> 00:05:57,800 And as they do so, they can wreak havoc 80 00:05:57,880 --> 00:05:59,640 or sculpt the evolution of the galaxy 81 00:05:59,720 --> 00:06:01,480 that they're propagating through. 82 00:06:03,880 --> 00:06:07,880 Sagittarius A-star's jets blasted gas out of the galaxy... 83 00:06:09,400 --> 00:06:14,240 ..creating the scars we see as the Fermi Bubbles. 84 00:06:14,320 --> 00:06:17,240 Now, whatever caused those jets seems to have turned off, 85 00:06:17,320 --> 00:06:19,320 it's not happening any more, 86 00:06:19,400 --> 00:06:22,320 and we're seeing sort of the leftovers of them. 87 00:06:22,400 --> 00:06:26,120 This is clearly a sign that sometime in the past few million years, 88 00:06:26,200 --> 00:06:29,360 the black hole in the centre of our galaxy, Sagittarius A-star, 89 00:06:29,440 --> 00:06:32,600 was actively feeding on material around it. 90 00:06:35,240 --> 00:06:38,480 Material was falling into it and blasting out this stuff. 91 00:06:41,800 --> 00:06:44,640 The jets left destruction in their wake. 92 00:06:49,000 --> 00:06:53,640 They may also have affected the growth of our entire galaxy. 93 00:06:55,040 --> 00:06:57,760 These structures at the centre of our galaxy are important 94 00:06:57,840 --> 00:07:00,640 because they can either shut off star formation 95 00:07:00,720 --> 00:07:02,720 or they can trigger star formation. 96 00:07:06,080 --> 00:07:09,920 As those jets propagate through the galaxy, they pile up gas, 97 00:07:10,000 --> 00:07:13,000 and that gas can be then triggered into star formation. 98 00:07:16,320 --> 00:07:20,960 But these jets can also impart so much heat, or energy feedback, 99 00:07:21,040 --> 00:07:23,800 into the environment, that they prevent star formation. 100 00:07:26,880 --> 00:07:29,800 So black holes, in many ways, conduct an orchestra, 101 00:07:29,880 --> 00:07:33,680 instructing or dictating when stars can and cannot form. 102 00:07:42,800 --> 00:07:47,360 In the centre of the Milky Way, star formation rates seem low. 103 00:07:49,880 --> 00:07:51,920 The jets could be responsible. 104 00:07:53,960 --> 00:07:59,640 But in 2017, the ALMA telescope discovered that change is coming. 105 00:08:02,480 --> 00:08:06,560 So ALMA's actually been able to peer into the heart of our galaxy 106 00:08:06,640 --> 00:08:10,080 and see that near all this destruction, 107 00:08:10,160 --> 00:08:13,080 there might actually be a new generation of stars forming. 108 00:08:15,280 --> 00:08:18,760 Today, our calm, supermassive black hole 109 00:08:18,840 --> 00:08:21,320 could be helping star formation in the core. 110 00:08:24,960 --> 00:08:28,120 But the Fermi Bubbles could be evidence of the time 111 00:08:28,200 --> 00:08:32,000 when Sagittarius A-star shut down star formation. 112 00:08:33,600 --> 00:08:37,280 Could the supermassive black hole roar back to life in the future? 113 00:08:39,240 --> 00:08:41,600 Sag A-star could roar back to life 114 00:08:41,680 --> 00:08:43,800 by just dumping some gas onto it. 115 00:08:43,880 --> 00:08:46,360 And there's a lot of gas at the centre of our galaxy, 116 00:08:46,440 --> 00:08:49,520 and it could wander into the proximity of Sagittarius A-star 117 00:08:49,600 --> 00:08:51,720 and ultimately fall onto the event horizon, 118 00:08:51,800 --> 00:08:53,800 and that would light it up. 119 00:08:58,080 --> 00:09:01,000 If Sagittarius A-star eats enough gas... 120 00:09:04,520 --> 00:09:07,320 ..it could shut down star formation in the galaxy 121 00:09:07,400 --> 00:09:09,400 for millions of years. 122 00:09:14,720 --> 00:09:18,040 It could also give off X-rays and gamma rays 123 00:09:18,120 --> 00:09:20,120 that may hit the Earth. 124 00:09:23,600 --> 00:09:27,160 Thankfully, our central supermassive black hole 125 00:09:27,240 --> 00:09:30,680 is pretty quiet, and massive feeding events, 126 00:09:30,760 --> 00:09:33,560 massive energy events, are very, very rare. 127 00:09:35,720 --> 00:09:38,440 We don't necessarily have much to worry about. 128 00:09:41,240 --> 00:09:44,760 Sagittarius A-star has reshaped our galaxy. 129 00:09:48,120 --> 00:09:50,760 If we want to survive in the universe, 130 00:09:50,840 --> 00:09:54,000 we need to know more about this monster black hole. 131 00:09:55,960 --> 00:10:00,840 The Event Horizon telescope is on a mission to do just that. 132 00:10:00,920 --> 00:10:04,000 Question is, will it succeed? 133 00:10:12,320 --> 00:10:16,960 The centre of the Milky Way is home to a supermassive black hole, 134 00:10:17,040 --> 00:10:19,880 Sagittarius A-star. 135 00:10:19,960 --> 00:10:21,960 At least, we think it is. 136 00:10:24,600 --> 00:10:28,440 We've never seen the supermassive black hole directly, 137 00:10:28,520 --> 00:10:32,000 but we have seen stars racing around the core. 138 00:10:33,080 --> 00:10:36,480 The speeds of the stars zipping around the centre 139 00:10:36,560 --> 00:10:39,240 of our Milky Way galaxy indicate 140 00:10:39,320 --> 00:10:43,040 that there's something very massive and very compact there. 141 00:10:43,120 --> 00:10:47,520 Indeed, four million times as massive as our Sun, 142 00:10:47,600 --> 00:10:51,480 in a volume smaller than that of our solar system. 143 00:10:51,560 --> 00:10:53,960 It's gotta be a black hole basically. 144 00:10:55,800 --> 00:10:59,520 By measuring the orbits of stars in our galaxy's centre... 145 00:11:02,120 --> 00:11:07,160 ..we estimate that Sagittarius A-star is over 100 times wider 146 00:11:07,240 --> 00:11:09,240 than our sun. 147 00:11:12,480 --> 00:11:16,240 But despite its size, the black hole is hidden. 