WEBVTT 1 00:00:03.399 --> 00:00:07.719 Welcome to Bedtime Astronomy. Explore the wonders of the cosmos 2 00:00:07.759 --> 00:00:12.279 with our soothing Bedtime Astronomie podcast. Each episode offers a 3 00:00:12.359 --> 00:00:16.320 gentle journey through the stars, planets, and beyond, perfect for 4 00:00:16.399 --> 00:00:20.239 unwinding after a long day. Let's travel through the mysteries 5 00:00:20.239 --> 00:00:22.440 of the universe as you drift off into a peaceful 6 00:00:22.480 --> 00:00:23.800 slumber under the night sky. 7 00:00:26.920 --> 00:00:29.600 I want you to start by just taking a moment 8 00:00:29.640 --> 00:00:35.560 to imagine something that completely defies our standard, everyday understanding 9 00:00:35.600 --> 00:00:37.840 of the cosmos, right because you know, when you look 10 00:00:37.920 --> 00:00:42.560 up at the night sky, it feels timeless, it feels static, eternal. Really, 11 00:00:43.079 --> 00:00:46.039 the constellations you see tonight are whether the exact same 12 00:00:46.079 --> 00:00:47.560 ones the ancient Greeks look at and. 13 00:00:47.600 --> 00:00:48.719 One's the Egyptians mapped out. 14 00:00:48.759 --> 00:00:51.799 Yeah, exactly. But right now, I want you to picture 15 00:00:52.520 --> 00:00:56.079 a super massive black hole, and it is sitting in 16 00:00:56.119 --> 00:01:00.000 the dark one point three billion light years away from 17 00:01:00.039 --> 00:01:01.759 where you were sitting right now. 18 00:01:01.600 --> 00:01:04.120 Which is a distance that's almost impossible for the human 19 00:01:04.120 --> 00:01:04.959 brain to process. 20 00:01:05.239 --> 00:01:07.680 It really is, just to put that distance into perspective. 21 00:01:07.760 --> 00:01:10.319 One point three billion years ago, the only life on 22 00:01:10.359 --> 00:01:14.079 Earth was microscopic, single celled organisms, floating in the ocean. Yep, 23 00:01:14.640 --> 00:01:16.680 the light from what we are about to discuss today 24 00:01:17.200 --> 00:01:19.680 has been traveling through the absolute void of space for 25 00:01:19.760 --> 00:01:22.000 all that time just to reach our telescopes. 26 00:01:22.079 --> 00:01:23.040 It's staggering. 27 00:01:23.400 --> 00:01:27.319 So this black hole is a cosmic engine of unimaginable power. 28 00:01:28.159 --> 00:01:32.239 Now imagine that engine seemingly reaching out and just pulling 29 00:01:32.239 --> 00:01:34.120 the plug on its own immense power. 30 00:01:33.920 --> 00:01:37.200 Supply, plunging into relative darkness right. 31 00:01:37.079 --> 00:01:39.439 Only to rapidly flip the switch back on and just 32 00:01:39.560 --> 00:01:40.400 roar back to life. 33 00:01:40.439 --> 00:01:42.760 I mean, it sounds like science fiction. You expect stars 34 00:01:42.799 --> 00:01:45.760 to twinkle, Sure, you might even expect a supernova to 35 00:01:45.799 --> 00:01:48.079 flare up and fade away over the course of say 36 00:01:48.280 --> 00:01:48.959 a few months. 37 00:01:49.040 --> 00:01:52.920 Sure, things we've observed before, right, But a super massive 38 00:01:53.000 --> 00:01:57.439 black hole, the gravitational anchor of an entire galaxy, suddenly 39 00:01:57.480 --> 00:01:59.719 throttling down and then powering back up. 40 00:02:00.079 --> 00:02:04.280 That completely rewrites our expectations of how these behemoths operate. 41 00:02:04.879 --> 00:02:07.920 We are talking about objects that dictate the evolution of 42 00:02:08.039 --> 00:02:11.919 billions of stars suddenly changing their fundamental. 43 00:02:11.319 --> 00:02:13.680 State, and we are just speaking in hypotheticals. Today we 44 00:02:13.719 --> 00:02:18.680 are looking at a very specific, very real celestial object, 45 00:02:18.960 --> 00:02:22.159 the galaxy catalog by astronomers is J tw one two 46 00:02:22.240 --> 00:02:25.439 four two three oh nine, though you will also hear 47 00:02:25.479 --> 00:02:28.479 it referred to by another astronomical survey designation, which is 48 00:02:28.840 --> 00:02:31.800 H one two three seven two three five two. Yes, 49 00:02:31.919 --> 00:02:34.240 catchy name, I know, right rolls right off the tongue. 