Supermassive Black Holes (2000)
Supermassive Black Holes
BBC2 9.00pm Thursday 30th November 2000
A violent end for Earth? In June 2000, astronomers made an extraordinary discovery. One that promises to solve one of the biggest problems in cosmology - how and why galaxies are created. Incredibly, the answer involves the most weird, destructive and terrifying objects in the Universe - supermassive black holes. Scientists are beginning to believe that these forces of pure destruction actually help trigger the birth of galaxies and therefore are at the heart of the creation of stars, planets and all life.
Supermassive black holes are so extraordinary that until recently, many people doubted that they existed at all. The idea of giant black holes the size of the Solar System seemed more like science fiction that reality - such monsters would be so powerful that they could destroy the very fabric of the Universe. But in the last five years a series of discoveries has changed our understanding of supermassive black holes and galaxies forever.
Using the powerful Hubble Space Telescope, scientists have been scanning nearby galaxies, searching for these giant black holes. It's a difficult job - by their very nature black holes swallow light - so can never be seen. So what scientists have been looking for is the effect of their massive gravity, hurling stars around them at immense speed. What they've found is more extraordinary than anyone could ever have imagined; not just evidence that these vast destructive monsters exist… but so far they're in every single galaxy toward which they have turned their telescopes. These giant agents of destruction appear to be common throughout the Universe. Scientists now think supermassive black holes are a fundamental part of what a galaxy actually is.
Lurking at the heart of every single galaxy is a giant black hole of apocalyptic proportions - and that includes our own galaxy, the Milky Way. Astronomer Andrea Ghez has been studying the heart of the Milky Way for the last five years. What she's discovered is irrefutable evidence for a giant black hole, 3 billion times the size of our own sun. A black hole that could destroy the entire Solar System. And as Horizon was filming in July 2000, Ghez got some terrifying images - of the giant monster sucking up gas and stars at the galaxy heart.
So what is this giant monster doing at the heart of our galaxy? What effect will this giant black hole 25,000 light years away have on us and the rest of the galaxy around it? These are questions that have been puzzling astronomers for the last few years - and in June, two separate groups of scientists found evidence that points to a startling answer. Rather than being destructive parasites, it seems that supermassive black holes may be essential in the very creation of the galaxies they live in.
Exactly how our galaxy was created has mystified astronomers and physicists for years. Although there have been many theories, there's little evidence to explain how the gas in the early Universe condensed to form the galaxy we see today. Now scientists realise they've been missing a vital ingredient - a supermassive black hole. The immense gravity of a giant black hole might trigger the gas to collapse in the first place. By churning up the gas around it, a giant black hole would trigger the birth of stars, planets and life itself. Despite being the most destructive thing in the Universe, scientists now think our supermassive black hole could be crucial in creating the galaxy as we know it.
The supermassive black hole in our own galaxy may be the reason we exist, but recent work suggests it may also be our end. At present Earth is so far away from the black hole that it can't affect us, but physicist John Dubinski thinks all that could change. In January 2000 he graphically simulated the final fate of our galaxy. In 3 billion years we will collide with the next door galaxy, Andromeda. The resulting apocalypse will force the Earth and our Solar System out of orbit. Dubinski has calculated a worrying 50:50 chance that we'll be sent hurtling in towards the black hole at the centre of this maelstrom. This would be fatal for the Earth.
NARRATOR (JOHN SHRAPNEL): Earlier this year two astronomers made an extraordinary discovery, one that is set to overturn our understanding of how the Universe formed.
PROF. KARL GEBHARDT (Nuker Team): We're never going to see a time like this in astronomy again.
PROF. LAURA FERRARESE (Rutgers University): Really the air is filled with new discoveries and new ideas.
NARRATOR: What they discovered was a very simple relationship, a relationship between the galaxy we live in and the most destructive force in the Universe. A supermassive black hole. It set the world of cosmology alight.
