1 00:00:00,000 --> 00:00:05,500 For centuries, scientists imagined objects so heavy and dense that their gravity 2 00:00:05,700 --> 00:00:09,500 might be strong enough to pull anything in — including light. 3 00:00:09,700 --> 00:00:12,500 They would be, quite literally, a black hole in space. 4 00:00:13,000 --> 00:00:18,500 But it’s only in the past few decades that astronomers have conclusively proved their existence. 5 00:00:18,700 --> 00:00:22,500 Today, Hubble lets scientists measure the effects of black holes, 6 00:00:22,700 --> 00:00:25,000 make images of their surroundings 7 00:00:25,200 --> 00:00:28,500 and glean fascinating insights into the evolution of our cosmos. 8 00:00:34,500 --> 00:00:38,500 Brought to you by the European Space Agency and NASA 9 00:00:44,500 --> 00:00:48,500 Hubblecast Episode 43: Hubble and Black Holes 10 00:00:52,000 --> 00:00:55,500 Presented by Dr J, aka Dr Joe Liske 11 00:00:58,000 --> 00:01:00,500 Hi everyone, and welcome to the Hubblecast. 12 00:01:00,800 --> 00:01:05,300 In science fiction, black holes are often portrayed as some kind of menacing threat 13 00:01:05,500 --> 00:01:07,500 to the safety of the whole Universe, 14 00:01:07,700 --> 00:01:11,800 like giant vacuum cleaners that somehow suck up all of existence. 15 00:01:14,500 --> 00:01:19,300 Now, in this episode, we’re going to separate the fiction from the facts and we’re going to look at 16 00:01:19,500 --> 00:01:23,500 the real science behind black holes and how Hubble has contributed to it. 17 00:01:29,700 --> 00:01:36,300 Black holes come in different sizes. We’ve had solid evidence for the smaller ones since the 1970s. 18 00:01:36,500 --> 00:01:40,300 These form when a huge star explodes at the end of its life. 19 00:01:40,500 --> 00:01:47,500 As the outer layers are blown away, the star’s core collapses in on itself forming an incredibly dense ball. 20 00:01:47,700 --> 00:01:50,500 For instance, a black hole with the same mass as the Sun 21 00:01:50,700 --> 00:01:53,700 would have a radius of only a few kilometres. 22 00:01:54,000 --> 00:01:58,000 These black holes won’t suck you in unless you get very close to them though. 23 00:01:58,300 --> 00:02:02,700 In fact, contrary to popular belief, a black hole the size of the Sun 24 00:02:03,000 --> 00:02:07,500 doesn’t actually exert any more gravitational pull than the Sun does. 25 00:02:07,800 --> 00:02:11,000 But these stellar black holes are just part of the story. 26 00:02:13,000 --> 00:02:15,500 Before Hubble was launched, 27 00:02:15,700 --> 00:02:17,700 astronomers had noticed that the centres of many galaxies 28 00:02:18,000 --> 00:02:22,000 were somehow much denser and brighter than they were expected to be. 29 00:02:22,300 --> 00:02:25,000 And so they speculated that there must be some kind of huge, 30 00:02:25,200 --> 00:02:28,000 massive objects lurking in the centres of these galaxies 31 00:02:28,200 --> 00:02:31,700 in order to provide the additional gravitational attraction. 32 00:02:32,000 --> 00:02:36,000 Now, could these objects be supermassive black holes, that is, 33 00:02:36,200 --> 00:02:40,500 black holes that are millions or even billions of times more massive than the stellar ones? 34 00:02:40,700 --> 00:02:46,500 Or was there perhaps a simpler, less exotic explanation, like giant star clusters? 35 00:02:47,500 --> 00:02:51,800 Frustratingly, at that time, the telescopes just weren’t quite powerful enough 36 00:02:52,000 --> 00:02:55,000 to see enough detail to solve the mystery. 37 00:02:57,500 --> 00:03:02,000 Fortunately, Hubble was on its way, along with a range of other high-tech telescopes. 38 00:03:02,300 --> 00:03:05,500 When the space telescope was being planned, 39 00:03:05,700 --> 00:03:08,800 the search for supermassive black holes was in fact one of its main objectives. 40 00:03:12,500 --> 00:03:18,500 Some of Hubble’s early observations in the 1990s were dedicated to these dense, bright galactic centres. 41 00:03:18,700 --> 00:03:24,500 Where ground-based telescopes were just seeing a sea of stars, Hubble was able to resolve the details. 42 00:03:24,800 --> 00:03:31,700 In fact, around the very centres of these galaxies, Hubble discovered rotating discs of gas and dust. 43 00:03:34,500 --> 00:03:39,000 When Hubble observed the disc at the centre of a nearby galaxy, Messier 87, 44 00:03:39,299 --> 00:03:43,500 the astronomers saw that its colour was not quite the same on both sides. 45 00:03:43,700 --> 00:03:47,500 One side was shifted towards blue and the other towards red, 46 00:03:47,700 --> 00:03:51,700 and this told the scientists that it must have been rotating very quickly. 47 00:03:52,000 --> 00:03:56,300 This is because the wavelength of light is changed by the motion of an object emitting it. 48 00:03:58,500 --> 00:04:03,000 Think about how the pitch of an ambulance siren drops as it drives past you, 49 00:04:03,200 --> 00:04:07,000 because the sound waves are more spaced out as the vehicle moves away. 50 00:04:07,300 --> 00:04:13,000 Similarly, if an object is moving towards you, the light’s wavelength is squashed, making it bluer; 51 00:04:13,200 --> 00:04:16,700 if it’s moving away, it’s stretched, making it redder. 