50 00:02:34.280 --> 00:02:36.280 But the mission for our conversation today is to truly 51 00:02:36.319 --> 00:02:38.520 try and grasp the sheer scale of what it means 52 00:02:38.520 --> 00:02:42.120 when an active galactic nucleus or an AGN suddenly changes 53 00:02:42.159 --> 00:02:42.759 its behavior. 54 00:02:43.039 --> 00:02:44.439 It's a huge paradigm shift. 55 00:02:44.560 --> 00:02:47.599 We are taking those beautiful, but you know, static pictures 56 00:02:47.639 --> 00:02:50.280 of space that we all have as our computer backgrounds, 57 00:02:50.520 --> 00:02:55.280 and we are stepping into a dynamic, real time cosmic mystery. 58 00:02:55.479 --> 00:02:59.280 Because for a long time, astrophysics was essentially an observational 59 00:02:59.400 --> 00:03:02.280 history sign right, We looked at things that took millions 60 00:03:02.280 --> 00:03:04.639 of years to change. So from a human perspective, they 61 00:03:04.719 --> 00:03:08.199 look frozen like a snapshot exactly. But HG one two 62 00:03:08.199 --> 00:03:11.000 three seven twenty two five to two forces us to 63 00:03:11.039 --> 00:03:14.759 look at the universe as a violently active, breathing environment. 64 00:03:15.520 --> 00:03:18.080 What we are examining here is a phenomenon known in 65 00:03:18.159 --> 00:03:21.520 astrophysics as a changing look AGN or a clay in 66 00:03:21.800 --> 00:03:25.000 lazy right, it is a classification that represents some of 67 00:03:25.000 --> 00:03:28.400 the most extreme rapid transformations we have ever witnessed in 68 00:03:28.439 --> 00:03:29.680 the centers of galaxies. 69 00:03:29.800 --> 00:03:32.039 So, okay, let's unpack this because before we can truly 70 00:03:32.120 --> 00:03:35.800 understand why this supermassive black hole eventually turned off and 71 00:03:35.840 --> 00:03:38.879 shocked the astronomical community, we need to understand what it 72 00:03:38.919 --> 00:03:40.560 actually looks like when it is fully on. 73 00:03:40.800 --> 00:03:42.520 Yes, baseline is important here. 74 00:03:42.800 --> 00:03:45.560 We need to understand the anatomy of this cosmic engine. 75 00:03:45.960 --> 00:03:48.879 If a black hole by definition lets no light escape, 76 00:03:49.240 --> 00:03:52.039 how can an active galactic nucleus be the brightest thing 77 00:03:52.039 --> 00:03:52.719 in the universe. 78 00:03:53.240 --> 00:03:55.800 Well, to understand a changing look agn we have to 79 00:03:55.879 --> 00:03:59.439 establish what an active galactic nucleus is at its core. 80 00:04:00.120 --> 00:04:03.240 At the heart of most, if not all, large galaxies, 81 00:04:03.280 --> 00:04:05.919 including our own Milky Way, by the way, lies a 82 00:04:05.960 --> 00:04:07.120 super massive black. 83 00:04:06.879 --> 00:04:10.520 Hole right Sagittarius a star for us exactly, And these 84 00:04:10.560 --> 00:04:13.719 can be millions or even billions of times the mass 85 00:04:13.719 --> 00:04:16.240 of our Sun, all crushed into a singularity. 86 00:04:16.360 --> 00:04:17.519 Mind bending it is. 87 00:04:18.000 --> 00:04:19.720 But as you point it out, a black hole by 88 00:04:19.759 --> 00:04:23.519 itself is just a massive gravitational sinkhole. It is invisible, 89 00:04:23.600 --> 00:04:26.399 it doesn't emit light. What makes an aging an active 90 00:04:26.480 --> 00:04:29.759 and incredibly luminous is the material surrounding it and the 91 00:04:29.759 --> 00:04:32.160 brutal physics of how that material falls in We. 92 00:04:32.120 --> 00:04:35.399 Are talking about gas, dust, maybe stars that wandered a 93 00:04:35.399 --> 00:04:38.240 bit too close and got shredded. Definitely shredded stars. Yes, 94 00:04:38.279 --> 00:04:41.120 but this material doesn't just fall straight down like an 95 00:04:41.160 --> 00:04:42.240 apple falling from a tree. 96 00:04:42.319 --> 00:04:44.920 Right, it can't, and that is due to a fundamental 97 00:04:44.959 --> 00:04:48.360 law of physics called the conservation of angular momentum. 98 00:04:48.480 --> 00:04:51.199 Okay, for anyone whose high school physics is a bit rusty, 99 00:04:51.480 --> 00:04:53.720 this is the classic ice skater effect. 