148 00:11:17,600 --> 00:11:20,680 One of the immediate challenges of actually observing black holes 149 00:11:20,760 --> 00:11:24,480 is the fact that they don't emit light, and so you can't see them. 150 00:11:24,560 --> 00:11:27,080 So we've never actually seen a black hole. 151 00:11:27,160 --> 00:11:29,400 We've only seen the stuff around a black hole. 152 00:11:29,480 --> 00:11:32,160 We have seen the effects of that black hole in parts 153 00:11:32,240 --> 00:11:34,240 on its ambient surroundings. 154 00:11:36,000 --> 00:11:39,120 That's where the Event Horizon Telescope came in. 155 00:11:40,360 --> 00:11:43,640 Its goal was to photograph Sagittarius A-star. 156 00:11:45,320 --> 00:11:49,240 Not the black hole itself, but its shadow. 157 00:11:50,720 --> 00:11:53,760 Around it is this gas that is moving around the black hole 158 00:11:53,840 --> 00:11:55,840 that is super-heated to millions of degrees. 159 00:11:55,920 --> 00:11:58,280 And what the Event Horizon telescope is trying to see 160 00:11:58,360 --> 00:12:00,360 is the shadow of the black hole. 161 00:12:01,960 --> 00:12:05,560 Light from the hot gas around Sagittarius A-star 162 00:12:05,640 --> 00:12:07,960 frames the giant shadow. 163 00:12:08,040 --> 00:12:11,520 It could be up to 93 million miles across. 164 00:12:13,200 --> 00:12:17,040 Problem is, Sagittarius A-star is so far away 165 00:12:17,120 --> 00:12:21,440 the supermassive black hole is still incredibly hard to see. 166 00:12:22,960 --> 00:12:24,960 Sagittarius A-star is big, 167 00:12:25,040 --> 00:12:27,640 but it's 26,000 light years away. 168 00:12:27,720 --> 00:12:30,360 You know, a single light year is six trillion miles. 169 00:12:30,440 --> 00:12:32,760 So this is a long, long walk. 170 00:12:32,840 --> 00:12:34,840 And even though it's big, 171 00:12:34,920 --> 00:12:36,960 that distance shrinks its apparent size 172 00:12:37,040 --> 00:12:39,360 to just a tiny little dot on the sky. 173 00:12:41,040 --> 00:12:45,520 To see the tiny dot, we need a telescope the size of the Earth. 174 00:12:47,160 --> 00:12:48,760 How do you possibly do that? 175 00:12:48,840 --> 00:12:50,600 You can't build that telescope, right? 176 00:12:52,160 --> 00:12:55,000 Well, there's a trick. You get a few different telescopes 177 00:12:55,080 --> 00:12:57,920 and you spread them out over the surface of the Earth. 178 00:13:03,680 --> 00:13:06,960 And when we add all of these sites together, 179 00:13:07,040 --> 00:13:10,160 we wind up being able to take an image of something 180 00:13:10,240 --> 00:13:12,240 that is really, really impossibly small. 181 00:13:14,560 --> 00:13:17,360 To gather enough light to see the small target, 182 00:13:17,440 --> 00:13:22,760 the team takes long exposure images of Sagittarius A-star's shadow. 183 00:13:27,240 --> 00:13:30,080 But there's a problem. 184 00:13:30,160 --> 00:13:32,880 The accretion disc moves too much 185 00:13:32,960 --> 00:13:36,840 for us to capture a clear image of Sagittarius A-star's shadow. 186 00:13:38,160 --> 00:13:40,600 When you're taking a long exposure of a person, 187 00:13:40,680 --> 00:13:43,240 you need them to be really, really still. 188 00:13:43,320 --> 00:13:46,560 Because if they're moving a lot, they're gonna blur the image out. 189 00:13:46,640 --> 00:13:50,640 And that kind of thing is happening when we observe Sagittarius A-star, 190 00:13:50,720 --> 00:13:52,760 because it is unwilling to sit still for us. 191 00:13:52,840 --> 00:13:54,560 It is booming and banging and flashing 192 00:13:54,640 --> 00:13:56,520 on the time scale of literally hours. 193 00:13:59,160 --> 00:14:01,400 As glowing material orbits the black hole 194 00:14:01,480 --> 00:14:03,680 at 30% the speed of light, 195 00:14:03,760 --> 00:14:06,480 Sagittarius A-star's shadow blurs. 196 00:14:09,360 --> 00:14:13,840 Future developments may allow us to see Sagittarius A-star clearly. 197 00:14:16,600 --> 00:14:19,560 For now, we can't capture an accurate image 198 00:14:19,640 --> 00:14:22,320 of our galaxy's supermassive black hole. 199 00:14:25,960 --> 00:14:30,560 But the hunt to see a supermassive black hole wasn't over. 200 00:14:30,640 --> 00:14:33,960 The Event Horizon telescope turned to another galaxy, 201 00:14:34,040 --> 00:14:38,200 54 million light years away, M87. 202 00:14:40,600 --> 00:14:43,200 M87 is an absolute beast of a galaxy. 203 00:14:43,280 --> 00:14:45,800 It's a so-called 'brightest cluster galaxy.' 204 00:14:45,880 --> 00:14:48,240 These are among the largest galaxies in the universe. 205 00:14:50,800 --> 00:14:54,720 And M87 is home to another supermassive black hole, 206 00:14:54,800 --> 00:14:58,000 the giant M87 star. 207 00:14:59,840 --> 00:15:02,320 M87 star is so massive 208 00:15:02,400 --> 00:15:05,280 that the gravitational region that's interesting 209 00:15:05,360 --> 00:15:07,600 is actually easier to image 210 00:15:07,680 --> 00:15:09,760 than the black hole in our own galaxy. 211 00:15:13,560 --> 00:15:19,080 M87 star is over 1,000 times more massive than Sagittarius A-star, 212 00:15:19,160 --> 00:15:22,160 and has a far larger accretion disc. 213 00:15:23,760 --> 00:15:27,280 When photographing a black hole, size matters, 214 00:15:27,360 --> 00:15:31,760 because big accretion discs project more stable light, 215 00:15:31,840 --> 00:15:34,400 so images of them don't blur as much. 216 00:15:38,240 --> 00:15:43,560 In April 2019, the Event Horizon team unveiled their image. 217 00:15:44,840 --> 00:15:48,040 We have seen what we thought was unseeable. 