52 00:04:16,700 --> 00:04:18,500 This is also known as the Doppler effect. 53 00:04:20,700 --> 00:04:25,000 So, by measuring how much the colours had shifted on either side of the disk, 54 00:04:25,000 --> 00:04:28,500 the astronomers were able to determine its speed of rotation. 55 00:04:28,800 --> 00:04:33,500 And it turned out that this disk was spinning at a rate of hundreds of kilometres per second. 56 00:04:33,800 --> 00:04:38,800 Now, this in turn allowed astronomers to deduce that, hidden at the very centre, 57 00:04:39,000 --> 00:04:43,500 there must be some kind of object with the mass of two to three billion times the mass of the Sun – 58 00:04:43,700 --> 00:04:48,500 and this was very likely a supermassive black hole. 59 00:04:49,700 --> 00:04:51,700 Now, along with a lot of other observations, 60 00:04:52,000 --> 00:04:55,500 this was a key piece of evidence that led to the notion that 61 00:04:55,800 --> 00:04:59,500 there is a supermassive black hole lurking at the centre of most, if not all, 62 00:04:59,700 --> 00:05:03,000 giant galaxies, including our own Milky Way. 63 00:05:06,000 --> 00:05:13,700 So far, so good. But this work was almost 20 years ago — what does it tell us about cutting-edge science today? 64 00:05:14,000 --> 00:05:20,500 Well, the science of black holes has moved along a lot since then. The mystery now isn’t whether they exist, 65 00:05:20,700 --> 00:05:23,300 but why they behave in the strange ways they do. 66 00:05:29,500 --> 00:05:33,000 For example, Hubble observations have helped to show that the mass 67 00:05:33,200 --> 00:05:38,500 of a supermassive black hole is closely related to the mass of its surrounding host galaxy. 68 00:05:38,900 --> 00:05:43,500 The bigger the black hole, the bigger the galaxy. Now the reason for this is totally unclear. 69 00:05:45,200 --> 00:05:49,000 A supermassive black hole is pretty big, and it packs a lot of punch, 70 00:05:49,200 --> 00:05:53,000 but you’ve got to remember that compared to its host galaxy it’s actually tiny. 71 00:05:54,000 --> 00:05:58,700 The region of space that is most obviously and most immediately influenced by 72 00:05:59,000 --> 00:06:03,800 a supermassive black hole is in fact about a million times smaller than its surrounding galaxy. 73 00:06:04,000 --> 00:06:08,500 That’s about the same size difference as between this coin and a whole city. 74 00:06:09,000 --> 00:06:14,500 So it’s pretty hard to imagine of any processes that might link the two in a long-lasting way. 75 00:06:19,700 --> 00:06:25,500 So a big area in science just now is trying to find out what’s going on here, and why the two are linked. 76 00:06:25,700 --> 00:06:31,700 Do black holes regulate the size of galaxies, or do galaxies regulate the size of black holes? 77 00:06:32,000 --> 00:06:35,300 Or is something altogether different happening? 78 00:06:35,500 --> 00:06:39,700 Just now, astronomers don’t know, but they’re working hard to find out. 79 00:06:46,800 --> 00:06:49,800 But there’s another interesting, and probably connected black hole mystery 80 00:06:50,800 --> 00:06:52,800 that has astronomers scratching their head. 81 00:06:53,000 --> 00:06:55,800 Now, when matter falls into a black hole, 82 00:06:55,800 --> 00:07:01,800 it forms this big swirling disc, which heats up and gives off a lot of powerful radiation. 83 00:07:02,000 --> 00:07:05,800 The more matter falls into the black hole, the more powerful the radiation. 84 00:07:06,500 --> 00:07:10,000 Now these active, accreting black holes are called quasars, 85 00:07:10,200 --> 00:07:13,700 and they are among the most luminous and most powerful objects in the Universe. 86 00:07:14,700 --> 00:07:20,000 The thing is, a quasar can get so greedy that its radiation is powerful enough 87 00:07:20,000 --> 00:07:23,000 to actually blow away all the gas and dust that’s feeding it. 88 00:07:23,200 --> 00:07:28,500 And so it seems there’s a natural upper limit to the rate at which a black hole can grow. 89 00:07:28,700 --> 00:07:34,000 Now, this implies that one wouldn’t expect to see any really big and really powerful 90 00:07:34,300 --> 00:07:36,000 quasars in the very early Universe, 91 00:07:36,300 --> 00:07:39,700 because there simply wouldn’t have been enough time to build up 92 00:07:40,000 --> 00:07:43,000 the supermassive black hole that is needed to power a quasar. 93 00:07:43,300 --> 00:07:48,700 But recent discoveries have in fact shown that quasars do exist in the early Universe 94 00:07:49,000 --> 00:07:54,000 only about a billion years after the Big Bang, which is much earlier than we had expected. 95 00:07:54,300 --> 00:07:59,000 And so there you have it: another mystery for astronomers to pore over. 96 00:08:03,000 --> 00:08:08,500 So, by helping to clear up one mystery about black holes, Hubble has opened up whole new research areas, 97 00:08:08,700 --> 00:08:13,000 and these will eventually give us a better understanding of the history of the Universe. 98 00:08:13,000 --> 00:08:16,300 This is Dr J signing off for the Hubblecast. 99 00:08:16,500 --> 00:08:20,700 Once again, nature surprises us beyond our wildest imagination. 100 00:08:21,000 --> 00:08:24,000 Transcribed by ESA/Hubble