100 00:04:53.399 --> 00:04:55.639 Right, that is the perfect way to visualize it. Yeah, 101 00:04:56.360 --> 00:04:58.439 when a figure skater is spinning on the ice with 102 00:04:58.480 --> 00:05:01.160 their arms outstretched, they rot tate at a certain speed, 103 00:05:01.720 --> 00:05:04.120 but when they pull their arms in tight to their body, 104 00:05:04.319 --> 00:05:08.720 they spin dramatically faster. The angular momentum has to be conserved, 105 00:05:08.800 --> 00:05:11.439 so a smaller radius means a higher velocity. 106 00:05:11.720 --> 00:05:13.360 So how does that apply to the black hole? 107 00:05:13.600 --> 00:05:17.839 Well, in space you have massive clouds of gas and 108 00:05:17.879 --> 00:05:22.480 stellar debris caught in the black hole's gravitational pull. Because 109 00:05:22.480 --> 00:05:26.680 that material always has some slight initial rotation, nothing is 110 00:05:26.759 --> 00:05:29.439 ever perfectly still as it gets pulled closer and closer 111 00:05:29.439 --> 00:05:32.560 to the black hole, it spins faster and faster, pulling 112 00:05:32.560 --> 00:05:36.160 the arms in exactly, and the centrifugal force causes this 113 00:05:36.240 --> 00:05:39.439 spherical cloud to flatten out into a disk. We call 114 00:05:39.480 --> 00:05:42.399 this an accretion disc acretion disc right. It is a 115 00:05:42.519 --> 00:05:46.879 rapidly swirling whirlpool of matter moving at significant fractions of 116 00:05:46.879 --> 00:05:47.560 the speed of light. 117 00:05:47.879 --> 00:05:51.199 Wow. And because the material in the inner part of 118 00:05:51.199 --> 00:05:53.720 the disc is closer to the black hole, it has 119 00:05:53.720 --> 00:05:56.000 to orbit much faster than the material in the outer 120 00:05:56.120 --> 00:05:57.759 edge just to keep from falling in. 121 00:05:57.920 --> 00:06:01.079 Yes, Kepler's laws in action at speed. 122 00:06:00.720 --> 00:06:04.079 Difference layers moving at different speeds that must create an 123 00:06:04.199 --> 00:06:05.759 unimaginable amount of friction. 124 00:06:06.040 --> 00:06:09.199 The friction is the engine. You have adjacent layers of 125 00:06:09.240 --> 00:06:13.240 gas scraping against each other at relativistic speeds so close 126 00:06:13.279 --> 00:06:16.160 to the speed alight exactly. But it is not just 127 00:06:16.439 --> 00:06:19.319 physical friction like you know, rubbing your hands together. We 128 00:06:19.360 --> 00:06:21.000 are dealing with plasma here. 129 00:06:20.920 --> 00:06:23.879 Okay, so gas where the electrons have been stripped from 130 00:06:23.920 --> 00:06:24.639 their atoms. Right. 131 00:06:24.680 --> 00:06:27.759 And because it's plasma, that means it is highly magnetized. 132 00:06:28.120 --> 00:06:32.120 The magnetic fields within the spinning plasma get twisted, stretched, 133 00:06:32.199 --> 00:06:34.720 and then they violently snap back in a process called 134 00:06:34.759 --> 00:06:38.759 magnetic reconnection. Oh wow, yeah, this acts as an incredibly 135 00:06:38.839 --> 00:06:43.279 powerful source of viscosity. The sheer mechanical and magnetic stress 136 00:06:43.360 --> 00:06:46.839 heats the material in the accretion disc to millions of degrees. 137 00:06:47.360 --> 00:06:51.160 It transforms from cold cosmic gas into a blindingly bright, 138 00:06:51.399 --> 00:06:52.680 superheated plasma. 139 00:06:52.759 --> 00:06:55.160 So the accretion disc itself is actually glowing. 140 00:06:55.279 --> 00:06:58.639 Radiantly glowing, and there is a halo of even hotter, 141 00:06:58.759 --> 00:07:02.120 thinner plasma hovering just above and below the inner disc, 142 00:07:02.519 --> 00:07:02.920 called the. 143 00:07:02.879 --> 00:07:06.000 Corona, like the Sun's corona, but way hotter, way hotter. 144 00:07:06.480 --> 00:07:11.160 The Agan corona reaches temperatures in the billions of degrees billions. 145 00:07:11.199 --> 00:07:12.279 That's just wow. Yeah. 146 00:07:12.920 --> 00:07:16.600 Together, the accretion disc and the corona radiate an astonishing 147 00:07:16.600 --> 00:07:20.279 amount of energy across the entire electromagnetic spectrum, from radio 148 00:07:20.279 --> 00:07:22.720 waves all the way up to high energy X rays. 149 00:07:23.279 --> 00:07:28.199 That brilliant radiating structure is the active galactic nucleus, so. 150 00:07:28.199 --> 00:07:29.959 It's basically a giant power plant. 