218 00:15:48,120 --> 00:15:52,240 We have seen and taken a picture of a black hole. 219 00:15:52,320 --> 00:15:54,320 (APPLAUSE) 220 00:16:00,800 --> 00:16:03,600 I've been working on this project for almost six years now, 221 00:16:03,680 --> 00:16:05,160 and so this is something 222 00:16:05,240 --> 00:16:07,640 we've been looking forward to for a really long time. 223 00:16:09,280 --> 00:16:12,720 Capturing this image took decades of work 224 00:16:12,800 --> 00:16:16,240 by hundreds of scientists from all over the world. 225 00:16:18,880 --> 00:16:21,440 I was really stunned. 226 00:16:21,520 --> 00:16:23,800 Suddenly when you say, 'That's the real thing', 227 00:16:23,880 --> 00:16:26,960 that's amazing. It really affected me. 228 00:16:27,040 --> 00:16:31,720 This is something six and half billion times the mass of the sun, 229 00:16:31,800 --> 00:16:35,560 55 million light years away, and we're looking at it? 230 00:16:37,280 --> 00:16:40,360 So, when you look at the image, it's totally fine, 231 00:16:40,440 --> 00:16:42,280 you're totally forgiven for thinking, 232 00:16:42,360 --> 00:16:44,000 "Uh, it looks a little blurry." 233 00:16:44,080 --> 00:16:49,360 But I cannot reiterate enough how profound this image actually is. 234 00:16:50,800 --> 00:16:53,680 We are seeing just a hair's width away 235 00:16:53,760 --> 00:16:57,200 from a discontinuity in the fabric of space time itself. 236 00:16:57,280 --> 00:17:01,120 Actually seeing so close to an actual event horizon, 237 00:17:01,200 --> 00:17:04,720 a discontinuity in the fabric of space-time, never seemed possible. 238 00:17:07,240 --> 00:17:09,600 This image of the heart of a distant galaxy... 239 00:17:10,800 --> 00:17:14,320 ..helps us understand supermassive black holes like never before. 240 00:17:17,280 --> 00:17:20,240 When we observe supermassive black holes in other galaxies, 241 00:17:20,320 --> 00:17:22,320 including the one in M87, 242 00:17:22,400 --> 00:17:25,080 we're able to learn more about the big picture 243 00:17:25,160 --> 00:17:28,200 of how these massive black holes form and evolve over time. 244 00:17:28,280 --> 00:17:31,320 And that, in turn, helps us understand how our Milky Way galaxy 245 00:17:31,400 --> 00:17:33,760 and its supermassive black hole has formed. 246 00:17:36,080 --> 00:17:38,480 By studying, not just making images of black holes, 247 00:17:38,560 --> 00:17:40,560 but making videos of black holes, 248 00:17:40,640 --> 00:17:43,520 and seeing as that gas is spinning around it, 249 00:17:43,600 --> 00:17:45,600 we can try to map around a black hole 250 00:17:45,680 --> 00:17:47,920 more precisely, and learn about its dynamics. 251 00:17:49,560 --> 00:17:53,600 An image of Sagittarius A-star remains out of reach, 252 00:17:53,680 --> 00:17:57,760 but in 2018, it shows a deadly side to its character. 253 00:18:00,160 --> 00:18:02,960 The supermassive black hole's accretion disc 254 00:18:03,040 --> 00:18:06,360 releases huge, powerful flares, 255 00:18:06,440 --> 00:18:09,560 and they could be pointed right at us. 256 00:18:13,120 --> 00:18:17,240 In 2018, astronomers were studying a special star. 257 00:18:19,160 --> 00:18:21,240 The star is called S-2... 258 00:18:22,360 --> 00:18:26,280 ..and it passes close to Sagittarius A-star every 16 years. 259 00:18:28,120 --> 00:18:30,120 By studying this star's fly-by, 260 00:18:30,200 --> 00:18:33,120 we hope to learn more about Sagittarius A-star. 261 00:18:35,160 --> 00:18:38,160 We think that S2 may be the very closest star 262 00:18:38,240 --> 00:18:41,480 to the supermassive black hole in the centre of our galaxy. 263 00:18:41,560 --> 00:18:43,560 At closest approach to Sag A-star, 264 00:18:43,640 --> 00:18:47,800 S2 comes within 17 light hours or so of the surface. 265 00:18:50,800 --> 00:18:53,640 The supermassive black hole's powerful gravity 266 00:18:53,720 --> 00:18:57,480 accelerates the star to 17 million miles an hour. 267 00:18:59,000 --> 00:19:02,760 That's fast enough to travel from New York to LA in half a second. 268 00:19:04,720 --> 00:19:08,880 But it's not the star's speed that excites scientists. 269 00:19:10,160 --> 00:19:13,160 This is a great star, because it's on an elliptical orbit 270 00:19:13,240 --> 00:19:15,560 that takes it fairly far from the black hole, 271 00:19:15,640 --> 00:19:20,160 but every few years, it passes right above the supermassive black hole. 272 00:19:22,400 --> 00:19:25,920 As we tracked S2's swing around Sagittarius A-star, 273 00:19:26,000 --> 00:19:29,720 we detected powerful bursts of infrared light 274 00:19:29,800 --> 00:19:33,480 coming from the direction of the supermassive black hole. 275 00:19:36,520 --> 00:19:38,360 There's a blob of gas that's orbiting 276 00:19:38,440 --> 00:19:40,240 very close to the black hole, 277 00:19:40,320 --> 00:19:43,200 and it was flaring as it went around. 278 00:19:43,280 --> 00:19:46,760 There were three separate flares of light that they were able to detect. 279 00:19:48,680 --> 00:19:52,280 The flares didn't come directly from the supermassive black hole. 280 00:19:54,480 --> 00:19:57,040 They came from material around it. 281 00:20:00,000 --> 00:20:02,720 The flares that were discovered are thought to originate 282 00:20:02,800 --> 00:20:05,880 from magnetic storms in this very, very hot turbulent gas 283 00:20:05,960 --> 00:20:08,000 around the black hole. 284 00:20:10,160 --> 00:20:12,320 The extreme heat in the accretion disc 285 00:20:12,400 --> 00:20:15,760 strips electrons from atoms of gas. 286 00:20:15,840 --> 00:20:20,120 The stripped electrons and hot gas form a plasma, 287 00:20:20,200 --> 00:20:22,920 which creates powerful magnetic fields 288 00:20:23,000 --> 00:20:25,120 when accelerated to high speeds. 