KARL GEBHARDT: Six months ago people were not that excited about supermassive black holes. The general astronomer did not care that much about supermassive black holes. Now they have to and now they'd better!
NARRATOR: The ultimate aim of cosmology is to understand how the Universe was formed. One of the most important questions is how galaxies were created, because without them we wouldn't exist.
DR ANDREW FRUCHTER (Space Telescope Science Institute): Galaxies contain almost all the stars we see in the Universe and maybe the places where all stars in the Universe will be created and stars are what produce oxygen, carbon, planets, everything you need for life and without life you don't get astronomers.
NARRATOR: We see our galaxy, the Milky Way, as a band of stars in the sky. In fact it's a giant rotating disc 200,000 light years wide. It contains over 200 billion stars like our own sun, circling slowly around the centre, but we are just one in 125 billion galaxies of different shapes and sizes spinning through space. Yet scientists haven't been able to explain how a single one of these galaxies was created.
ANDREW FRUCHTER: Galaxy formation is a very complicated process. It involves gravity and it involves large balls of gas colliding , it involves the dynamics of stars, it involves the chemistry of the gas coming together.
NARRATOR: All we know is that when the Universe was young there were no stars or planets, just great swirling clouds of hydrogen gas. The mystery is how each of these clouds turned into: the complex galaxies of stars we see today.
ANDREW FRUCHTER: We just don't know how they do it, how galaxies formed out of the, the ionised hot gas that filled the Universe is still physics that we do not really understand yet.
NARRATOR: Exactly how galaxies were created has troubled the world's leading astronomers and physicists for decades, but 6 months ago scientists found evidence for an extraordinary answer. The Nuker team is a group of world respected astronomers, but they're not galaxy experts. They are experts in the most violent and destructive forces known to science: supermassive black holes. Until recently, supermassive black holes were mere theory. These are giant black holes of apocalyptic proportion.
KARL GEBHARDT: Supermassive black holes are a million to a billion times the mass of, of a, of a typical black hole.
PROF. SANDRA FABER (Nuker Team): They could fill a whole solar system.
NARRATOR: A supermassive black hole is quite simply gravity gone mad. An object of such concentrated matter its gravitational pull is insatiable. Nothing can escape it, not even light itself. Anything that gets close - gas, stars and entire solar systems - are sucked into oblivion. It even destroys the very fabric of the Universe. If you think of the Universe as a space-time web, the gravity of ordinary stars and planets creates a dent in this web, but the immense gravity of a supermassive black hole is so destructive that it distorts space-time to breaking point. At the heart of a supermassive black hole is one of the most mysterious things in physics - the singularity, a point where space, time and all known laws of physics fall apart.
SANDRA FABER: What happens at the centre of the singularity is a complete mystery and solving it is going to require new physics that we just don't have right now. Some people think that you can fall through the singularity and pop out in another part of the Universe. The theories for the singularity are, some of them are very, very radical. We just don't know.
NARRATOR: Supermassive black holes are so bizarre that until recently many scientists doubted they existed at all. They were an extreme idea, dreamt up to explain a very rare and distant type of galaxy: active galaxies. These are amongst the brightest objects in the Universe. These galaxies have a brilliant burning core with vast jets of energy spurting out of the centre. This ferocious heart of brilliant hot gas is called a quasar. Scientists thought this whirling mass might be caused by a giant black hole sucking up gas and stars, literally feeding on the centre of the galaxy.
PROF. JOHN KORMENDY (Nuker Team): The idea is that the quasars that we see that look so bright are not the black hole, the supermassive black hole, they are the gas that's just about to fall into the supermassive black hole, that's going around it, shining very brightly just before it disappears down the black hole.
NARRATOR: A giant black hole would have a gravitational pull so overwhelming it would hurl gas and stars around it at almost the speed of light. The violent clashing would heat the gas up to over a million degrees.
JOHN KORMENDY: The gas rubs against itself essentially and gets extremely hot and extremely hot gas shines very brightly.