151 00:07:30.279 --> 00:07:33.000 It is. The engine is so efficient at converting mass 152 00:07:33.000 --> 00:07:36.600 into energy that a single agn can easily outshine the 153 00:07:36.600 --> 00:07:40.240 combined light of every single star in its entire host galaxy. 154 00:07:40.279 --> 00:07:41.839 It really want you to hold onto that image. If 155 00:07:41.839 --> 00:07:44.439 you're listening a structure around a black hole that is 156 00:07:44.439 --> 00:07:47.959 brighter than billions of stars combined, which brings us to 157 00:07:48.000 --> 00:07:49.199 the timescale anomaly. 158 00:07:49.439 --> 00:07:50.319 Yes, the anomaly. 159 00:07:50.759 --> 00:07:53.759 This is the core fact that makes HG one two 160 00:07:53.800 --> 00:07:58.160 three seven twenty two fifty two seven so absolutely mind bending. Usually, 161 00:07:58.199 --> 00:08:00.279 when we talk about a super massive black hole going 162 00:08:00.319 --> 00:08:02.959 through feeding episodes, periods where it consumes more matter and 163 00:08:03.000 --> 00:08:06.800 gets brighter or consumes less and dims, astronomers expect those 164 00:08:06.879 --> 00:08:10.000 changes in luminosity to take tens of thousand or even 165 00:08:10.000 --> 00:08:11.360 millions of years. 166 00:08:11.680 --> 00:08:14.360 Right, The physics simply demands time because of the sheer 167 00:08:14.399 --> 00:08:16.439 physical size of the objects involved. 168 00:08:16.480 --> 00:08:18.480 They're just too big to change quickly exactly. 169 00:08:18.519 --> 00:08:21.959 And accretion disk around a supermassive black hole isn't measured 170 00:08:21.959 --> 00:08:24.959 in miles or kilometers. It is measured in late days 171 00:08:25.040 --> 00:08:25.800 or light months. 172 00:08:26.000 --> 00:08:26.959 Wait light months. 173 00:08:27.120 --> 00:08:29.839 Yes, it might take light traveling one hundred and eighty 174 00:08:29.879 --> 00:08:32.799 six thousand miles per second several months just across from 175 00:08:32.799 --> 00:08:35.000 one side of the disc to the other. So if 176 00:08:35.039 --> 00:08:37.639 a process is going to change the overall structure and 177 00:08:37.840 --> 00:08:40.759 energy output of something that vast it is limited by 178 00:08:40.759 --> 00:08:42.639 the speed of light and the suit of sound within 179 00:08:42.679 --> 00:08:45.320 the plasma. So a human lifetime is a blink of 180 00:08:45.360 --> 00:08:47.799 an eye compared to the feeding cycle of an agn 181 00:08:47.840 --> 00:08:50.799 At least, that was the standard assumption across astrophysics. 182 00:08:51.399 --> 00:08:54.639 Wait, so if this disc is measured in light months across, 183 00:08:55.200 --> 00:08:58.080 how could it possibly change fast enough for us to notice? 184 00:08:58.399 --> 00:09:00.480 I mean, the data for H one two, three, seven 185 00:09:01.000 --> 00:09:03.759 fifty two completely shattered that assumption. 186 00:09:03.879 --> 00:09:05.120 Completely shattered it. 187 00:09:05.200 --> 00:09:08.039 Observers didn't watch it dim over a millennium. They watched 188 00:09:08.080 --> 00:09:11.480 its X ray brightness completely collapse, dropping by a staggering 189 00:09:11.480 --> 00:09:14.039 factor of seventeen in just eighteen months. 190 00:09:14.080 --> 00:09:16.759 The factor of seventeen drop in eighteen months for an 191 00:09:16.799 --> 00:09:20.159 object of this magnitude is well, it's practically a free fall. 192 00:09:20.600 --> 00:09:23.720 It is structural failure on a galactic scale, happening in 193 00:09:23.799 --> 00:09:24.320 real time. 194 00:09:24.399 --> 00:09:26.519 To put that into perspective for you, it is like 195 00:09:26.639 --> 00:09:28.720 going out to your favorite national park to look at 196 00:09:28.720 --> 00:09:32.600 a massive, towering mountain range like the Rockies or the Alps, 197 00:09:32.960 --> 00:09:35.799 and watching the largest peak erode into flat dust over 198 00:09:35.840 --> 00:09:36.639 a single weekend. 199 00:09:36.720 --> 00:09:38.159 That's a really good way to think about it. 200 00:09:38.159 --> 00:09:41.919 It's a timescale that makes perfect sense for human events, 201 00:09:41.960 --> 00:09:44.639 you know, building a house, completing a work project, but 202 00:09:44.799 --> 00:09:49.080 it is shockingly, almost impossibly fast for astrophysics. 