289 00:20:26,640 --> 00:20:30,360 Because some supermassive black holes have these super-heated, 290 00:20:30,440 --> 00:20:33,640 rapidly spinning vortices of gas swirling around them, 291 00:20:33,720 --> 00:20:38,160 you get these very, very powerful, very tightly wound magnetic fields. 292 00:20:40,360 --> 00:20:42,720 And there's energy stored in that magnetic field. 293 00:20:42,800 --> 00:20:45,360 It's like a bunch of piano wires all tangled up, 294 00:20:45,440 --> 00:20:48,280 and if these things interact with each other, they can snap. 295 00:20:48,360 --> 00:20:50,600 And when they snap, that energy is released. 296 00:20:55,200 --> 00:20:57,360 You'll get this enormous release of energy 297 00:20:57,440 --> 00:21:00,360 as these coils of magnetic fields effectively snap, 298 00:21:00,440 --> 00:21:02,800 and when they do so... 299 00:21:04,040 --> 00:21:06,040 ..just like on the surface of our sun, 300 00:21:06,120 --> 00:21:08,120 they release an enormous flare of gas. 301 00:21:12,040 --> 00:21:15,920 These powerful flares can be millions of miles wide... 302 00:21:17,640 --> 00:21:20,960 ..and come packed with super-heated gas and plasma. 303 00:21:24,600 --> 00:21:27,560 Solar flares release as much energy 304 00:21:27,640 --> 00:21:30,440 as ten million volcanic explosions. 305 00:21:35,520 --> 00:21:38,240 Flares from Sagittarius A-star's accretion disc... 306 00:21:40,520 --> 00:21:44,720 ..are like millions of solar flares all going off at once. 307 00:21:48,160 --> 00:21:51,520 It's kinda like comparing a nuclear weapon to a firecracker. 308 00:21:54,160 --> 00:21:59,160 Sagittarius A-star's flares release intense blasts of radiation, 309 00:21:59,240 --> 00:22:01,720 but by watching the flares from Earth, 310 00:22:01,800 --> 00:22:03,800 we can learn about the orientation 311 00:22:03,880 --> 00:22:06,840 of the supermassive black hole's accretion disc. 312 00:22:09,200 --> 00:22:12,680 This gas that's in this accretion disc around the black hole 313 00:22:12,760 --> 00:22:16,320 is like a friendly helper, shining a flashlight back toward Earth, 314 00:22:16,400 --> 00:22:18,880 and we can watch the orbit of these flashlights 315 00:22:18,960 --> 00:22:21,280 and help understand the orientation of gas 316 00:22:21,360 --> 00:22:23,360 that swirls around the black hole. 317 00:22:27,040 --> 00:22:29,080 We think we're getting a bird's-eye view of it, 318 00:22:29,160 --> 00:22:31,160 and looking down the barrel, 319 00:22:31,240 --> 00:22:33,840 or looking at the accretion disc basically face on. 320 00:22:33,920 --> 00:22:35,760 That means that any material 321 00:22:35,840 --> 00:22:38,080 that gets blasted away from the black hole 322 00:22:38,160 --> 00:22:40,360 could be aimed right at us. 323 00:22:43,800 --> 00:22:47,160 Should we be worried about the flares reaching Earth? 324 00:22:48,880 --> 00:22:50,720 It sounds worrisome, 325 00:22:50,800 --> 00:22:53,440 this blob of gas emitting these huge flares of light, 326 00:22:53,520 --> 00:22:57,040 but you gotta realise, this is 26,000 light years away. 327 00:22:57,120 --> 00:23:00,200 That is a long way. It took an extremely sensitive detector 328 00:23:00,280 --> 00:23:03,400 on one of the largest telescopes on Earth 329 00:23:03,480 --> 00:23:05,840 to be able to see this at all. 330 00:23:05,920 --> 00:23:08,320 Earth is safe, for now, 331 00:23:08,400 --> 00:23:11,240 but the more we learn about the galaxy's centre, 332 00:23:11,320 --> 00:23:13,320 the more terrifying it becomes. 333 00:23:14,960 --> 00:23:18,760 We know of Sagittarius A-star, the central supermassive black hole, 334 00:23:18,840 --> 00:23:23,200 but now we're beginning to suspect that it might not be alone. 335 00:23:25,200 --> 00:23:28,440 A dangerous swarm of black holes could be racing 336 00:23:28,520 --> 00:23:31,160 around the centre of the Milky Way. 337 00:23:31,240 --> 00:23:34,360 Thousands more may be hiding from sight. 338 00:23:39,720 --> 00:23:43,280 The supermassive black hole, Sagittarius A-star, 339 00:23:43,360 --> 00:23:45,720 dominates the centre of the Milky Way... 340 00:23:48,000 --> 00:23:50,320 ..affecting star formation... 341 00:23:52,200 --> 00:23:55,560 ..and carving out vast gas bubbles in space. 342 00:23:57,720 --> 00:24:02,320 But Sagittarius A-star may not be the only black hole in town, 343 00:24:02,400 --> 00:24:05,040 or even the most dangerous. 344 00:24:07,480 --> 00:24:10,320 We've known for a long time that there's a supermassive black hole 345 00:24:10,400 --> 00:24:12,400 in the very heart of our galaxy, 346 00:24:12,480 --> 00:24:15,360 but there may be an angry swarm of smaller black holes 347 00:24:15,440 --> 00:24:18,200 buzzing all around it. 348 00:24:18,280 --> 00:24:22,680 In April 2018, astronomers, led by Columbia University, 349 00:24:22,760 --> 00:24:25,560 revealed the results of a hunting mission 350 00:24:25,640 --> 00:24:27,840 in the centre of the galaxy. 351 00:24:27,920 --> 00:24:31,600 They'd used 12 years of Chandra Observatory data 352 00:24:31,680 --> 00:24:34,480 to seek out stellar mass black holes. 353 00:24:35,920 --> 00:24:38,600 Black holes that are made from the death of stars, 354 00:24:38,680 --> 00:24:42,360 from supernova explosions, are called stellar mass black holes. 355 00:24:47,960 --> 00:24:52,360 These are made from stars that were many times the mass of the sun. 356 00:24:55,760 --> 00:24:58,360 Finding stellar mass black holes is tough. 357 00:25:00,280 --> 00:25:03,200 Light can't escape a black hole's gravity, 358 00:25:03,280 --> 00:25:05,280 so we can't see them directly. 