1 00:00:00,700 --> 00:00:02,835 [music playing] 2 00:00:02,936 --> 00:00:04,236 NARRATOR: There's a killer lurking 3 00:00:04,337 --> 00:00:11,777 in our galaxy, a star ready to explode into a supernova. 4 00:00:11,878 --> 00:00:13,545 These are the most visually stunning 5 00:00:13,646 --> 00:00:15,214 events in the universe. 6 00:00:15,315 --> 00:00:18,150 NARRATOR: Seen from Earth, it would have a terrible beauty. 7 00:00:18,251 --> 00:00:20,953 But for us, it could be fatal. 8 00:00:21,054 --> 00:00:24,390 In a few seconds, it can release as much energy 9 00:00:24,491 --> 00:00:27,993 as the sun will over its entire lifetime. 10 00:00:28,094 --> 00:00:31,663 NARRATOR: We're trying to hunt it down, but it's lying low. 11 00:00:31,765 --> 00:00:33,899 We haven't seen a supernova in the Milky Way 12 00:00:34,000 --> 00:00:36,335 in over 400 years. 13 00:00:36,436 --> 00:00:38,070 NARRATOR: It could be anywhere. 14 00:00:41,608 --> 00:00:45,244 It is nearly impossible to predict where and when 15 00:00:45,345 --> 00:00:46,879 the next supernova will happen. 16 00:00:48,748 --> 00:00:52,718 NARRATOR: The hunt is on to find the next supernova 17 00:00:52,819 --> 00:00:55,054 before it finds us. 18 00:00:59,659 --> 00:01:01,593 [booming] 19 00:01:12,672 --> 00:01:18,377 October 2019, one of the brightest stars in the sky 20 00:01:18,478 --> 00:01:19,778 looks dangerously unstable. 21 00:01:23,183 --> 00:01:25,484 If you look at the constellation of Orion, 22 00:01:25,585 --> 00:01:30,923 one of the shoulders of Orion is a star that is obviously red. 23 00:01:31,024 --> 00:01:32,257 This is Betelgeuse. 24 00:01:38,998 --> 00:01:40,599 I could go into my backyard and see it. 25 00:01:40,700 --> 00:01:42,534 You could clearly see that it was getting dimmer. 26 00:01:44,571 --> 00:01:45,404 NARRATOR: Is this a warning? 27 00:01:47,173 --> 00:01:52,544 Is Betelgeuse about to die in a massive cosmic explosion, 28 00:01:52,645 --> 00:01:53,479 a supernova? 29 00:01:55,315 --> 00:01:58,750 We've been studying this star for hundreds of years. 30 00:01:58,852 --> 00:02:03,155 And one thing we're sure about is that it's big, very big. 31 00:02:05,592 --> 00:02:09,061 Betelgeuse is a massive star, maybe 15 or 20 times 32 00:02:09,162 --> 00:02:10,629 the mass of our sun. 33 00:02:10,730 --> 00:02:12,397 And it's near the end of its life. 34 00:02:14,501 --> 00:02:19,037 It is a massive, enormous, luminous star. 35 00:02:19,139 --> 00:02:21,306 And one day, it's going to go boom. 36 00:02:24,444 --> 00:02:28,480 NARRATOR: Betelgeuse is on our list of supernova candidates 37 00:02:28,581 --> 00:02:29,815 because of this massive size. 38 00:02:32,252 --> 00:02:33,819 The bigger star they are, actually the 39 00:02:33,920 --> 00:02:34,720 shorter the lifespan. 40 00:02:38,758 --> 00:02:40,659 NARRATOR: The lifespan of a star depends 41 00:02:40,760 --> 00:02:43,929 on a delicate balance between two competing forces-- 42 00:02:45,365 --> 00:02:49,668 Gravity pulling in and heat and pressure pushing out. 43 00:02:51,137 --> 00:02:54,106 Stars exist because they're held up. 44 00:02:54,207 --> 00:02:56,241 They're not held up by pillars. 45 00:02:56,342 --> 00:02:59,111 They're held up by energy flowing out of the core 46 00:02:59,212 --> 00:03:00,846 toward the surface of the star. 47 00:03:00,947 --> 00:03:04,183 That stops the gravitational contraction. 48 00:03:04,317 --> 00:03:05,517 MICHELLE THALLER: Stars get their energy 49 00:03:05,618 --> 00:03:08,120 from nuclear fusion reactions right in the core. 50 00:03:08,221 --> 00:03:10,822 And the most basic one is taking two hydrogen atoms 51 00:03:10,924 --> 00:03:12,791 and slamming them together to form a helium atom. 52 00:03:14,227 --> 00:03:16,428 And you might think, OK, the more hydrogen you have, 53 00:03:16,529 --> 00:03:19,431 the more stuff you have, maybe the longer the start will live. 54 00:03:19,532 --> 00:03:22,034 Turns out it's exactly opposite. 55 00:03:22,135 --> 00:03:24,469 NARRATOR: The reason-- gravity. 56 00:03:24,571 --> 00:03:27,072 The more mass a star has, the stronger 57 00:03:27,173 --> 00:03:31,009 its gravity, gravity that crushes its hydrogen 58 00:03:31,110 --> 00:03:32,811 atoms closer together. 59 00:03:34,280 --> 00:03:35,714 As you crush things more and more, 60 00:03:35,815 --> 00:03:38,283 the temperature gets hotter and hotter and hotter. 61 00:03:38,384 --> 00:03:41,153 And the nuclear fusion reactions burn faster. 62 00:03:41,254 --> 00:03:44,523 So bigger stars burn their fuel very, very quickly 63 00:03:44,624 --> 00:03:45,958 and live short lives. 64 00:03:46,059 --> 00:03:48,694 Smaller stars burn their fuel much more slowly 65 00:03:48,795 --> 00:03:50,662 and live long, protracted lives. 66 00:03:50,763 --> 00:03:52,764 So when you are a big star, you live fast 67 00:03:52,865 --> 00:03:53,565 and you die young. 68 00:04:01,641 --> 00:04:06,612 NARRATOR: Betelgeuse burns brighter than 125,000 suns. 69 00:04:06,713 --> 00:04:10,048 But now it's running out of its hydrogen fuel. 70 00:04:10,149 --> 00:04:13,919 So it's burning whatever it has left just to stay alive. 71 00:04:17,457 --> 00:04:20,225 Stars are basically factories for burning 72 00:04:20,326 --> 00:04:21,727 hydrogen into helium. 73 00:04:21,828 --> 00:04:23,929 And then, once the helium is burned, 74 00:04:24,030 --> 00:04:27,566 they start burning heavier and heavier elements, like carbon 75 00:04:27,667 --> 00:04:32,004 and nitrogen and oxygen. 76 00:04:32,105 --> 00:04:34,873 It's a little like, you burn something, you get ash. 77 00:04:34,974 --> 00:04:36,508 But then if you crush the ash enough, 78 00:04:36,609 --> 00:04:38,377 you could burn it again. 79 00:04:38,511 --> 00:04:41,179 And then you crush it some more, and you can burn it yet again. 80 00:04:43,416 --> 00:04:45,250 NARRATOR: But this process can't go on forever. 81 00:04:46,986 --> 00:04:51,423 As the size of the atomic nuclei being fused together grows, 82 00:04:51,524 --> 00:04:53,425 the amount of energy released falls. 83 00:04:54,994 --> 00:04:58,263 The fuel the star needs to resist the pull of gravity 84 00:04:58,364 --> 00:04:59,064 is running out. 85 00:05:00,533 --> 00:05:02,601 Unfortunately, the amount of energy 86 00:05:02,702 --> 00:05:05,737 you can extract by putting two nuclei together 87 00:05:05,838 --> 00:05:10,742 gets smaller and smaller the bigger the nuclei are until you 88 00:05:10,843 --> 00:05:14,613 come to making iron, and iron, it turns out, 89 00:05:14,714 --> 00:05:16,648 is the last thing you can make that way. 90 00:05:16,749 --> 00:05:19,518 The problem with iron is, when you fuse it, 91 00:05:19,619 --> 00:05:21,386 it doesn't make energy. 