203 00:09:49.240 --> 00:09:52.080 It really highlights a crucial point about how limited our 204 00:09:52.120 --> 00:09:55.519 observational window of the universe really is. What do you mean, Well, 205 00:09:55.679 --> 00:09:58.440 we have only had X ray telescopes in orbit for 206 00:09:58.480 --> 00:10:02.919 a few decades. Even within that narrow window, this galaxy 207 00:10:02.960 --> 00:10:05.879 proved that catastrophic structural changes can happen in a. 208 00:10:05.799 --> 00:10:08.440 Flash, which is terrifying and exciting all at once. 209 00:10:08.519 --> 00:10:12.000 Exactly now. To picture this properly, we should also clarify 210 00:10:12.080 --> 00:10:14.480 the specific type of galaxy we are looking at. H 211 00:10:14.559 --> 00:10:16.840 E one two three seven twenty two five to two 212 00:10:17.000 --> 00:10:19.720 is not a quasar. It is a seafert. 213 00:10:19.360 --> 00:10:22.600 Galaxy, right, and that distinction changes how we actually view 214 00:10:22.639 --> 00:10:23.840 it through our telescope. 215 00:10:23.879 --> 00:10:28.120 It does. Both quasars and seafert galaxies possess extremely bright 216 00:10:28.360 --> 00:10:33.039 active nuclei powered by supermassive black holes. However, a quasar 217 00:10:33.120 --> 00:10:35.120 represents the most extreme end of the. 218 00:10:35.080 --> 00:10:37.320 Spectrum the gas guzzlers of the universe. 219 00:10:37.440 --> 00:10:41.000 Yes, it is so unfathomably bright that its light completely 220 00:10:41.159 --> 00:10:43.919 washes out the host galaxy surrounding it. When you look 221 00:10:43.960 --> 00:10:46.480 at a quasar. Through a telescope, you essentially just see 222 00:10:46.480 --> 00:10:48.200 a blinding star like point of light. 223 00:10:48.360 --> 00:10:49.840 You can't see the actual galaxy. 224 00:10:50.080 --> 00:10:52.639 No, you can't see the spiral arms or the galaxy 225 00:10:52.679 --> 00:10:57.559 itself without highly specialized equipment blocking that central glare. But 226 00:10:57.600 --> 00:11:02.080 a seafert galaxy is slightly luminous relatively speaking. It is 227 00:11:02.159 --> 00:11:05.120 hosted in a spiral galaxy, much like our own Milky Way, 228 00:11:05.240 --> 00:11:08.759 but with an agn operating at a let's say, a 229 00:11:08.840 --> 00:11:09.399 lower gear. 230 00:11:09.519 --> 00:11:11.879 So with the seafert the host galaxy doesn't get completely 231 00:11:11.960 --> 00:11:14.000 blown out by the central glare exactly. 232 00:11:14.200 --> 00:11:18.720 With a seafert galaxy, the beautiful swirling galactic structure, the 233 00:11:18.759 --> 00:11:22.240 spiral arms, the dark dust lanes, the regions of active 234 00:11:22.279 --> 00:11:26.840 star formation, it all remains clearly visible to our telescopes 235 00:11:26.879 --> 00:11:28.960 alongside the intensely bright core. 236 00:11:29.120 --> 00:11:30.919 So it's not just a floating flashlight, right. 237 00:11:30.960 --> 00:11:34.120 It gives astronomers a much clearer context of the environment 238 00:11:34.200 --> 00:11:36.559 surrounding the black hole. We can see the neighborhood, not 239 00:11:36.639 --> 00:11:37.320 just the floodlight. 240 00:11:37.480 --> 00:11:39.399 I love that. So imagine you are looking through a 241 00:11:39.440 --> 00:11:43.320 powerful telescope. You see this glowing, magnificent spiral galaxy, a 242 00:11:43.360 --> 00:11:46.320 pinwheel of billions of stars floating in the dark. The 243 00:11:46.399 --> 00:11:49.519 very center is a blindingly bright core the accretion disk 244 00:11:49.559 --> 00:11:52.799 of the supermassive black hole actively feeding. And then, over 245 00:11:52.840 --> 00:11:54.399 the course of just a year and a half, which 246 00:11:54.399 --> 00:11:57.279 is barely any time at all, you watch that blinding 247 00:11:57.320 --> 00:12:00.960 center suddenly dim to a tiny fraction of its original strength, 248 00:12:01.279 --> 00:12:03.679 while the rest of the galaxy around it keeps turning 249 00:12:04.000 --> 00:12:05.120 completely undisturbed. 