359 00:25:08,720 --> 00:25:12,480 And stellar mass black holes are only tens of miles wide, 360 00:25:12,560 --> 00:25:15,000 making them almost impossible to detect. 361 00:25:18,240 --> 00:25:22,400 So astronomers look for a special type of stellar mass black hole. 362 00:25:26,360 --> 00:25:30,040 One of the ways that we look for stellar mass black holes 363 00:25:30,120 --> 00:25:33,000 is that they often are vampires, 364 00:25:33,080 --> 00:25:35,960 eating a companion star. 365 00:25:39,720 --> 00:25:43,800 These vampires are part of a binary pair, 366 00:25:43,880 --> 00:25:48,280 a stellar mass black hole in orbit with a living star, 367 00:25:48,360 --> 00:25:51,480 the black hole feasting on its partner. 368 00:25:53,280 --> 00:25:56,880 That black hole is like a very, very deadly parasite for that star. 369 00:25:56,960 --> 00:25:59,720 It is ripping mass off the surface of that star, 370 00:25:59,800 --> 00:26:03,200 and that matter is raining down toward the black hole itself. 371 00:26:06,960 --> 00:26:09,040 And that material lights up, 372 00:26:09,120 --> 00:26:11,680 so this allows us to hunt for black holes, 373 00:26:11,760 --> 00:26:14,600 not through taking pictures of black holes directly, 374 00:26:14,680 --> 00:26:18,480 but through seeing the material falling to its doom. 375 00:26:19,720 --> 00:26:23,640 Problem is, gas and dust spread throughout the galaxy 376 00:26:23,720 --> 00:26:28,200 stops visible light from the binary pair reaching Earth. 377 00:26:28,280 --> 00:26:31,520 But the binary pair release another type of light 378 00:26:31,600 --> 00:26:36,720 that passes through the gas and dust more easily, X-rays. 379 00:26:36,800 --> 00:26:39,000 The system itself is emitting X-rays, 380 00:26:39,080 --> 00:26:41,240 so they're called 'X-ray binaries.' 381 00:26:41,320 --> 00:26:44,000 So, these are useful because the X-ray emission 382 00:26:44,080 --> 00:26:47,680 can be very powerful, and can be potentially seen from the Earth, 383 00:26:47,760 --> 00:26:51,520 even though the binary's very far away, say, at the galactic centre. 384 00:26:54,560 --> 00:26:58,000 The glowing discs of material and X-ray binary systems 385 00:26:58,080 --> 00:27:00,600 are almost a million times smaller 386 00:27:00,680 --> 00:27:03,680 than the accretion disc surrounding Sagittarius A-star, 387 00:27:03,760 --> 00:27:08,960 too small for us to see the material swirling around them in detail. 388 00:27:11,600 --> 00:27:15,560 So we see the X-ray binaries as pinpricks of X-ray light. 389 00:27:18,840 --> 00:27:22,520 Astronomers detect 12 of these X-ray binaries 390 00:27:22,600 --> 00:27:25,680 in a small, three-lightyear wide patch of space 391 00:27:25,760 --> 00:27:27,760 at the galactic centre. 392 00:27:27,840 --> 00:27:30,640 And that means that there could be a much larger collection 393 00:27:30,720 --> 00:27:33,200 of these relatively tiny stellar mass black holes 394 00:27:33,280 --> 00:27:35,280 in the heart of our galaxy. 395 00:27:37,240 --> 00:27:39,600 If black holes form the way we think they do, 396 00:27:39,680 --> 00:27:42,560 there are very likely maybe swarms of black holes 397 00:27:42,640 --> 00:27:44,640 racing around Sagittarius A-star. 398 00:27:47,320 --> 00:27:50,880 But X-ray binaries that are powerful enough for us to detect 399 00:27:50,960 --> 00:27:53,560 are incredibly rare. 400 00:27:55,440 --> 00:27:59,800 We estimate that for the dozen X-ray binaries discovered, 401 00:27:59,880 --> 00:28:02,360 there could be up to one thousand more. 402 00:28:07,040 --> 00:28:11,120 In total, there could be 20,000 stellar mass black holes 403 00:28:11,200 --> 00:28:14,240 in this three-lightyear region of space 404 00:28:23,680 --> 00:28:27,040 They may be small, but they're dangerous. 405 00:28:27,120 --> 00:28:29,120 Stellar mass black holes 406 00:28:29,200 --> 00:28:31,920 are by far deadlier than supermassive black holes. 407 00:28:32,000 --> 00:28:36,240 Stellar mass black holes are more likely to tear things apart 408 00:28:36,320 --> 00:28:39,120 when they get too close. 409 00:28:39,200 --> 00:28:43,520 These stellar mass black holes are only tens of miles wide, 410 00:28:43,600 --> 00:28:46,680 but are several times the mass of the sun, 411 00:28:46,760 --> 00:28:50,240 and that makes their gravitational pull increase rapidly 412 00:28:50,320 --> 00:28:52,600 as you get close to one. 413 00:28:52,680 --> 00:28:55,520 Even 100 miles from the event horizon, 414 00:28:55,600 --> 00:28:59,080 the gravitational pull can be thousands of times stronger 415 00:28:59,160 --> 00:29:02,080 just a single yard closer to the black hole. 416 00:29:04,280 --> 00:29:08,120 Strong enough to tear you or your spaceship apart. 417 00:29:17,160 --> 00:29:22,000 Supermassive black holes like Sagittarius A-star are different. 418 00:29:22,080 --> 00:29:25,560 It's over four million times the mass of the sun, 419 00:29:25,640 --> 00:29:28,280 and its event horizon is so large 420 00:29:28,360 --> 00:29:30,720 that the gravitational forces increase 421 00:29:30,800 --> 00:29:33,240 much more slowly as you get near. 422 00:29:35,400 --> 00:29:37,720 The event horizon extends for so far 423 00:29:37,800 --> 00:29:40,160 that you can be sucked in without knowing it, 424 00:29:40,240 --> 00:29:42,720 and you're lost, and you're not ripped apart, 425 00:29:42,800 --> 00:29:44,800 as you pass through the event horizon. 426 00:29:46,000 --> 00:29:48,880 You can be trapped forever, but not realise it yet. 427 00:29:51,640 --> 00:29:55,640 Why are these black holes swarming in the galaxy's centre? 