92 00:05:21,487 --> 00:05:22,854 It takes it away. 93 00:05:22,955 --> 00:05:26,958 So when the star builds up that iron core, it's doomed. 94 00:05:27,060 --> 00:05:29,995 HAKEEM OLUSEYI: It can no longer create energy in its core 95 00:05:30,096 --> 00:05:33,198 to flow out toward the surface strong enough 96 00:05:33,299 --> 00:05:34,700 to keep it from collapsing. 97 00:05:34,801 --> 00:05:36,601 So collapse is what they do. 98 00:05:39,672 --> 00:05:41,406 NARRATOR: In a fraction of a second, 99 00:05:41,507 --> 00:05:45,477 the star's core collapses down from the size of a planet 100 00:05:45,578 --> 00:05:47,479 to about the size of a small city. 101 00:05:48,881 --> 00:05:51,583 And when that happens, all hell breaks loose. 102 00:05:56,723 --> 00:05:58,323 NARRATOR: A huge amount of energy 103 00:05:58,424 --> 00:06:00,859 is suddenly released, which forces 104 00:06:00,960 --> 00:06:03,662 the collapsing layers back out. 105 00:06:03,763 --> 00:06:08,266 The result-- an enormous explosion we call a supernova. 106 00:06:13,139 --> 00:06:14,573 JAMES BULLOCK: The shockwave from a supernova 107 00:06:14,674 --> 00:06:17,542 rips out at thousands of miles per second. 108 00:06:17,643 --> 00:06:19,044 And for a brief period of time, they're 109 00:06:19,145 --> 00:06:20,479 brighter than an entire galaxy. 110 00:06:23,549 --> 00:06:26,118 NARRATOR: A supernova could devastate life on Earth. 111 00:06:28,721 --> 00:06:32,924 And the evidence can be found at the bottom of our oceans. 112 00:06:34,160 --> 00:06:35,427 MICHELLE THALLER: There are layers and layers 113 00:06:35,528 --> 00:06:36,828 of silt that have built up. 114 00:06:36,929 --> 00:06:40,465 And there seem to be a layer, about 2.6 million years ago, 115 00:06:40,566 --> 00:06:43,201 that was enriched in a very strange chemical element, 116 00:06:43,302 --> 00:06:45,370 something called iron-60. 117 00:06:45,471 --> 00:06:48,373 PHIL PLAIT: Iron-60 is a radioactive isotope of iron, 118 00:06:48,474 --> 00:06:51,510 and it doesn't last very long, just a few million years. 119 00:06:51,611 --> 00:06:54,846 And the only place that we know of that can make iron-60 120 00:06:54,947 --> 00:06:57,983 is a supernova in an exploding star. 121 00:07:05,525 --> 00:07:09,161 That means there must have been a supernova close enough 122 00:07:09,262 --> 00:07:12,664 to the Earth within the past couple of million years 123 00:07:12,765 --> 00:07:16,001 to have physically deposited material on our planet. 124 00:07:17,603 --> 00:07:19,371 That freaks me out. 125 00:07:21,240 --> 00:07:24,242 NARRATOR: The sign of this shocking assault on our planet 126 00:07:24,343 --> 00:07:28,380 is a thin layer of this very rare type of iron. 127 00:07:28,481 --> 00:07:31,817 We find it in the mud of every ocean floor 128 00:07:31,918 --> 00:07:34,753 and always at the same depth. 129 00:07:34,854 --> 00:07:37,556 This interstellar dust must have drenched 130 00:07:37,657 --> 00:07:43,295 our world in one enormous burst 2.6 million years ago. 131 00:07:43,396 --> 00:07:44,896 It was a terrible time. 132 00:07:44,997 --> 00:07:50,068 A third of large animal species in the sea suddenly died out. 133 00:07:50,169 --> 00:07:51,470 There were some pretty amazing fish. 134 00:07:51,571 --> 00:07:54,139 Probably the most amazing is the megalodon, 135 00:07:54,240 --> 00:07:57,843 a giant shark-- teeth the size of dinner plates and so on. 136 00:07:57,944 --> 00:08:00,712 But they went extinct 2.6 million years ago 137 00:08:00,813 --> 00:08:02,380 at the end of the Pliocene. 138 00:08:02,482 --> 00:08:03,181 What happened? 139 00:08:05,985 --> 00:08:08,353 PHIL PLAIT: A lot of sea creatures died. 140 00:08:08,454 --> 00:08:10,689 And a lot of them were in shallow waters, 141 00:08:10,790 --> 00:08:14,392 whereas deep-water animals tended to survive. 142 00:08:14,494 --> 00:08:16,428 That sounds kind of like a supernova. 143 00:08:16,529 --> 00:08:19,297 That can do things that would affect our atmosphere, 144 00:08:19,398 --> 00:08:21,800 would affect shallow water, but not deeper water. 145 00:08:23,970 --> 00:08:26,805 NARRATOR: Supernovas create huge amounts of cosmic rays. 146 00:08:29,242 --> 00:08:30,976 When they crash into other atoms, 147 00:08:31,077 --> 00:08:35,146 they break up and produce showers of dangerous shrapnel 148 00:08:35,248 --> 00:08:35,947 called muons. 149 00:08:38,217 --> 00:08:41,520 These charged particles are similar to electrons, 150 00:08:41,621 --> 00:08:43,121 only 200 times heavier. 151 00:08:45,057 --> 00:08:48,193 So they penetrate more deeply and cause more damage. 152 00:08:50,229 --> 00:08:52,130 They can pierce through our atmosphere, 153 00:08:52,231 --> 00:08:54,933 pierce through our skin, get into a cell, 154 00:08:55,034 --> 00:08:56,801 and disrupt the DNA. 155 00:08:56,903 --> 00:08:59,471 They'll go right through a mouse but deposit 156 00:08:59,572 --> 00:09:01,540 in the body of a larger animal. 157 00:09:01,641 --> 00:09:03,808 So the impact on an animal the size of a megalodon, 158 00:09:03,910 --> 00:09:05,410 say, could be pretty extreme. 159 00:09:08,014 --> 00:09:12,717 NARRATOR: Muons can shatter DNA, causing mutations and cancer. 160 00:09:12,818 --> 00:09:14,819 But their power weakens as they travel 161 00:09:14,921 --> 00:09:17,856 through water, which may be why only 162 00:09:17,957 --> 00:09:19,257 deep sea creatures survived. 163 00:09:20,993 --> 00:09:24,062 The extinction really tells us that we're not 164 00:09:24,163 --> 00:09:26,598 separate and apart from the universe and the goings 165 00:09:26,699 --> 00:09:27,732 on up there, right? 166 00:09:27,833 --> 00:09:29,501 Supernova going off and things like that-- 167 00:09:29,602 --> 00:09:30,702 OK, it's a pretty light show. 168 00:09:30,803 --> 00:09:31,836 No. 169 00:09:31,938 --> 00:09:34,406 It is a direct impact to life on Earth and us. 170 00:09:36,242 --> 00:09:38,910 NARRATOR: So are we in danger of extinction? 171 00:09:40,346 --> 00:09:42,581 Is Betelgeuse about to explode? 172 00:09:54,794 --> 00:09:57,429 When stars explode as supernovas, 173 00:09:57,530 --> 00:10:00,465 they can devastate planets hundreds of light years away. 174 00:10:02,268 --> 00:10:06,237 Betelgeuse is about 550 light years from Earth. 175 00:10:06,339 --> 00:10:10,141 So, when it dramatically dimmed in 2019, 176 00:10:10,242 --> 00:10:11,743 scientists were concerned. 177 00:10:14,680 --> 00:10:16,181 But Betelgeuse has dimmed before. 178 00:10:18,451 --> 00:10:20,085 ALEX FILIPPENKO: Betelgeuse varies 179 00:10:20,186 --> 00:10:21,920 quite a lot over the years. 180 00:10:22,021 --> 00:10:24,255 There are some cycles, and sometimes 181 00:10:24,357 --> 00:10:26,958 these cycles come together, and you get a deep minimum. 