250 00:12:05.399 --> 00:12:07.399 It's a haunting visual, it really is. 251 00:12:08.120 --> 00:12:10.759 But a visual can only tell you so much. A 252 00:12:10.799 --> 00:12:13.440 picture might tell you a galaxy got dimmer, but it 253 00:12:13.440 --> 00:12:16.159 won't tell you the physics of why or how the 254 00:12:16.159 --> 00:12:20.240 structure around the black hole changed. For that, astronomers have 255 00:12:20.320 --> 00:12:23.639 to decode the light itself using spectral fingerprints, right. 256 00:12:23.519 --> 00:12:26.519 Because we don't just look at photographs to understand what's happening. 257 00:12:26.759 --> 00:12:30.080 We rely on spectroscopy, which is arguably the most powerful 258 00:12:30.120 --> 00:12:33.519 tool in all of astronomy period absolutely to understand the 259 00:12:33.600 --> 00:12:36.240 changing look aspect of this galaxy, we have to break 260 00:12:36.240 --> 00:12:39.519 down how we classify active galactic nuclei into type one 261 00:12:39.559 --> 00:12:40.120 and type two. 262 00:12:40.279 --> 00:12:41.919 Okay, for those of us who haven't played with a 263 00:12:41.960 --> 00:12:46.000 prism since elementary school science class, how does reading the 264 00:12:46.080 --> 00:12:48.840 light actually tell us what is happening near a black 265 00:12:48.879 --> 00:12:52.000 hole one point three billion light years away? 266 00:12:52.159 --> 00:12:55.240 Well, think about Isaac Newton passing sunlight through a glass 267 00:12:55.240 --> 00:12:59.679 prism and seeing a rainbow. Spectroscopy is essentially the advanced 268 00:12:59.720 --> 00:13:02.679 version of that. When astronomers pass the light from a 269 00:13:02.679 --> 00:13:06.279 distinct galaxy through a spectrograph attached to a telescope, it 270 00:13:06.360 --> 00:13:10.720 breaks that light down into its constituent wavelengths, creating a spectrum. 271 00:13:10.799 --> 00:13:13.039 Okay, so a very detailed rainbow. 272 00:13:12.879 --> 00:13:16.679 Right, But it's not a perfectly smooth, continuous rainbow. When 273 00:13:16.679 --> 00:13:20.759 you look closely, you see specific distinct bright lines at 274 00:13:20.919 --> 00:13:24.200 very precise wavelengths. These are called emission lines. 275 00:13:23.919 --> 00:13:26.440 And these lines act as a barcode for the universe. 276 00:13:26.120 --> 00:13:30.879 Don't they exactly like a barcode. Every chemical element hydrogen, helium, carbon, 277 00:13:31.039 --> 00:13:35.000 iron has a unique atomic structure. When the electrons and 278 00:13:35.039 --> 00:13:37.399 these atoms are heated up by the intense energy of 279 00:13:37.399 --> 00:13:39.519 the black hole, they jump to higher. 280 00:13:39.399 --> 00:13:41.879 Energy level, They get excited, right, and. 281 00:13:41.840 --> 00:13:44.559 When they fall back down, they release a photon of 282 00:13:44.639 --> 00:13:48.320 light at a very specific unchanging wavelength. So by looking 283 00:13:48.360 --> 00:13:50.879 at where these bright lines appear on our spectrum, we 284 00:13:51.000 --> 00:13:53.519 know exactly what kind of gas is glowing in that 285 00:13:53.559 --> 00:13:54.360 distant galaxy. 286 00:13:54.440 --> 00:13:55.120 That's incredible. 287 00:13:55.120 --> 00:13:57.600 But the spectrum tells us more than just chemistry. It 288 00:13:57.639 --> 00:13:58.639 tells us velocity. 289 00:13:59.000 --> 00:14:01.600 Oh right, this is where the Doppler effect comes in. 290 00:14:02.120 --> 00:14:04.080 I think most of us understand the Doppler effect when 291 00:14:04.080 --> 00:14:06.879 it comes to sound. You hear an ambulance coming towards you, 292 00:14:07.360 --> 00:14:10.200 and the siren sounds high pitch because the sound waves 293 00:14:10.200 --> 00:14:13.279 are being compressed as it passes and drives away. The 294 00:14:13.360 --> 00:14:17.200 pitch drops noticeably because the sound waves are being stretched out. 295 00:14:17.639 --> 00:14:18.960 But how does that apply to light? 296 00:14:19.279 --> 00:14:22.279 It works the exact same way. If a cloud of 297 00:14:22.360 --> 00:14:25.919 glowing gas and space is moving rapidly toward our telescopes 298 00:14:25.960 --> 00:14:29.519 on Earth light waves, it emits get slightly compressed. We 299 00:14:29.600 --> 00:14:32.399 call this blue shift because the light is shifted toward 300 00:14:32.440 --> 00:14:35.320 the shorter, bluer end of the spectrum. And if the 301 00:14:35.360 --> 00:14:37.720 gas is moving rapidly away from us, the light waves 302 00:14:37.720 --> 00:14:41.440 are stretched out, shifting them toward the longer, redder wavelength. 