428 00:29:55,720 --> 00:29:59,200 It appears they've migrated from the rest of the Milky Way. 429 00:30:01,080 --> 00:30:03,160 Through a process called dynamical friction, 430 00:30:03,240 --> 00:30:05,720 black holes can actually sink to the centres of galaxies 431 00:30:05,800 --> 00:30:08,320 very, very rapidly, like dropping a stone into a pond. 432 00:30:09,720 --> 00:30:13,000 What that means is that an errant, wandering black hole 433 00:30:13,080 --> 00:30:16,240 might eventually find its way toward the centre of our own galaxy, 434 00:30:16,320 --> 00:30:18,320 where Sagittarius A-star resides. 435 00:30:20,160 --> 00:30:23,440 As stellar mass black holes orbit the galaxy, 436 00:30:23,520 --> 00:30:28,280 they interact gravitationally with stars and clouds of gas and dust. 437 00:30:31,160 --> 00:30:33,360 These interactions push the black holes 438 00:30:33,440 --> 00:30:35,720 towards the centre of the galaxy... 439 00:30:37,360 --> 00:30:39,360 ..where the black holes swarm. 440 00:30:44,000 --> 00:30:47,680 A swarm of stellar mass black holes sounds deadly, 441 00:30:47,760 --> 00:30:52,600 but it may not be the most lethal thing in the centre of the Milky Way. 442 00:30:55,120 --> 00:30:57,640 A surprising observation indicates 443 00:30:57,720 --> 00:31:01,440 that there is a lot of antimatter in the centre of our galaxy. 444 00:31:04,040 --> 00:31:07,960 In 2017, astronomers tried to solve 445 00:31:08,040 --> 00:31:10,600 a decades old cosmic mystery. 446 00:31:13,640 --> 00:31:17,880 Unexplained high energy radiation streaming through our galaxy. 447 00:31:22,200 --> 00:31:25,000 At first, we didn't know where it came from... 448 00:31:27,200 --> 00:31:30,160 ..but we discovered it was gamma radiation 449 00:31:30,240 --> 00:31:33,400 coming from somewhere in the centre of the Milky Way. 450 00:31:36,120 --> 00:31:38,840 The question is, what's making these gamma rays? 451 00:31:38,920 --> 00:31:42,160 That's hard to do. It's not like you can rub your hands together 452 00:31:42,240 --> 00:31:44,640 and generate gamma rays. 453 00:31:44,720 --> 00:31:47,560 When we took a closer look at the gamma rays, 454 00:31:47,640 --> 00:31:49,720 we discovered the signature 455 00:31:49,800 --> 00:31:53,240 of the most explosive substance in the universe, 456 00:31:53,320 --> 00:31:55,320 antimatter. 457 00:31:57,920 --> 00:32:02,560 Antimatter is like normal matter, but with opposite charge. 458 00:32:02,640 --> 00:32:05,480 That's it. It's matter's evil twin. 459 00:32:07,880 --> 00:32:11,920 When evil twin meets good twin, it is not a happy reunion. 460 00:32:15,880 --> 00:32:19,960 Antimatter is scary. It's not like you wanna have some in your kitchen. 461 00:32:20,040 --> 00:32:23,720 This stuff is very, very explosive, if you wanna think of it that way. 462 00:32:23,800 --> 00:32:27,920 If it touches normal matter, it releases a huge amount of energy. 463 00:32:29,640 --> 00:32:33,960 When matter and antimatter combine, they annihilate each other 464 00:32:34,040 --> 00:32:36,520 and transform into high energy radiation, 465 00:32:36,600 --> 00:32:38,640 just like the gamma rays 466 00:32:38,720 --> 00:32:41,360 seen streaming out of the centre of the Milky Way. 467 00:32:43,840 --> 00:32:46,320 We see antimatter throughout the galaxy, 468 00:32:46,400 --> 00:32:48,720 but strangely, the galactic centre 469 00:32:48,800 --> 00:32:53,000 seemed to have 40% more antimatter than anywhere else. 470 00:32:55,400 --> 00:32:57,400 Right now, in the heart of our galaxy, 471 00:32:57,480 --> 00:33:00,320 we actually observe fountains of antimatter 472 00:33:00,400 --> 00:33:04,800 that are producing ten trillion tons of antimatter every second. 473 00:33:06,360 --> 00:33:09,320 One of the big questions that we've wondered about for a very long time 474 00:33:09,400 --> 00:33:11,920 is what's the origin of this stuff? 475 00:33:14,360 --> 00:33:16,880 Initially, there were several suspects. 476 00:33:18,040 --> 00:33:20,040 One possible source of antimatter 477 00:33:20,120 --> 00:33:23,400 is the central supermassive black hole, Sagittarius A-star. 478 00:33:23,480 --> 00:33:26,480 Matter can be swirling around this, and it can have such high energy 479 00:33:26,560 --> 00:33:29,120 that it can create antimatter. 480 00:33:31,960 --> 00:33:34,880 But the antimatter isn't coming from a single point. 481 00:33:34,960 --> 00:33:38,160 It's spread across thousands of light years of space. 482 00:33:40,280 --> 00:33:44,560 So Sagittarius A-star can't be the source of the gamma ray stream. 483 00:33:48,120 --> 00:33:50,840 Another suspect was dark matter. 484 00:33:53,600 --> 00:33:57,080 One of the biggest mysteries in the universe right now is dark matter. 485 00:33:57,160 --> 00:33:59,920 We know that the majority of mass in the universe 486 00:34:00,000 --> 00:34:03,360 is not in the same form that we are, it's not made of atoms. 487 00:34:03,440 --> 00:34:05,960 But whatever sort of particle it is or may be, 488 00:34:06,040 --> 00:34:09,000 if these things collide, they can produce antimatter, 489 00:34:09,080 --> 00:34:11,080 and that will produce the gamma rays. 490 00:34:11,160 --> 00:34:13,760 So it's possible that as we look into the heart of the galaxy 491 00:34:13,840 --> 00:34:15,840 and see these extra gamma rays, 492 00:34:15,920 --> 00:34:18,000 that's the signal that dark matter is there. 493 00:34:21,480 --> 00:34:24,480 But the gamma ray stream we detected 494 00:34:24,560 --> 00:34:27,640 is too weak to have been created by dark matter. 495 00:34:30,840 --> 00:34:33,600 Then we had a breakthrough. 496 00:34:33,680 --> 00:34:38,400 We discovered that a special metal called Titanium-44 497 00:34:38,480 --> 00:34:41,800 could be responsible for the gamma ray stream. 