NARRATOR: In reality, although a quasar burns brightly, it is actually impossible to see if there's a black hole in the middle. Paradoxically the black hole is made invisible by the fact that it swallows light. So for years no-one could be certain if supermassive black holes really did exist at the heart of these strange active galaxies. The Nukers have spent the last two decades hunting for these elusive monsters. The first problem they faced was to prove that supermassive black holes existed at all. What they were to discover would be stranger than most people could have imagined. One of the first of the Nukers to try to find one was Alan Dressler. In 1983 he came to the Palomar Telescope in California, convinced that he'd found a way to prove that supermassive black holes exist.
DR. ALAN DRESSLER (Nuker Team): You can't see a black hole directly - that's what makes it a black hole - so what you're looking for is evidence of its gravity, you're looking at how it pulls on the stars that are coming nearby.
NARRATOR: Dressler knew that although a black hole is invisible, its immense gravity would hurl stars around it at over 500,000 kilometres an hour. By measuring how fast these stars were moving, he could prove if there really was a black hole at the centre of an active galaxy.
ALAN DRESSLER: I picked a galaxy nearby which is called NGC1068, an active galaxy, which meant that it probably had a supermassive black hole in it, at least that's what we wanted to prove.
NARRATOR: To be certain that the stars were moving unnaturally fast in NGC1068, Dressler wanted to compare them with stars in a normal galaxy, without a black hole. Stars circling around a weak centre of gravity would move at half the speed. So for this comparison he chose the very ordinary galaxy next door to us, Andromeda, with a quiet, inactive centre like our own. To measure the speed of the stars in these two very different galaxies, Dressler used an instrument called a spectroscope. This looks at the changing pattern of light coming from stars as they rotate around the galaxy core. The spectroscope shows the centre of the galaxy as a white band and the movement of stars around the core is traced by a dark vertical line. If the stars at the galaxy's centre are circling slowly then the dark band would show hardly any change, but if they're travelling at great speed, whizzing towards and away from us either side of a supermassive black hole then the dark band should show a sudden shift across the centre of the galaxy.
ALAN DRESSLER: I would expect to see some rather rapid change in this dark line so that there'd be a very big change in the speed from one side of the galaxy to the other, very suddenly, right over the centre, and that would show that the stars were moving very rapidly in the centre of the galaxy because of the influence of a great mass in the centre, the supermassive black hole.
NARRATOR: Over the next few nights Dressler measured the speed of the stars in NGC1068 and in Andromeda. When the results came down from the telescope he saw something that was completely unexpected. The picture from the active galaxy, where he hoped to find a black hole, was unreadable. NGC1068 was just too far away for the telescope to get a clear picture. The surprise came from Andromeda, the quiet, normal galaxy right next to us.
ALAN DRESSLER: I was astonished when I found what I was looking for, but not where I was looking for it. This jog in this dark band shows that on one side the stars were moving very rapidly away from us at 150 kilometres a second, which is 500,000 kilometres an hour.
NARRATOR: Dressler thought there could only be one thing that would cause the stars to move this fast: a supermassive black hole and he wasn't alone. Fellow Nuker John Kormendy had found exactly the same thing.
JOHN KORMENDY: The moment I could see that wiggle I knew essentially instantly that there was a very good chance that this would be a supermassive black hole. When you see something like that you know you're on to something.
NARRATOR: They'd found evidence of the most terrifying force in nature, but worryingly it wasn't in some far-off active galaxy. This supermassive black hole was in the very ordinary galaxy right next door to us. Andromeda seemed to have a black hole but no bright quasar.
ALAN DRESSLER: If there was a supermassive black hole why wasn't it shining? That suggested that there was not stuff falling in. Maybe lots of galaxies could have a dormant phase where they had a supermassive black hole but they weren't being fed so they weren't shining.