303 00:14:41.759 --> 00:14:44.919 This is red shift. Now apply this to a swirling 304 00:14:44.919 --> 00:14:48.480 accretion disk and the clouds of gas orbiting the black hole. 305 00:14:48.799 --> 00:14:50.519 Well, the gas on one side of the black hole 306 00:14:50.559 --> 00:14:52.480 is rotating toward us YEA and the gas on the 307 00:14:52.519 --> 00:14:53.720 other side is rotating away. 308 00:14:53.879 --> 00:14:56.840 Right. Let's start with a type one ag in. When 309 00:14:56.840 --> 00:14:59.279 we look at a type one, the spectrograph shows us 310 00:14:59.399 --> 00:15:03.679 two types of emission lines, broad lines and. 311 00:15:03.679 --> 00:15:05.440 Narrow lines broad and narrow. 312 00:15:05.639 --> 00:15:09.159 The physical reality behind this is fascinating. The broad lines 313 00:15:09.159 --> 00:15:13.279 are created by gas that is swirling extremely fast, very 314 00:15:13.399 --> 00:15:16.679 very close to the supermassive black hole, in a region 315 00:15:16.799 --> 00:15:19.440 heavily dominated by the black hole's immense. 316 00:15:19.039 --> 00:15:20.799 Gravity, so deep in the gravity. 317 00:15:20.840 --> 00:15:24.840 Well, yes, according to Kepler's laws of orbital motion, things 318 00:15:24.919 --> 00:15:29.000 closer to a massive body must orbit faster. We are 319 00:15:29.039 --> 00:15:32.320 talking about gas moving at thousands of kilometers per second. 320 00:15:32.399 --> 00:15:32.840 Wow. 321 00:15:32.919 --> 00:15:36.519 Because the gas is moving so incredibly rapidly, the Doppler 322 00:15:36.559 --> 00:15:39.679 effect severely smears the wavelength of the light it emits. 323 00:15:40.000 --> 00:15:42.320 The light from the gas moving toward us is heavily 324 00:15:42.360 --> 00:15:45.799 blue shifted, and the light moving away is heavily red shifted. 325 00:15:46.240 --> 00:15:49.200 When you combine all that light into one signal, it 326 00:15:49.240 --> 00:15:52.480 creates a wide, fat, broad peak on our spectrograph. 327 00:15:52.559 --> 00:15:56.200 Ah. So when an astronomer sees a broad line, they 328 00:15:56.240 --> 00:15:59.360 know they are looking at incredibly fast gas deep in 329 00:15:59.399 --> 00:16:01.600 the danger zone right next to the engine. 330 00:16:01.720 --> 00:16:04.360 Yes, and the narrow lines and a type one spectrum 331 00:16:04.360 --> 00:16:07.159 come from clouds of gas that are located much further. 332 00:16:06.919 --> 00:16:08.879 Away from the black hole, where it's safer. 333 00:16:08.840 --> 00:16:13.039 Where the gravitational pull is significantly weaker. Yeah, the gas 334 00:16:13.120 --> 00:16:16.320 is moving much slower, maybe just hundreds of kilometers per 335 00:16:16.399 --> 00:16:21.080 second because the velocity is lower, the Doppler shifting is minimal. 336 00:16:21.519 --> 00:16:23.200 The light isn't smeared out. 337 00:16:23.399 --> 00:16:25.200 It's a sharper peak exactly. 338 00:16:25.200 --> 00:16:28.200 It creates a sharp, tall, narrow peak on the graph. 339 00:16:28.559 --> 00:16:32.360 So a type one AGN shows both the fast broad 340 00:16:32.519 --> 00:16:35.279 inner gas and the slow narrow outer. 341 00:16:35.159 --> 00:16:36.159 Gas, and a type two. 342 00:16:36.279 --> 00:16:38.840 A type two AGN shows only the narrow emission lines. 343 00:16:38.879 --> 00:16:41.720 The broad lines are completely absent from the barcode. 344 00:16:41.840 --> 00:16:42.320 Interesting. 345 00:16:42.480 --> 00:16:45.200 This means we are only seeing the slow moving gas 346 00:16:45.320 --> 00:16:48.120 far away from the black hole. We aren't detecting the 347 00:16:48.159 --> 00:16:50.200 fast moving gas right next to the engine at all. 348 00:16:50.320 --> 00:16:53.399 Okay, hold on to a late person. Classifying a galaxy 349 00:16:53.440 --> 00:16:55.799 as type one or type two makes total sense. Either 350 00:16:55.840 --> 00:16:57.960 you see the fast inner gas or you don't. But 351 00:16:58.000 --> 00:17:00.360 the universe rarely deals in just black and white. 