182 00:10:28,728 --> 00:10:31,863 NARRATOR: So dimming is part of the star's natural cycle 183 00:10:31,998 --> 00:10:33,465 as it nears the end of its life. 184 00:10:36,002 --> 00:10:39,704 But to get a full picture, we took Betelgeuse's temperature. 185 00:10:41,474 --> 00:10:43,642 If the star was dimming, that would mean that the surface 186 00:10:43,743 --> 00:10:45,210 was cooling over time. 187 00:10:45,311 --> 00:10:47,512 We actually made measurements of the temperature of Betelgeuse 188 00:10:47,613 --> 00:10:49,014 and found out that wasn't happening. 189 00:10:49,115 --> 00:10:50,315 It hardly cooled at all. 190 00:10:50,416 --> 00:10:53,118 It cooled, like, 50 or 100 degrees. 191 00:10:53,219 --> 00:10:55,286 You might expect a much, much more 192 00:10:55,388 --> 00:10:57,489 dramatic change in the surface temperature 193 00:10:57,590 --> 00:10:58,423 if it were about to explode. 194 00:11:01,827 --> 00:11:04,295 NARRATOR: So, if Betelgeuse wasn't cooling 195 00:11:04,397 --> 00:11:06,464 much, what was making it dim? 196 00:11:08,668 --> 00:11:12,904 To take a closer look, we used a very large telescope 197 00:11:12,972 --> 00:11:16,141 and an exoplanet hunting instrument 198 00:11:16,242 --> 00:11:20,011 called SPHERE and came up with an extraordinary image. 199 00:11:23,249 --> 00:11:27,218 When I first saw this image of Betelgeuse, it blew me away. 200 00:11:27,319 --> 00:11:28,653 I almost gasped. 201 00:11:28,754 --> 00:11:30,955 I may have said a word I can't say on TV. 202 00:11:32,725 --> 00:11:34,025 That was very exciting. 203 00:11:35,761 --> 00:11:38,029 NARRATOR: The image reveals that, while the upper part 204 00:11:38,130 --> 00:11:41,166 of Betelgeuse was still bright, the lower 205 00:11:41,267 --> 00:11:44,169 part was noticeably dimmer. 206 00:11:44,270 --> 00:11:47,072 We had images of Betelgeuse from before, 207 00:11:47,173 --> 00:11:49,474 and we were able to compare the new ones with it. 208 00:11:49,575 --> 00:11:52,343 And so you could see that half of Betelgeuse 209 00:11:52,445 --> 00:11:53,878 looked pretty much the same. 210 00:11:53,979 --> 00:11:56,748 But the other half was significantly dimmer. 211 00:11:56,849 --> 00:12:00,018 And what could make a star dim that quickly? 212 00:12:00,119 --> 00:12:02,721 And remember how big this star is. 213 00:12:02,822 --> 00:12:04,956 Nothing happens on Betelgeuse quickly. 214 00:12:05,057 --> 00:12:07,659 So this must be something happening right on the surface. 215 00:12:10,863 --> 00:12:12,797 NARRATOR: As heavier material like silicone 216 00:12:12,898 --> 00:12:16,067 emerges from the surface of Betelgeuse, 217 00:12:16,168 --> 00:12:19,304 it cools and condenses. 218 00:12:19,405 --> 00:12:21,139 It's kind of like sticking the hose in the wrong end 219 00:12:21,240 --> 00:12:22,307 of your vacuum cleaner. 220 00:12:22,408 --> 00:12:24,142 Instead of pulling stuff in, [imitates explosion] 221 00:12:24,243 --> 00:12:26,044 it blows all this dust out into space. 222 00:12:28,481 --> 00:12:31,783 NARRATOR: Betelgeuse has cosmic indigestion 223 00:12:31,884 --> 00:12:36,621 and is belching dust, which makes the star seem dim. 224 00:12:36,722 --> 00:12:37,522 But it's not over. 225 00:12:39,191 --> 00:12:42,393 All through 2020, Betelgeuse first brightened 226 00:12:42,495 --> 00:12:44,195 and then dimmed again. 227 00:12:45,998 --> 00:12:50,001 So astronomers are watching this massive star with bated breath. 228 00:12:52,171 --> 00:12:53,471 It's going to explode. 229 00:12:53,572 --> 00:12:55,640 The question is, when? 230 00:12:55,741 --> 00:12:58,376 It's probably sometime in the next 100,000 years. 231 00:12:58,477 --> 00:12:59,544 But it could be tomorrow. 232 00:12:59,645 --> 00:13:01,112 It could have already exploded and we're 233 00:13:01,213 --> 00:13:03,515 just waiting to see the light. 234 00:13:03,616 --> 00:13:06,751 NARRATOR: With luck, if Betelgeuse blows, all we'll see 235 00:13:06,852 --> 00:13:08,086 is a beautiful light show. 236 00:13:10,656 --> 00:13:13,691 At a distance of 550 light years, 237 00:13:13,793 --> 00:13:16,327 it's probably too far to do serious damage. 238 00:13:19,031 --> 00:13:22,634 But is there another star we should worry about? 239 00:13:26,472 --> 00:13:30,408 A closer star, just 150 light years from Earth, 240 00:13:30,509 --> 00:13:34,946 could do us some major damage, a star like IK Pegasi. 241 00:13:37,416 --> 00:13:40,451 But it isn't this star which we can see in our night 242 00:13:40,553 --> 00:13:41,986 sky that's the threat. 243 00:13:44,056 --> 00:13:47,659 The main star is only about 1.6 times the mass of the sun. 244 00:13:47,760 --> 00:13:50,161 That's nowhere near enough mass to go supernova. 245 00:13:50,262 --> 00:13:54,432 And yet, we think it is the progenitor for a supernova. 246 00:13:54,500 --> 00:13:55,200 How can that be? 247 00:13:56,802 --> 00:13:58,403 NARRATOR: The main star isn't alone. 248 00:13:59,805 --> 00:14:02,540 It has a more dangerous accomplice. 249 00:14:05,411 --> 00:14:08,746 There's another star there orbiting the larger star. 250 00:14:08,848 --> 00:14:10,748 PHIL PLAIT: And this is what we call a binary system-- 251 00:14:10,850 --> 00:14:12,250 Two stars orbiting each other. 252 00:14:13,652 --> 00:14:15,253 Right now, the system is stable. 253 00:14:15,354 --> 00:14:17,422 But things aren't always going to be the way they are now, 254 00:14:17,523 --> 00:14:20,358 and sometime in the future, things 255 00:14:20,459 --> 00:14:21,993 are going to change a lot. 256 00:14:24,396 --> 00:14:26,264 NARRATOR: IK Pegasi is really made up 257 00:14:26,365 --> 00:14:34,339 of IK Pegasi A, a large white star, and its accomplice, 258 00:14:34,440 --> 00:14:41,812 a white dwarf called IK Pegasi B. This tiny star 259 00:14:41,881 --> 00:14:43,248 is the real threat to Earth. 260 00:14:45,217 --> 00:14:49,721 You can think of a white dwarf as a zombie. 261 00:14:49,822 --> 00:14:53,658 You know, it's a dead star, and they can eat living stars. 262 00:14:53,759 --> 00:14:57,362 If there's a normal star like the sun near a white dwarf, 263 00:14:57,463 --> 00:15:00,298 the white dwarf has very, very intense gravity. 264 00:15:00,399 --> 00:15:03,401 It can literally pull material off that normal star, 265 00:15:03,502 --> 00:15:06,104 and that material will then pile up on the surface 266 00:15:06,205 --> 00:15:07,071 of the white dwarf. 267 00:15:07,172 --> 00:15:09,707 So it really is eating a living star. 268 00:15:12,611 --> 00:15:14,746 NARRATOR: These stars orbit each other just 269 00:15:14,847 --> 00:15:18,049 18.5 million miles apart. 270 00:15:18,150 --> 00:15:21,552 That's closer than Mercury is to our sun. 271 00:15:21,654 --> 00:15:26,457 But they're not interacting with each other, yet. 272 00:15:26,558 --> 00:15:28,960 The problem is, sometime in the future, 273 00:15:29,061 --> 00:15:31,329 that normal star is going to run out of fuel. 274 00:15:31,430 --> 00:15:34,165 And when it does, it's going to expand into a red giant. 