NARRATOR: A few theorists had predicted this very thing: supermassive black holes could exist in two states. When it's feeding a giant black hole creates a bright burning gas disk around it and then for some reason it stops feeding, leaving a dark, deadly core lurking menacingly in the centre of the galaxy and one of these dark, silent monsters had been found in our neighbouring galaxy. The discovery of a massive black hole lurking so close to us made headlines around the world, but many scientists found the news impossible to believe. They didn't think the evidence was good enough for such an extreme idea. Even the Nukers themselves began to have doubts.
JOHN KORMENDY: There is always the danger that instead of being a black hole, it's a dense cluster of something else that's dark, that's not a black hole.
PROF. DOUG RICHSTONE (Nuker Team): I thought there was a fair chance that we'd made some terrible bone-headed mistake and that somebody within a year was going to write a paper and show that we were a bunch of idiots and we would feel terrible about it.
NARRATOR: To convince the sceptics, they needed to find more supermassive black holes in many more galaxies. For this they needed to look further into space. So they turned to Hubble Space Telescope. From 1994 Hubble began a systematic survey of the centres of distant galaxies searching for the tell-tale signature of stars speeding around a supermassive black hole.
(ACTUALITY SCIENTIFIC CHAT)
Astronomers started by looking at an active galaxy, M87. As expected it had a giant feeding black hole shooting a great jet of energy into space. But it was when the search broadened out to include inactive galaxies as well, that something incredible happened. In every galaxy scientists looked at they found evidence for a supermassive black hole.
SANDRA FABER: NGC3115.
JOHN KORMENDY: NGC3377.
KARL GEBHARDT: NGC3379.
DOUG RICHSTONE: M31 and M32.
KARL GEBHARDT: In total there's probably 20-30 or so black holes that have been found.
NARRATOR: Supermassive black holes were supposed to be rare, but Hubble was finding them everywhere, both feeding in active galaxies and lurking quietly in ordinary galaxies.
SANDRA FABER: Pretty soon we got used to the idea that everything we would look at would have a black hole in it. You know, after the first three or four cases we were beginning to wonder: does every one have a black hole.
DOUG RICHSTONE: One by one we were seeing this picture sort of emerge out of the fog that, that every galaxy, or almost every galaxy, had a supermassive black hole in it. It was really quite astonishing.
NARRATOR: Far from being rare freaks of nature, the Nukers began to suspect that all galaxies could have giant black holes at their hearts. If this was true it would revolutionise ideas of what a galaxy actually is. More disturbingly, it meant there could be a supermassive black hole lurking at the heart of our very own galaxy, the Milky Way. Andrea Ghez has been coming to Hawaii for the last five years, trying to find out if there's a supermassive black hole in the middle of the Milky Way.
PROF. ANDREA GHEZ (University of California, Los Angeles): When I first started thinking about astronomy it never occurred to me that there might be a supermassive black hole at the centre of our galaxy. The idea was that galaxies rotated just around the mass of the centre, which was just stars and gas and dust and nothing particularly exotic.
NARRATOR: Andrea Ghez has been using a telescope even more powerful than Hubble - the Keck Telescope, perched 14,000ft up on the sacred mountain of Mauna Kea. The Keck telescope is the biggest optical telescope in the world. It has a vast mirror, 10 metres across, made up of 36 segments of highly polished aluminised glass.
ANDREA GHEZ: The Keck telescope is a fabulous telescope to use. It's great because it's large. This is a case where bigger is better. You get to collect a lot of photons, you can see very faint things and it allows you to see very fine details.
NARRATOR: Four times a year, Ghez focuses the telescope on the stars at the very heart of our Milky Way. She's looking for the tell-tale high speeds that reveal the presence of a black hole. The centre of the Milky Way is so near and the Keck telescope so powerful Ghez is able to see closer into the centre of the galaxy than anyone has ever done before.
ANDREA GHEZ: Here's an example of one of the images we got just last night. The seeing was, it was kind of a typical night, not the best night, not the worst night. Each one of these blobs here is a star and what you see is each star is distorted - that's what the atmosphere does. It's like looking through a pond, like you want to look at a penny at the bottom of a pond and the water's moving, it looks all distorted and it looks different every time you look, so this is one exposure and the next exposure looks like this.