498 00:34:46,000 --> 00:34:49,120 Titanium-44 is a highly radioactive element. 499 00:34:49,200 --> 00:34:52,440 That means that it wants to decay into other types of nuclei. 500 00:34:55,880 --> 00:35:00,520 When Titanium-44 decays, it gives off antimatter. 501 00:35:00,600 --> 00:35:03,640 But to produce the antimatter seen in the galaxy's core, 502 00:35:03,720 --> 00:35:06,600 you would need a lot of Titanium-44. 503 00:35:07,960 --> 00:35:12,160 It could be created in rare energetic events, 504 00:35:12,240 --> 00:35:15,600 in the collision of two dead stars, 505 00:35:15,680 --> 00:35:18,480 white dwarfs. 506 00:35:18,560 --> 00:35:20,080 A white dwarf star 507 00:35:20,160 --> 00:35:22,320 is a star that did not have enough mass when it died 508 00:35:22,400 --> 00:35:24,400 to actually become a supernova. 509 00:35:24,480 --> 00:35:26,840 It just sort of cools off as a dead little cinder. 510 00:35:26,920 --> 00:35:28,560 But what if you have two white dwarfs 511 00:35:28,640 --> 00:35:30,400 that are orbiting around each other? 512 00:35:30,480 --> 00:35:33,360 And as they come closer and closer and collide, 513 00:35:33,440 --> 00:35:35,640 all of a sudden now you have enough mass 514 00:35:35,720 --> 00:35:37,920 to actually kick a supernova explosion off. 515 00:35:42,720 --> 00:35:44,720 These particular kinds of supernovae 516 00:35:44,800 --> 00:35:47,080 are very good at producing Titanium-44. 517 00:35:49,520 --> 00:35:52,200 So these kinds of supernovas are very, very good 518 00:35:52,280 --> 00:35:54,280 at making antimatter. 519 00:35:56,360 --> 00:35:59,720 These supernovas erupt in the core of the galaxy 520 00:35:59,800 --> 00:36:02,120 once every 2,000 years. 521 00:36:04,640 --> 00:36:07,800 But outside of the core, in the disc of the galaxy 522 00:36:07,880 --> 00:36:09,880 where our solar system orbits... 523 00:36:12,040 --> 00:36:15,160 ..these supernovas happen three times as often. 524 00:36:17,000 --> 00:36:20,080 So, the gamma ray observations were wrong, 525 00:36:20,160 --> 00:36:23,280 there isn't more antimatter in the heart of the galaxy. 526 00:36:24,600 --> 00:36:28,960 It's our region of the galaxy that contains the most antimatter. 527 00:36:31,840 --> 00:36:34,720 The question is, are we in danger? 528 00:36:36,440 --> 00:36:38,080 If you take an ounce of matter 529 00:36:38,160 --> 00:36:40,240 and an ounce of antimatter and collide them, 530 00:36:40,320 --> 00:36:43,000 you're generating a megaton of energy, 531 00:36:43,080 --> 00:36:47,120 the equivalent of a million tons of TNT exploding. 532 00:36:48,480 --> 00:36:51,880 So you don't need much antimatter to generate a vast amount of energy. 533 00:36:53,240 --> 00:36:55,520 But the thing you have to remember is we live 534 00:36:55,600 --> 00:36:59,120 in this wonderful dramatic environment of the larger universe. 535 00:36:59,200 --> 00:37:01,640 It's not dangerous. It's very far away from us, 536 00:37:01,720 --> 00:37:03,720 and it's fascinating... 537 00:37:05,920 --> 00:37:09,080 ..that all of this antimatter's being produced in our galaxy. 538 00:37:09,160 --> 00:37:11,560 So just sit back and enjoy the fireworks. 539 00:37:19,000 --> 00:37:23,280 The Milky Way is around 100,000 light years across, 540 00:37:23,360 --> 00:37:27,360 and it's home to at least 200 billion stars, 541 00:37:27,440 --> 00:37:30,920 but it hasn't always been this large. 542 00:37:31,000 --> 00:37:34,000 We know that our Milky Way galaxy 543 00:37:34,080 --> 00:37:36,400 grew to the size it is now, which is huge, 544 00:37:36,480 --> 00:37:38,920 by eating other galaxies. 545 00:37:39,000 --> 00:37:41,800 And some of these galaxies would have had 546 00:37:41,880 --> 00:37:44,360 supermassive black holes in their centres. 547 00:37:47,760 --> 00:37:51,440 When the Milky Way's gravity pulled in smaller galaxies, 548 00:37:51,520 --> 00:37:55,200 most of their material merged with the Milky Way. 549 00:37:57,160 --> 00:37:59,640 But some material, like stars, 550 00:37:59,720 --> 00:38:04,200 could've been slung tens of thousands of light years out of the Milky Way. 551 00:38:07,160 --> 00:38:09,320 This could even have happened 552 00:38:09,400 --> 00:38:12,360 to a smaller galaxy's supermassive black hole. 553 00:38:12,440 --> 00:38:16,920 It is entirely possible there are supermassive black holes 554 00:38:17,000 --> 00:38:20,000 wandering around out there, not in the centre. 555 00:38:22,680 --> 00:38:24,800 So how could it be possible that there's actually 556 00:38:24,880 --> 00:38:26,880 a supermassive black hole close to us, 557 00:38:26,960 --> 00:38:29,040 wandering around, but we never even see it? 558 00:38:29,120 --> 00:38:32,560 Well, remember, 'black hole' means it's really, really black. 559 00:38:32,640 --> 00:38:35,280 It actually absorbs radiation and any energy, 560 00:38:35,360 --> 00:38:37,960 so unless something is falling into a black hole 561 00:38:38,040 --> 00:38:40,280 or orbiting around it, you're not gonna see it. 562 00:38:41,600 --> 00:38:44,320 And so if this supermassive black hole were hypothetically 563 00:38:44,400 --> 00:38:46,400 wandering the outskirts of our galaxy, 564 00:38:46,480 --> 00:38:49,960 well, there's a lot less gas there for that black hole to run into, 565 00:38:50,040 --> 00:38:53,520 and if there's no gas around that black hole, we will not see it. 566 00:38:56,120 --> 00:38:58,520 The rogue supermassive black hole 567 00:38:58,600 --> 00:39:02,280 may not stay in the outskirts of the galaxy forever, 568 00:39:02,360 --> 00:39:06,720 gravitational interactions slowly pull it back into the Milky Way. 569 00:39:08,280 --> 00:39:10,520 Billions of years later, 570 00:39:10,600 --> 00:39:14,000 the supermassive black hole arrives in the centre. 