275 00:15:36,268 --> 00:15:38,169 NARRATOR: When it gets to the end of its life, 276 00:15:38,270 --> 00:15:43,241 IK Pegasi A will cool and swell up to become a red giant. 277 00:15:45,377 --> 00:15:48,613 And that's it, no big explosion. 278 00:15:48,714 --> 00:15:51,749 It won't become a supernova. 279 00:15:51,850 --> 00:15:57,055 But that's just when it's accomplice, IK Pegasi B, 280 00:15:57,156 --> 00:15:57,855 will start to feed. 281 00:15:59,992 --> 00:16:02,460 A lot of that material will gravitationally 282 00:16:02,561 --> 00:16:04,896 be attracted to the white dwarf and fall under the surface. 283 00:16:06,632 --> 00:16:08,399 NARRATOR: As the white dwarf pulls material 284 00:16:08,500 --> 00:16:10,802 from its bloated red giant neighbor, 285 00:16:10,903 --> 00:16:12,003 it gets more and more massive. 286 00:16:13,706 --> 00:16:16,441 It's gravitational pull increases, 287 00:16:16,542 --> 00:16:17,976 so it feeds even faster. 288 00:16:21,880 --> 00:16:24,782 Eventually, it can no longer support its own weight. 289 00:16:26,919 --> 00:16:29,087 The core of the star is actually very dense. 290 00:16:29,188 --> 00:16:31,289 In fact, if you had, like, a teaspoon of material, 291 00:16:31,390 --> 00:16:34,058 it would weigh about as much as an 18-wheel truck. 292 00:16:34,159 --> 00:16:36,661 And it's basically right at the limit of normal matter 293 00:16:36,762 --> 00:16:38,863 being able to hold up at that density. 294 00:16:38,964 --> 00:16:41,132 You dump more and more stuff onto it, 295 00:16:41,233 --> 00:16:43,568 and eventually there's a limit that's reached. 296 00:16:43,669 --> 00:16:47,538 And it either collapses or, more generally, blows up. 297 00:16:53,612 --> 00:16:56,948 NARRATOR: When, this happens IK Pegasi will 298 00:16:57,016 --> 00:17:00,752 be brighter than the full moon in our sky 299 00:17:00,853 --> 00:17:03,554 because it's only 150 light years away. 300 00:17:07,292 --> 00:17:10,595 Having a supernova 150 light years sounds like a bad idea, 301 00:17:10,696 --> 00:17:11,629 and it is. 302 00:17:11,730 --> 00:17:13,264 That's close enough that it might 303 00:17:13,399 --> 00:17:15,199 have some physical effects on the Earth. 304 00:17:19,038 --> 00:17:21,873 NARRATOR: Right now, IK Pegasi is about as far 305 00:17:21,974 --> 00:17:25,043 from Earth as the supernova suspected 306 00:17:25,144 --> 00:17:26,511 of killing off the megalodon. 307 00:17:29,281 --> 00:17:31,916 So how worried should we be? 308 00:17:33,919 --> 00:17:36,220 The good news is the IK Peg system 309 00:17:36,321 --> 00:17:38,489 is moving away from the sun and the Earth 310 00:17:38,590 --> 00:17:40,892 right now at a decent clip. 311 00:17:40,993 --> 00:17:43,494 So if it's not going to blow up for a while, that means 312 00:17:43,595 --> 00:17:47,398 it could be on the other side of the galaxy by the time it does. 313 00:17:47,466 --> 00:17:49,767 By that time, we'll be completely safe. 314 00:17:49,868 --> 00:17:52,437 As an astronomer and an astronomer who has studied 315 00:17:52,538 --> 00:17:55,106 supernovas professionally, having them 316 00:17:55,207 --> 00:17:58,042 far away is fine with me, close enough that we can study them 317 00:17:58,143 --> 00:18:00,511 well but not so close that I can study 318 00:18:00,612 --> 00:18:03,314 them personally on a physical level on my own body. 319 00:18:03,415 --> 00:18:04,115 Yeah, no. 320 00:18:09,688 --> 00:18:12,757 NARRATOR: A close supernova would be devastating for life 321 00:18:12,858 --> 00:18:13,558 on Earth. 322 00:18:14,927 --> 00:18:19,597 Will there be any warning signs before one 323 00:18:19,698 --> 00:18:22,667 of our prime suspects is about to blow? 324 00:18:32,244 --> 00:18:34,078 [booming] 325 00:18:40,119 --> 00:18:42,720 To find a supernova warning signal, 326 00:18:42,821 --> 00:18:46,424 we need to know what's happening deep inside the core 327 00:18:46,525 --> 00:18:47,625 of an exploding star. 328 00:18:49,361 --> 00:18:51,796 At the very beginning of a supernova explosion, 329 00:18:51,897 --> 00:18:54,465 the core of a massive star is collapsing. 330 00:18:54,533 --> 00:18:56,434 There's no more nuclear fusion going on, 331 00:18:56,535 --> 00:18:59,170 and it is compressing to higher and higher densities. 332 00:19:00,706 --> 00:19:02,373 NARRATOR: The star's gravity crushes 333 00:19:02,474 --> 00:19:05,510 protons and electrons so close together 334 00:19:05,611 --> 00:19:07,311 they merge to form neutrons. 335 00:19:09,181 --> 00:19:12,850 The star's core becomes one of the densest materials 336 00:19:12,951 --> 00:19:14,585 in the universe. 337 00:19:14,686 --> 00:19:17,688 It's like a gigantic atomic nucleus-- 338 00:19:17,789 --> 00:19:22,460 Roughly half a million Earths compressed into the volume, 339 00:19:22,561 --> 00:19:24,262 the size of a city. 340 00:19:24,363 --> 00:19:27,198 That's really, really dense stuff. 341 00:19:27,299 --> 00:19:29,367 If you had about a teaspoon full of material, 342 00:19:29,468 --> 00:19:31,669 that would be about as much mass as Mount Everest. 343 00:19:36,909 --> 00:19:39,644 NARRATOR: Forcing protons and electrons together 344 00:19:39,745 --> 00:19:42,914 releases a huge amount of energy in the form 345 00:19:43,015 --> 00:19:47,985 of tiny, elusive, subatomic particles called neutrinos. 346 00:19:49,755 --> 00:19:51,189 Neutrinos are one of the most abundant 347 00:19:51,290 --> 00:19:52,423 particles in the universe. 348 00:19:52,524 --> 00:19:54,292 But they don't interact with things very much at all. 349 00:19:56,795 --> 00:19:59,030 HAKEEM OLUSEYI: Neutrinos are often called ghost particles 350 00:19:59,131 --> 00:20:00,665 because they do what ghosts do. 351 00:20:00,766 --> 00:20:02,466 They walk through walls. 352 00:20:02,568 --> 00:20:04,468 But neutrinos walk through us. 353 00:20:04,570 --> 00:20:05,570 They walk through the planet. 354 00:20:05,671 --> 00:20:07,471 They walk through stars. 355 00:20:07,573 --> 00:20:09,140 They're super ghosts. 356 00:20:09,241 --> 00:20:11,342 [whooshing] 357 00:20:11,443 --> 00:20:14,011 NARRATOR: At first, these neutrinos can fly straight 358 00:20:14,112 --> 00:20:15,846 out of the core of the star. 359 00:20:15,948 --> 00:20:19,450 But, as the star collapses, it gets so dense 360 00:20:19,551 --> 00:20:21,752 that some neutrinos get trapped and 361 00:20:21,853 --> 00:20:25,089 their energy turned into heat. 362 00:20:25,190 --> 00:20:28,392 And that creates a shockwave that rips the star apart. 363 00:20:28,493 --> 00:20:31,796 And the ensuing explosion is brighter than billions of stars 364 00:20:31,897 --> 00:20:32,597 all put together. 