NARRATOR: By superimposing thousands of these pictures taken overnight, the computer can compensate for the atmosphere's distortion producing a detailed picture of the centre of the galaxy.
ANDREA GHEZ: You can see the position of the stars very accurately. If we go into the centre here and rescale it, we actually see that there are fainter stars towards the centre of our field of view and these stars are extremely important. It's the motion of these stars that reveal the presence of the black hole.
NARRATOR: Ghez has been following the motions of these stars for the last five years. If there was no black hole they'd be moving very slowly, but she's discovered they're circling at speeds of over 1,000 kilometres a second.
ANDREA GHEZ: These stars that we've been watching are 2 light weeks from the, from the centre of our galaxy, so their motion, the fact they are going 1,000 kilometres per second tells us that within 2 light weeks there's 2 million times the mass of the sun of matter there.
NARRATOR: There's only one thing in the Universe this dense. Lying at the centre of this necklace of spinning stars is a supermassive black hole. You can't see it, but it's there. The most destructive force in the Universe is lurking at the heart of our very own galaxy, the Milky Way. The puzzle for cosmologists now is what affect it has on the galaxy around it. If, as it now seems, every single galaxy has a black hole at its heart, this can't be a coincidence. Perhaps black holes are an essential part of what galaxies are and how they work.
SANDRA FABER: Well now that we know they're in every galaxy, the question is what do they do? Are they fundamental, or are they just a frill? Do they really influence the life of galaxies, or is it the other way round - do galaxies influence them? That's what we're trying to find out.
NARRATOR: So the Nuker team set out to find out if there was any relationship between supermassive black holes and the galaxies around them.
JOHN KORMENDY: When you're studying an object that you know almost nothing about, the first thing you want to do is find some regular pattern of behaviour, because that sort of thing can teach you new science.
NARRATOR: One of the first things they noticed was a strange link between the size of the galaxy and the size of the black hole in the middle.
DR. JOHN MAGGORIAN (Nuker Team): We found that there's a relationship between the mass of the black hole and the mass of the surrounding host galaxy in the sense that small galaxies have small black holes of around a million solar masses and big galaxies have big black holes of round a billion solar masses.
NARRATOR: Every single black hole was almost exactly in proportion to the size of its galaxy. No matter how big or small, bizarrely the galaxy always had a black hole half a percent of its entire mass.
JOHN MAGGORIAN: This was surprising and immediately leads to questions: why?
NARRATOR: No-one had expected that black hole size and galaxy size could possibly be related. It suggested some mysterious invisible connection between a galaxy and its black hole, but what this was a mystery scientists would have to wait three years to solve.
The first breakthrough came when a new instrument was added to Hubble Space Telescope.
This dramatically accelerated the discovery of new black holes, giving scientists a wealth of new potential leads to follow. For three years the data has been coming down to Earth. Amongst those who've been sifting through it are two young competing researchers. What they were to discover this year would turn the world of cosmology on its head.
LAURA FERRARESE: Every day I go to work I don't really know what's going to happen, but I can count that it's going to be something exciting every single day.
KARL GEBHARDT: These past six months have been phenomenal in terms of black hole research. We've been extremely excited, we're finding these black holes in, in, in numbers that we had never been able to do before.
NARRATOR: Karl Gebhardt and Laura Ferrarese were trying to find the fundamental connection linking black holes and their galaxies, so they searched through all the different galaxy characteristics looking for any new links that might give a clue. But it wasn't until they looked at a property called sigma that the mystery began to unravel.
LAURA FERRARESE: Sigma is a just a very, very fancy name for something that's actually very simple.
NARRATOR: Sigma is the speed at which the stars are circling in the outer reaches of the galaxy. The stars at the edge of the galaxy are so far away from the black hole that they're completely unaffected by its gravity.