571 00:39:18,440 --> 00:39:23,000 On the way in, it could cause havoc for solar systems like our own. 572 00:39:24,440 --> 00:39:27,400 If a solar system is disturbed 573 00:39:27,480 --> 00:39:30,200 by a wandering black hole... 574 00:39:31,840 --> 00:39:35,920 ..then the nice, ordered architecture, 575 00:39:36,000 --> 00:39:40,440 or the orbits of its planets, are completely disrupted. 576 00:39:42,280 --> 00:39:47,280 In our solar system, the orbits of the planets are perfectly ordered. 577 00:39:48,760 --> 00:39:51,920 If a black hole came even a little too close, 578 00:39:52,000 --> 00:39:54,920 its gravity could throw the planets into chaos. 579 00:39:57,800 --> 00:40:01,280 Some planets might be flung out of our solar system, 580 00:40:01,360 --> 00:40:05,000 others might be plunged down into the nuclear furnace of the sun. 581 00:40:12,600 --> 00:40:16,000 Some planets could face the ultimate destruction... 582 00:40:18,400 --> 00:40:21,200 ..being eaten by the rogue black hole. 583 00:40:26,280 --> 00:40:29,000 If we can't see them, 584 00:40:29,080 --> 00:40:32,000 could one be about to wander through our solar system? 585 00:40:34,640 --> 00:40:38,800 So, hypothetically, a wandering black hole could, 586 00:40:38,880 --> 00:40:41,040 yes, impact our solar system, 587 00:40:41,120 --> 00:40:44,600 but I can't overstate how profoundly unlikely that is to happen. 588 00:40:44,680 --> 00:40:46,680 Galaxies are mostly empty space. 589 00:40:46,760 --> 00:40:50,800 We are just this speck in this cosmic vortex that is our galaxy, 590 00:40:50,880 --> 00:40:55,240 and so our solar system is a really, really, really small target. 591 00:40:58,440 --> 00:41:01,320 When this rogue supermassive black hole meets up 592 00:41:01,400 --> 00:41:04,920 with Sagittarius A-star, the fuse is lit. 593 00:41:07,520 --> 00:41:10,200 The pair spiral towards each other... 594 00:41:13,160 --> 00:41:15,720 ..spinning faster and faster, 595 00:41:15,800 --> 00:41:18,040 reaching up to half the speed of light. 596 00:41:23,400 --> 00:41:26,840 Finally, the two black holes merge. 597 00:41:32,960 --> 00:41:36,360 You would have an enormously energetic event on your hands. 598 00:41:36,440 --> 00:41:40,080 Those supermassive black holes could, in principle, merge together, 599 00:41:40,160 --> 00:41:42,960 create a huge blast of gravitational waves, 600 00:41:43,040 --> 00:41:46,600 accompanied by a profoundly energetic flash of light, 601 00:41:46,680 --> 00:41:49,200 that could, in principle, endanger all life on Earth. 602 00:41:55,760 --> 00:42:00,360 It's literally a stretching and contracting of space itself. 603 00:42:00,440 --> 00:42:02,640 It's like grabbing the framework of space 604 00:42:02,720 --> 00:42:04,720 and just shaking it really hard. 605 00:42:04,800 --> 00:42:08,600 And if this happens in our galaxy, the amount of energy emitted, 606 00:42:08,680 --> 00:42:11,520 that would be... That would be bad. 607 00:42:14,240 --> 00:42:17,480 When the black holes collide, they release more energy 608 00:42:17,560 --> 00:42:20,240 than all of the stars in the universe combined. 609 00:42:28,320 --> 00:42:31,400 Should we be panicked about this? And the answer is, no. 610 00:42:31,480 --> 00:42:34,400 The Earth has been orbiting the sun for four and a half billion years 611 00:42:34,480 --> 00:42:36,480 without any incident, right? 612 00:42:36,560 --> 00:42:38,840 We're pretty safe from them. 613 00:42:38,920 --> 00:42:43,360 If we were around to see the two black holes collide, 614 00:42:43,440 --> 00:42:47,200 we'd witness the most destructive light show in history. 615 00:42:50,000 --> 00:42:53,160 But for now, the centre of our galaxy 616 00:42:53,240 --> 00:42:55,600 is relatively quiet, 617 00:42:55,680 --> 00:42:58,600 but it's still a terrible place to be. 618 00:43:02,520 --> 00:43:05,640 The centre of our Milky Way is not a friendly place. 619 00:43:05,720 --> 00:43:09,480 It's nowhere you wanna be. It's a bad neighbourhood. 620 00:43:09,560 --> 00:43:13,600 You've got tons of stars, tons of radiation, 621 00:43:13,680 --> 00:43:17,440 stars are being born and dying and exploding. 622 00:43:17,520 --> 00:43:19,680 You've got the central supermassive black hole, 623 00:43:19,760 --> 00:43:22,160 you've got a potential swarm of black holes, 624 00:43:22,240 --> 00:43:24,560 you've got accretion discs, you've got flares, 625 00:43:24,640 --> 00:43:27,280 you've got magnetic outbursts, you've got jets. 626 00:43:28,720 --> 00:43:31,120 Let's just stay out here in the suburbs, all right? 627 00:43:32,600 --> 00:43:34,600 The centre of our galaxy 628 00:43:34,680 --> 00:43:38,040 is one of the most nightmarish places in the cosmos. 629 00:43:40,840 --> 00:43:44,400 It's also home to some of the most incredible forces 630 00:43:44,480 --> 00:43:46,480 the universe has to offer. 631 00:43:49,280 --> 00:43:52,000 Whatever the future holds for our galaxy... 632 00:43:53,960 --> 00:43:57,920 ..the core of the Milky Way will be at the centre of it all. 633 00:43:59,440 --> 00:44:03,040 Our home galaxy, the Milky Way, is our safe harbour, 634 00:44:03,120 --> 00:44:05,720 our island in this vast cosmic ocean. 635 00:44:05,800 --> 00:44:08,480 And so to understand the heart of our galaxy 636 00:44:08,560 --> 00:44:12,840 is to understand our home in this cosmic void. 637 00:44:12,920 --> 00:44:15,840 Subtitles by Deluxe 55097

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