365 00:20:36,301 --> 00:20:40,471 NARRATOR: This light show may be spectacular, but it's only 1% 366 00:20:40,572 --> 00:20:43,040 of the energy released in a supernova. 367 00:20:43,141 --> 00:20:47,378 The rest is in the form of a massive burst of neutrinos. 368 00:20:47,479 --> 00:20:52,550 So neutrinos could act as a supernova early warning system. 369 00:20:52,651 --> 00:20:54,218 At least that's the idea. 370 00:20:57,422 --> 00:21:02,293 On February 24th, 1987, that idea was tested. 371 00:21:05,063 --> 00:21:08,032 An astronomer was doing a routine survey of a dwarf 372 00:21:08,133 --> 00:21:09,734 galaxy close to ours. 373 00:21:12,004 --> 00:21:15,206 He was taking pictures of it, develops the pictures, 374 00:21:15,307 --> 00:21:17,608 and says, hey, there's a star here 375 00:21:17,709 --> 00:21:19,710 that wasn't there yesterday. 376 00:21:22,881 --> 00:21:25,516 He basically got up, walked outside, and looked and went, 377 00:21:25,617 --> 00:21:27,485 oh, there's that star. 378 00:21:27,586 --> 00:21:30,655 And it turns out he had discovered a supernova. 379 00:21:32,924 --> 00:21:34,425 NARRATOR: Because it was the first supernova 380 00:21:34,526 --> 00:21:41,065 spotted that year, it was called Supernova 1987A. 381 00:21:41,166 --> 00:21:45,836 1987A a was an amazing event in the world of astronomy. 382 00:21:45,937 --> 00:21:50,041 Essentially, a supernova went off in our own backyard. 383 00:21:50,142 --> 00:21:52,209 PAUL SUTTER: It was very close to us, 384 00:21:52,311 --> 00:21:55,746 occurring in a neighbor galaxy of the Milky Way. 385 00:21:55,847 --> 00:21:59,750 And so it was the brightest thing seen in our skies 386 00:21:59,851 --> 00:22:01,852 since the invention of the telescope. 387 00:22:04,923 --> 00:22:08,659 NARRATOR: Supernova 1987A blazed with the power 388 00:22:08,760 --> 00:22:10,461 of 100 million suns. 389 00:22:11,797 --> 00:22:14,365 But that wasn't the most exciting part. 390 00:22:14,466 --> 00:22:17,668 For the first time, we received an early warning 391 00:22:17,769 --> 00:22:20,938 that a supernova was about to appear three hours 392 00:22:21,039 --> 00:22:22,940 before it lit up our night sky. 393 00:22:24,409 --> 00:22:27,578 Neutrino observatories around the world 394 00:22:27,679 --> 00:22:31,515 saw a sudden surge in neutrinos from the same direction 395 00:22:31,616 --> 00:22:32,316 on the sky. 396 00:22:38,223 --> 00:22:41,759 NARRATOR: Neutrinos' ability to zip across the galaxy, 397 00:22:41,860 --> 00:22:45,062 slipping through stars and planets like ghosts, 398 00:22:45,163 --> 00:22:48,032 gives them an unbeatable head start during a supernova. 399 00:22:50,268 --> 00:22:53,938 The neutrinos are released in the very earliest moments 400 00:22:54,039 --> 00:22:55,473 of this supernova blast. 401 00:22:55,574 --> 00:22:58,709 And they slip through the atmosphere of the star 402 00:22:58,810 --> 00:23:01,078 before it goes boom. 403 00:23:03,582 --> 00:23:06,717 NARRATOR: Neutrinos can escape in as little as 10 seconds. 404 00:23:08,787 --> 00:23:10,888 But it can take hours for the shockwave 405 00:23:10,989 --> 00:23:15,192 to travel right through the star and blast off the outer layers, 406 00:23:15,293 --> 00:23:17,261 revealing the light. 407 00:23:17,362 --> 00:23:19,530 The result is that we see neutrinos 408 00:23:19,631 --> 00:23:22,633 from a supernova explosion before we see the actual light. 409 00:23:28,540 --> 00:23:31,642 So if we want to spot the next supernova explosion, 410 00:23:31,743 --> 00:23:34,645 we've got to be paying attention to the neutrinos. 411 00:23:38,016 --> 00:23:40,684 NARRATOR: Astronomers set up the SuperNova Early 412 00:23:40,786 --> 00:23:46,190 Warning System, a network of neutrino detectors 413 00:23:46,291 --> 00:23:47,158 all around the world. 414 00:23:49,361 --> 00:23:51,595 It should give astronomers several hours notice 415 00:23:51,730 --> 00:23:53,164 of an impending supernova. 416 00:23:56,401 --> 00:23:58,869 But, so far, nothing. 417 00:23:58,970 --> 00:24:01,172 No supernovas have occurred near enough 418 00:24:01,273 --> 00:24:02,306 for the system to detect. 419 00:24:04,576 --> 00:24:07,411 Neutrinos are like the friend that never comes. 420 00:24:07,512 --> 00:24:09,079 We're sitting here waiting for him. 421 00:24:09,181 --> 00:24:11,215 But we don't know when it's going to actually happen. 422 00:24:14,319 --> 00:24:18,389 NARRATOR: But when they do come, we might be in trouble 423 00:24:18,490 --> 00:24:21,826 because some supernovas are armed with the most powerful 424 00:24:21,927 --> 00:24:23,227 weapon in the universe-- 425 00:24:25,130 --> 00:24:25,830 Gamma rays. 426 00:24:30,302 --> 00:24:32,102 [booming] 427 00:24:38,076 --> 00:24:41,178 Our hunt for the Milky Way's next supernova 428 00:24:41,279 --> 00:24:43,514 has identified some potential suspects-- 429 00:24:45,217 --> 00:24:50,888 Very massive, lonely stars and stars with smaller sidekicks. 430 00:24:53,391 --> 00:24:58,162 In 2018, astronomers found a system called Apep 431 00:24:58,263 --> 00:25:04,635 8,000 light years away with two very massive stars, each one 432 00:25:04,736 --> 00:25:06,237 about as massive as Betelgeuse. 433 00:25:08,573 --> 00:25:13,677 These are giant stars nearing the end of their lives 434 00:25:13,778 --> 00:25:19,083 with massive outer layers of gas that continually contract 435 00:25:19,184 --> 00:25:21,318 and heat up again and again. 436 00:25:23,054 --> 00:25:24,255 HAKEEM OLUSEYI: They become really 437 00:25:24,356 --> 00:25:26,957 huge and bloated and swollen, and they're 438 00:25:27,058 --> 00:25:28,993 prone to huge outbursts. 439 00:25:31,863 --> 00:25:35,900 NARRATOR: These unstable stars are called Wolf-Rayet stars. 440 00:25:39,838 --> 00:25:44,608 They're very rare and so hot and bright they emit more radiation 441 00:25:44,709 --> 00:25:47,611 than a million sunlike stars. 442 00:25:47,712 --> 00:25:51,348 This intense energy is blasting their outer layers off 443 00:25:51,449 --> 00:25:52,149 into space. 444 00:25:53,685 --> 00:25:56,587 Mass loss has been occurring from the star, 445 00:25:56,688 --> 00:26:00,658 so much so that you've actually lost all the hydrogen 446 00:26:00,759 --> 00:26:03,127 that wasn't burned into helium. 447 00:26:03,228 --> 00:26:06,096 So now you have a star that's made entirely 448 00:26:06,197 --> 00:26:08,432 of helium and heavier elements. 449 00:26:08,533 --> 00:26:11,168 NARRATOR: With no hydrogen left, these massive stars 450 00:26:11,269 --> 00:26:13,070 are running low on usable fuel. 451 00:26:15,640 --> 00:26:18,208 They're like ticking time bombs, made 452 00:26:18,310 --> 00:26:22,012 even more dangerous because they're spinning so fast. 453 00:26:24,049 --> 00:26:26,417 It's spinning so quickly, it's on the verge 454 00:26:26,518 --> 00:26:28,419 of ripping itself apart. 