352 00:17:00.480 --> 00:17:00.879 Very true. 353 00:17:00.960 --> 00:17:05.079 Astronomers have these micromeasurements. We have intermediate subtypes sitting right 354 00:17:05.079 --> 00:17:07.720 in between, like type one point two, one point five, 355 00:17:07.799 --> 00:17:10.079 one point eight, and one point nine. I gotta say 356 00:17:10.400 --> 00:17:12.599 arguing over a one point eight versus a one point 357 00:17:12.680 --> 00:17:17.519 nine sounds like splitting cosmic hares. Why does this micro 358 00:17:17.640 --> 00:17:20.640 level classification actually matter to our understanding of how the 359 00:17:20.640 --> 00:17:21.319 black hole. 360 00:17:21.160 --> 00:17:25.200 Operates because it reveals the thermodynamics of the engine shutting down. 361 00:17:25.640 --> 00:17:29.240 The intermediate types are where the taxonomy gets incredibly precise. 362 00:17:29.720 --> 00:17:33.240 They are ranked purely by how strong the broad emission 363 00:17:33.240 --> 00:17:36.440 lines are relative to the narrow ones. Okay, and astronomers 364 00:17:36.440 --> 00:17:39.400 look for very specific chemical signatures to make this judgment, 365 00:17:39.559 --> 00:17:43.799 particularly two specific hydrogen emission lines known as hydrogen alpha 366 00:17:43.920 --> 00:17:46.720 or H alpha, and hydrogen beta or H beta. 367 00:17:46.839 --> 00:17:49.200 Let's break this down. Why focus on those two hydrogen 368 00:17:49.240 --> 00:17:50.680 lines specifically. 369 00:17:50.200 --> 00:17:52.519 Well, hydrogen is the most abundant element in the universe, 370 00:17:52.519 --> 00:17:55.079 so its signals are the loudest. In the atomic structure 371 00:17:55.079 --> 00:17:57.880 of hydrogen, an electron jumping from the third energy level 372 00:17:57.920 --> 00:18:00.480 down to the second produces the H alpha line, which 373 00:18:00.519 --> 00:18:03.240 is a specific shade of red. An electron jumping from 374 00:18:03.279 --> 00:18:05.759 the fourth energy level down to the second produces the 375 00:18:05.920 --> 00:18:09.240 H beta line, which is a specific shade of blue. Green. 376 00:18:10.039 --> 00:18:15.000 Here's the crucial physical difference. It takes significantly more energy 377 00:18:15.000 --> 00:18:18.039 from the black holes radiation to kick an electron up 378 00:18:18.039 --> 00:18:20.440 to that fourth level to make H beta than it 379 00:18:20.480 --> 00:18:22.359 does to kick it to the third level to make 380 00:18:22.519 --> 00:18:23.039 H alpha. 381 00:18:23.240 --> 00:18:25.759 Ah. So the H beta line requires a hotter, more 382 00:18:25.880 --> 00:18:27.720 energetic engine to exist. 383 00:18:27.440 --> 00:18:31.079 Precisely, so, as an AGN starts to lose power the 384 00:18:31.200 --> 00:18:34.920 environment cools down, it doesn't have the intense energy required 385 00:18:34.960 --> 00:18:38.079 to excite the hydrogen atoms to that fourth energy level anymore. 386 00:18:38.160 --> 00:18:40.599 So what happens the broad H beta line starts to 387 00:18:40.599 --> 00:18:43.920 fade away. First, in a type one point eight AGN, 388 00:18:44.319 --> 00:18:47.640 you can still clearly see one broad line, the hydrogen 389 00:18:47.640 --> 00:18:50.079 alcil line, because the engine still has enough juice for that. 390 00:18:50.559 --> 00:18:53.400 But the broad hydrogen beta line has become very weak. 391 00:18:53.759 --> 00:18:55.160 It's just barely hanging on. 392 00:18:55.359 --> 00:18:58.400 It's like watching a city experience a rolling blackout. The 393 00:18:58.400 --> 00:19:01.319 most power hungry industrial sector shut down first, while the 394 00:19:01.319 --> 00:19:02.920 residential lights stay on a little longer. 395 00:19:03.039 --> 00:19:05.079 That is a highly accurate way of view it. Actually, 396 00:19:05.200 --> 00:19:08.119 as the temperature continues to drop, the power hungry H 397 00:19:08.160 --> 00:19:11.559 beta line vanishes entirely. When you show just the broad 398 00:19:11.720 --> 00:19:14.000 H offal line, you are classified as a type one. 399 00:19:13.839 --> 00:19:15.920 Point nine is one broad line left yep. 400 00:19:16.119 --> 00:19:19.079 If the engine cools even further and that final broad 401 00:19:19.240 --> 00:19:21.880