455 00:26:28,520 --> 00:26:30,955 And this means that, when this thing blows, 456 00:26:31,056 --> 00:26:32,323 it's going to blow hard. 457 00:26:34,125 --> 00:26:38,395 NARRATOR: When a star goes supernova, its core collapses. 458 00:26:38,496 --> 00:26:40,698 The smaller it gets, the faster it spins. 459 00:26:42,400 --> 00:26:47,037 Some cores collapse into fast, spinning neutron stars. 460 00:26:47,138 --> 00:26:51,208 Heavier ones, like Apep, collapse into even denser 461 00:26:51,309 --> 00:26:52,910 and more mysterious objects-- 462 00:26:54,245 --> 00:26:55,012 Black holes. 463 00:26:58,049 --> 00:27:01,385 The immense gravity within Apep's collapsing core 464 00:27:01,486 --> 00:27:05,689 will drag back some of the gas and dust into a spinning disk. 465 00:27:08,660 --> 00:27:11,495 As the material falls on to the core, 466 00:27:11,596 --> 00:27:14,198 it compresses and it speeds up. 467 00:27:15,700 --> 00:27:17,534 NARRATOR: The dying star spins faster 468 00:27:17,636 --> 00:27:19,603 and faster as it collapses. 469 00:27:21,473 --> 00:27:25,309 And this incredible rotation drives the creation 470 00:27:25,410 --> 00:27:29,413 of massive magnetic fields that are capable of funneling 471 00:27:29,514 --> 00:27:32,983 material around and up and out in the form 472 00:27:33,084 --> 00:27:35,419 of huge beams of radiation. 473 00:27:38,423 --> 00:27:41,158 So the energy from the supernova collapse, 474 00:27:41,259 --> 00:27:44,361 instead of being admitted spherically in every direction, 475 00:27:44,462 --> 00:27:46,930 comes at us in a tightly focused beam. 476 00:27:48,266 --> 00:27:50,734 Like a laser from the Death Star, 477 00:27:50,835 --> 00:27:52,803 it is pointed in one direction. 478 00:27:56,174 --> 00:27:58,075 NARRATOR: This is a gamma ray burst. 479 00:28:00,178 --> 00:28:02,546 PHIL PLAIT: It is the single scariest thing 480 00:28:02,647 --> 00:28:04,515 the universe has to offer. 481 00:28:04,616 --> 00:28:08,719 This is an explosion so powerful that, in a few seconds 482 00:28:08,820 --> 00:28:11,789 or minutes, it can release as much energy 483 00:28:11,890 --> 00:28:14,792 as the sun will over its entire lifetime. 484 00:28:16,528 --> 00:28:20,364 You do not want to get caught in a gamma ray burst. 485 00:28:20,465 --> 00:28:21,465 Let's just put it that way. 486 00:28:27,372 --> 00:28:29,039 NARRATOR: The impact of a nearby gamma ray 487 00:28:29,140 --> 00:28:31,842 burst on our home planet is almost 488 00:28:31,943 --> 00:28:33,210 too terrible to think about. 489 00:28:34,546 --> 00:28:37,181 It would be a very bad day for Earth. 490 00:28:40,585 --> 00:28:43,287 Earth's atmosphere could be partly blown away, 491 00:28:43,388 --> 00:28:44,888 and there could be chemical reactions 492 00:28:44,989 --> 00:28:46,356 in the atmosphere that would form 493 00:28:46,458 --> 00:28:48,325 all kinds of noxious products. 494 00:28:51,930 --> 00:28:53,330 NARRATOR: A gamma ray burst from Apep 495 00:28:53,431 --> 00:28:56,300 might last only 10 seconds, but its impact 496 00:28:56,434 --> 00:28:57,167 would last for decades. 497 00:28:58,570 --> 00:29:03,040 The generation of nitrogen oxide from a gamma ray burst 498 00:29:03,141 --> 00:29:04,541 would be disastrous. 499 00:29:04,642 --> 00:29:06,143 In the upper atmosphere, it would 500 00:29:06,244 --> 00:29:07,878 eat away at our ozone layer. 501 00:29:07,979 --> 00:29:12,683 In the lower atmosphere, it would come out as acid rain. 502 00:29:12,817 --> 00:29:15,953 And the acid rain would destroy our crops. 503 00:29:18,323 --> 00:29:22,025 NARRATOR: Nitrogen dioxide also filters out sunlight, 504 00:29:22,127 --> 00:29:24,895 turning the skies dark and cooling the Earth enough 505 00:29:24,996 --> 00:29:26,930 to trigger a new Ice Age. 506 00:29:30,502 --> 00:29:34,104 Any life on the land, in the shallow parts of the sea, 507 00:29:34,205 --> 00:29:37,441 or that live near the sea surface would be done. 508 00:29:37,542 --> 00:29:40,911 In fact, it would ultimately result in extinction. 509 00:29:45,183 --> 00:29:49,219 NARRATOR: Blasted by ultraviolet radiation from our sun, 510 00:29:49,320 --> 00:29:53,690 freezing cold and hungry, humanity's future 511 00:29:53,792 --> 00:29:54,525 would be bleak. 512 00:29:58,029 --> 00:30:02,132 So we really need to know, when Apep goes supernova 513 00:30:02,233 --> 00:30:06,937 and produces its deadly beam of gamma rays, 514 00:30:07,038 --> 00:30:08,238 are we in its line of fire? 515 00:30:10,542 --> 00:30:12,910 The good news is that we are probably 516 00:30:13,011 --> 00:30:16,446 not right in the direct firing line of Apep. 517 00:30:16,548 --> 00:30:19,817 PAUL SUTTER: The axis of rotation of the Apep system 518 00:30:19,918 --> 00:30:22,820 is pointed 30 degrees away from us. 519 00:30:22,921 --> 00:30:26,390 So if it does blow, it's likely that the jets 520 00:30:26,491 --> 00:30:28,025 are going to miss us. 521 00:30:28,126 --> 00:30:30,727 It makes me feel better that this gamma 522 00:30:30,829 --> 00:30:32,429 ray burst isn't pointing at us. 523 00:30:32,530 --> 00:30:35,732 But, of course, there are many other cosmic catastrophes 524 00:30:35,834 --> 00:30:38,168 potentially waiting to get us. 525 00:30:38,269 --> 00:30:42,506 NARRATOR: Apep is on the edge of an enormous explosion. 526 00:30:42,607 --> 00:30:45,209 Its huge gravity and incredible spin 527 00:30:45,310 --> 00:30:48,178 should produce a spectacular supernova. 528 00:30:51,015 --> 00:30:55,385 But what if some stars are too big to blow? 529 00:31:11,202 --> 00:31:18,375 Galaxy NGC 6946-- a local galaxy just 20 million light years 530 00:31:18,476 --> 00:31:23,046 away and well known to supernova detectives. 531 00:31:23,147 --> 00:31:26,216 It's the fireworks galaxy because it has produced so many 532 00:31:26,317 --> 00:31:28,485 supernovas in the past century. 533 00:31:28,553 --> 00:31:30,654 And they notice that one star that they 534 00:31:30,755 --> 00:31:34,124 thought would become a supernova instead blinked out. 535 00:31:36,294 --> 00:31:37,995 NARRATOR: The star under investigation 536 00:31:38,096 --> 00:31:45,035 is N6946-BH1, a cosmic heavyweight 25 537 00:31:45,136 --> 00:31:46,870 times the mass of our sun. 538 00:31:49,040 --> 00:31:51,742 That's way more than the eight solar masses we 539 00:31:51,843 --> 00:31:53,777 thought guaranteed a supernova. 540 00:31:55,113 --> 00:31:57,347 This is a very massive, very luminous 541 00:31:57,448 --> 00:32:00,851 star, the prototype of what you expect 542 00:32:00,952 --> 00:32:03,921 to explode as a supernova. 543 00:32:04,022 --> 00:32:05,756 MICHELLE THALLER: And over the last couple of years, 544 00:32:05,857 --> 00:32:07,524 its brightness has been changing. 545 00:32:07,625 --> 00:32:10,327 Maybe the star was beginning to go a bit unstable. 546 00:32:10,428 --> 00:32:12,329 But then, right in front of our eyes, 547 00:32:12,430 --> 00:32:15,365 this star just completely disappeared. 548 00:32:20,204 --> 00:32:22,005 This is a huge mystery. 549 00:32:22,106 --> 00:32:23,307 Why didn't this thing blow up? 550 00:32:24,943 --> 00:32:27,311 NARRATOR: How could a star just disappear? 551 00:32:28,613 --> 00:32:30,147 There had to be something left behind. 552 00:32:32,884 --> 00:32:35,953 So astronomers began a search for evidence 553 00:32:36,054 --> 00:32:37,354 and found a crucial clue. 554 00:32:39,791 --> 00:32:41,458 When you look in the infrared, you 555 00:32:41,559 --> 00:32:42,893 can still see some light there. 556 00:32:42,994 --> 00:32:44,962 So there was something happening there. 557 00:32:45,063 --> 00:32:45,762 But what? 558 00:32:47,699 --> 00:32:50,867 NARRATOR: We think the infrared light is heat coming off 559 00:32:50,969 --> 00:32:52,569 the debris of the dead star. 560 00:32:56,007 --> 00:32:59,042 Something is pulling it inwards, something 561 00:32:59,143 --> 00:33:02,646 powerful but also invisible-- 562 00:33:02,747 --> 00:33:03,447 A black hole. 563 00:33:05,750 --> 00:33:07,517 The outer stuff from the star is still 564 00:33:07,618 --> 00:33:09,753 falling on to that black hole, and it's 565 00:33:09,854 --> 00:33:11,288 powering a little bit of light. 566 00:33:11,389 --> 00:33:13,457 A little bit of the infrared light still gets out. 567 00:33:18,029 --> 00:33:19,563 NARRATOR: How can a giant star become 568 00:33:19,664 --> 00:33:23,600 a black hole without exploding into a supernova first? 569 00:33:26,537 --> 00:33:30,273 The answer lies in how dying stars burn their fuel. 570 00:33:32,310 --> 00:33:35,479 For stars that are about, say, 20 times the mass of the sun, 571 00:33:35,613 --> 00:33:37,681 you're actually going to burn things convectively. 572 00:33:37,782 --> 00:33:41,318 That means the gases inside the core are moving around. 573 00:33:41,419 --> 00:33:45,355 A good analogy is water in a boiling pot of water. 574 00:33:45,456 --> 00:33:47,157 You've got your potatoes up here. 575 00:33:47,258 --> 00:33:48,525 You're trying to boil them. 576 00:33:48,626 --> 00:33:51,795 You've got convective cells of water that are heated. 577 00:33:51,896 --> 00:33:53,930 Bring the heat up to the top. 578 00:33:54,032 --> 00:33:55,432 Get the potatoes hot. 579 00:33:55,533 --> 00:33:59,403 And then those blobs of water cool down, become denser, 580 00:33:59,504 --> 00:34:01,071 and settle down to the bottom again 581 00:34:01,172 --> 00:34:02,539 where they're heated once more. 582 00:34:04,876 --> 00:34:06,777 NARRATOR: As fusion turns hydrogen 583 00:34:06,878 --> 00:34:11,014 to helium and then to carbon, convection mixes 584 00:34:11,115 --> 00:34:12,616 the carbon so it burns up. 585 00:34:15,620 --> 00:34:18,388 PAUL SUTTER: Convection cells work inside 586 00:34:18,489 --> 00:34:21,958 of a star like massive elevators that 587 00:34:22,060 --> 00:34:24,961 take hot gas from the central regions, 588 00:34:25,063 --> 00:34:28,365 bring it up to the surface, allow it to cool, and then 589 00:34:28,466 --> 00:34:30,734 pull that material back down. 590 00:34:30,835 --> 00:34:35,338 They're constantly churning back and forth inside of a star. 591 00:34:37,675 --> 00:34:40,510 NARRATOR: But stars more massive than roughly 20 times 592 00:34:40,611 --> 00:34:45,482 the mass of the sun, like N6946-BH1, 593 00:34:45,583 --> 00:34:47,084 don't burn carbon this way. 594 00:34:49,520 --> 00:34:51,621 Instead of mixing, the heavier atoms 595 00:34:51,722 --> 00:34:54,991 created by the fusion reactions just start to pile up. 596 00:34:57,562 --> 00:34:58,862 That means there's a layer of very 597 00:34:58,963 --> 00:35:02,065 dense material building up on just the surface of the core. 598 00:35:02,166 --> 00:35:04,034 All of the stuff is just ready to collapse. 599 00:35:06,504 --> 00:35:08,772 It's possible that, if you have enough mass sitting 600 00:35:08,873 --> 00:35:11,007 around, the collapse is so powerful 601 00:35:11,109 --> 00:35:13,143 that it actually collapses into a black hole 602 00:35:13,244 --> 00:35:15,912 before any supernova goes off. 603 00:35:16,013 --> 00:35:18,615 That, then, is a failed supernova. 604 00:35:18,716 --> 00:35:20,717 It's a star that pretty much directly 605 00:35:20,818 --> 00:35:22,385 collapses to form a black hole. 606 00:35:26,591 --> 00:35:30,827 NARRATOR: If many of the massive stars we expect to go supernova 607 00:35:30,928 --> 00:35:33,029 won't, that's a problem. 608 00:35:36,601 --> 00:35:39,402 We used to think we had the basics of supernovas cracked. 609 00:35:39,504 --> 00:35:41,605 Any time you have a star more massive than eight times 610 00:35:41,706 --> 00:35:43,373 the mass of the sun, it was destined 611 00:35:43,474 --> 00:35:45,175 to explode as a supernova. 612 00:35:45,276 --> 00:35:47,911 And then along comes a star that screws everything up. 613 00:35:50,515 --> 00:35:52,616 NARRATOR: To make things worse, we found no clear 614 00:35:52,717 --> 00:35:55,051 distinction between stars that go out with 615 00:35:55,153 --> 00:35:56,887 a bang and those that don't. 616 00:35:59,824 --> 00:36:05,462 As many as 30% of massive stars could die without exploding. 617 00:36:05,563 --> 00:36:08,131 Our search for the next killer supernova 618 00:36:08,232 --> 00:36:09,132 is getting even harder. 619 00:36:10,568 --> 00:36:12,869 PHIL PLAIT: Stars blow up when we don't expect them to. 620 00:36:12,970 --> 00:36:15,605 They don't blow up when we expect them to. 621 00:36:15,706 --> 00:36:17,741 They can have several stars orbiting each other, 622 00:36:17,842 --> 00:36:19,609 and the one that blows up isn't necessarily the one 623 00:36:19,710 --> 00:36:20,410 you think it will. 624 00:36:23,714 --> 00:36:27,050 NARRATOR: So right now we can't identify a prime suspect, 625 00:36:27,151 --> 00:36:28,285 but the hunt continues. 626 00:36:30,755 --> 00:36:31,922 PAUL SUTTER: As far as we know, there 627 00:36:32,023 --> 00:36:34,257 are no life-threatening stars out there, 628 00:36:34,358 --> 00:36:37,928 but we haven't done a complete survey. 629 00:36:38,029 --> 00:36:41,398 So please keep funding astronomy so we can keep looking. 630 00:36:43,668 --> 00:36:44,935 NARRATOR: Supernovas destroy. 631 00:36:47,138 --> 00:36:49,105 But can they also create? 632 00:36:52,476 --> 00:36:56,580 Did a supernova spark humanity's rise to dominate 633 00:36:56,681 --> 00:36:59,416 our world and our solar system? 634 00:37:11,462 --> 00:37:19,202 Supernovas are spectacular, devastating, and frightening. 635 00:37:21,806 --> 00:37:24,608 But without them, we wouldn't exist. 636 00:37:26,277 --> 00:37:28,712 The iron in your blood and the calcium in your bones 637 00:37:28,813 --> 00:37:32,983 was literally forged inside of a star that exploded billions 638 00:37:33,084 --> 00:37:35,018 of years ago as a supernova. 639 00:37:35,119 --> 00:37:38,054 And I think this is one of the most beautiful and the most 640 00:37:38,155 --> 00:37:40,991 profound things that we've learned in astronomy, 641 00:37:41,092 --> 00:37:44,894 that we're literally viscerally connected to the cosmos 642 00:37:44,996 --> 00:37:46,730 and the cosmos is connected to us. 643 00:37:49,767 --> 00:37:52,369 With every breath, we are inhaling 644 00:37:52,470 --> 00:37:55,205 oxygen that was created in a supernova explosion. 645 00:37:55,306 --> 00:37:57,307 [booming] 646 00:37:59,210 --> 00:38:02,979 This is almost literally a cosmic cycle of life. 647 00:38:04,949 --> 00:38:07,183 [booming] 648 00:38:09,620 --> 00:38:11,187 NARRATOR: And the supernova may even 649 00:38:11,289 --> 00:38:14,758 be responsible for the dawning of our intelligence 650 00:38:14,859 --> 00:38:17,894 by causing lightning. 651 00:38:17,995 --> 00:38:19,262 It might sound rather incredible, 652 00:38:19,363 --> 00:38:21,498 but a supernova might actually influence, directly, weather 653 00:38:21,599 --> 00:38:22,732 right here on the Earth. 654 00:38:22,833 --> 00:38:26,503 The cosmic rays from a supernova will create charges 655 00:38:26,604 --> 00:38:28,371 in the lower atmosphere. 656 00:38:28,472 --> 00:38:30,840 That energy will break apart molecules, 657 00:38:30,941 --> 00:38:34,678 excite atoms and molecules, and it will ionize them. 658 00:38:34,779 --> 00:38:38,748 And an ionized atmosphere means that now 659 00:38:38,849 --> 00:38:40,550 it can conduct electricity. 660 00:38:40,651 --> 00:38:43,453 So it probably increased lightning across the planet. 661 00:38:46,991 --> 00:38:49,893 NARRATOR: It's possible the same gamma ray burst that caused 662 00:38:49,994 --> 00:38:54,264 a mass extinction 2.6 million years ago 663 00:38:54,365 --> 00:38:58,168 also affected Earth's atmosphere, 664 00:38:58,269 --> 00:39:01,571 triggering tremendous bursts of lightning, 665 00:39:01,672 --> 00:39:04,841 which caused forest fires. 666 00:39:06,477 --> 00:39:09,212 HAKEEM OLUSEYI: We have evidence of widespread fires 667 00:39:09,313 --> 00:39:10,313 at this time. 668 00:39:10,414 --> 00:39:12,882 So it could be that lightning was increased, 669 00:39:12,983 --> 00:39:14,918 and that created more fires. 670 00:39:15,019 --> 00:39:18,054 And those fires could have leveled forests and savannas, 671 00:39:18,155 --> 00:39:19,422 creating grasslands. 672 00:39:21,092 --> 00:39:25,195 NARRATOR: So how could this change/boost our intelligence? 673 00:39:25,296 --> 00:39:29,599 With their forest homes burnt, our ancestors, early hominids, 674 00:39:29,700 --> 00:39:33,937 had to adapt to life out in the open, which meant standing up. 675 00:39:35,072 --> 00:39:36,106 DAN DURDA: You're living in a savanna 676 00:39:36,207 --> 00:39:39,075 where there's lions and leopards and cheetahs, 677 00:39:39,176 --> 00:39:41,244 and the savanna is mostly grassland. 678 00:39:41,345 --> 00:39:44,214 It's a lot more efficient, perhaps, on two feet. 679 00:39:44,315 --> 00:39:45,148 You can run. 680 00:39:45,249 --> 00:39:46,416 And moving on two feet might have 681 00:39:46,517 --> 00:39:48,651 been the survival mechanism. 682 00:39:50,154 --> 00:39:52,956 NARRATOR: Standing upright also triggered the most important 683 00:39:53,057 --> 00:39:54,124 change in our history. 684 00:39:56,093 --> 00:39:57,527 DAN DURDA: Walking around on two feet 685 00:39:57,628 --> 00:40:00,864 freed our hands to be able to start doing things. 686 00:40:00,965 --> 00:40:03,099 And as you-- you know, of course, you can imagine that, 687 00:40:03,200 --> 00:40:05,468 as you start doing things, that drives your brain to more 688 00:40:05,569 --> 00:40:06,903 complexity as you're trying to figure 689 00:40:07,004 --> 00:40:08,171 out how to manipulate things. 690 00:40:08,272 --> 00:40:09,939 And this is perhaps the biggest 691 00:40:10,040 --> 00:40:12,842 evolutionary leap, because, without it, we 692 00:40:12,943 --> 00:40:13,943 don't get tool use. 693 00:40:14,044 --> 00:40:15,178 We don't get fire. 694 00:40:15,279 --> 00:40:16,780 We don't get intelligence. 695 00:40:18,749 --> 00:40:20,850 NARRATOR: As our ancient ancestors adapted 696 00:40:20,951 --> 00:40:24,053 to their new habitat, they took their first steps 697 00:40:24,155 --> 00:40:25,488 toward world domination. 698 00:40:27,258 --> 00:40:28,858 At least, that's the theory. 699 00:40:30,528 --> 00:40:33,830 The idea presented here is this would be the dawn 700 00:40:33,931 --> 00:40:36,032 of modern humans as we see it. 701 00:40:36,133 --> 00:40:38,435 And we would owe that to lightning 702 00:40:38,536 --> 00:40:40,937 created from a gamma ray burst. 703 00:40:41,038 --> 00:40:41,738 That's nuts. 704 00:40:45,509 --> 00:40:48,711 NARRATOR: Supernovas are extraordinary. 705 00:40:48,813 --> 00:40:50,547 They launched our journey into the cosmos. 706 00:40:51,916 --> 00:40:54,651 And in time, a supernova may end it. 707 00:40:57,621 --> 00:41:01,658 We're searching hard to spot which one it could be. 708 00:41:01,759 --> 00:41:04,894 But, for now, the only way we'll know for sure 709 00:41:04,995 --> 00:41:06,596 is when it lights up our sky. 710 00:41:09,433 --> 00:41:13,570 While a supernova might appear to be the death of a star, 711 00:41:13,671 --> 00:41:16,239 the beauty of it is that it's really a story 712 00:41:16,340 --> 00:41:17,407 about beginnings, as well. 713 00:41:23,347 --> 00:41:26,082 Supernovae giveth, and they taketh away. 714 00:41:26,183 --> 00:41:28,518 Without supernovae, the Earth wouldn't exist 715 00:41:28,619 --> 00:41:29,719 and we wouldn't exist. 716 00:41:32,323 --> 00:41:35,158 I actually do imagine standing out on a nice winter night, 717 00:41:35,259 --> 00:41:36,926 looking up at Betelgeuse, and actually 718 00:41:37,027 --> 00:41:38,761 seeing the thing explode. 719 00:41:38,863 --> 00:41:40,864 There would be this bright light. 720 00:41:40,965 --> 00:41:42,765 I can imagine my face lighting up. 721 00:41:42,867 --> 00:41:44,133 I would really lose it. 722 00:41:47,438 --> 00:41:48,705 I would love to see a supernova up close, 723 00:41:48,806 --> 00:41:49,506 DAN DURDA: Right? 724 00:41:49,607 --> 00:41:50,740 I mean, what a light show. 725 00:41:50,841 --> 00:41:52,809 But there's no way I would want to be that close 726 00:41:52,910 --> 00:41:54,811 because I don't want to die.