JOHN KORMENDY: Those stars don't feel the black hole, they feel the rest of the stars in the galaxy, they don't know or care that the black hole is there. If you took the black hole away from the galaxy they'd be moving at exactly the same speeds.
NARRATOR: This has lead scientists to believe there couldn't possibly by any connection between the size of the black hole and the speed of the stars at the edge of the galaxy. They were about to be proved wrong. As the two researchers went through the new data, they first had to calculate the mass of each black hole. Then they found out the speed at which the stars were moving at the edge of the galaxy and plotted all these figures on a graph.
KARL GEBHARDT: As they came in I would take that new black hole mass and the sigma for that galaxy and add it to my plot.
NARRATOR: There should be no relationship between the two, yet as they added each new point marking the speed of the stars against the mass of the black hole, a clear pattern started to emerge. To their amazement the points lay in an obvious band across the graph. The properties were clearly related: the bigger the black hole, the faster the speed of the stars at the edge of the galaxy.
LAURA FERRARESE: What we discovered is that the supermassive black holes at the centre of galaxies and the galaxies themselves are really very tightly intertwined.
NARRATOR: The stars on the edge of the galaxy have no physical connection with the black hole. Yet somehow their speed is tightly bound with the size of the black hole billions of miles away. If the two things aren't physically linked now, it means they must have been at some point in the past.
KARL GEBHARDT: The fact that we see there's such a tight relationship between the speed of the stars and the black hole in the middle is a probe to what happened early on in the galaxy.
JOHN KORMENDY: It screams at you something that you don't yet understand about the connection between galaxy formation and black hole formation.
NARRATOR: The relationship points at an extraordinary idea: that galaxies and their giant black holes could be linked from birth. In fact, scientists thought that supermassive black holes might even be involved in the formation of the galaxies themselves.
SANDRA FABER: This correlation's the most important thing we've learned about supermassive black holes so far. Astronomers are always looking for correlations. Whenever you find one that's really tight, like this one, it's a sign that there's some basic physics there that you need to look for.
NARRATOR: As it happens, the physics that might explain what was going on had been suggested years before: by theorists Martin Rees and Jo Silk. Jo Silk has spent much of his life trying to solve the mystery of galaxy formation. Three years ago it became clear that he'd been missing a vital ingredient. If there was a black hole in every galaxy, then scientists would need to explain what it was doing there.
PROF. JOSEPH SILK (University of Oxford): We had to rethink our ideas of how galaxies were made. To understand the first light of the Universe we really have to include the role of these supermassive black holes in galaxy formation.
NARRATOR: All previous ideas of galaxy formation had assumed that gas in the early Universe simply condensed to form stars and galaxies. Silk and Rees came up with a completely different idea. They proposed that the centre of each early gas cloud could have collapsed to form a giant black hole. The black hole would immediately start feeding on the gas around it, creating a brilliant quasar. Silk realised that the energy from this newly formed quasar would create intense temperature changes in the surrounding gas. This would cause the gas around the black hole and its newly formed quasar to condense into stars, which means, in effect, that the black hole could have helped to trigger the birth of the galaxy.
JOSEPH SILK: We think of black holes normally as being destructive influences on their surroundings. In this case they're creative, they're having a very positive impact on the formation of the galaxies.
NARRATOR: But there was more. This theory predicted when and why the black hole would eventually stop feeding and go quiet. They calculated that this would happen when the feeding black hole grew so large that the vast amount of energy spewing from its bright quasar would literally force the rest of the galaxy out of its reach.
JOSEPH SILK: It has the effect of pushing a wind against the surrounding gas and driving the surrounding gas away like a snowplough.
NARRATOR: With only its hot whirling quasar within its reach, the black hole would swallow that up and then stop feeding. It would be left invisible at the centre of the galaxy. Silk and Rees calculated that this moment when the black hole pushed the surrounding galaxy away, would depend, bizarrely, on how fast the stars in the outer galaxy were moving. The faster these stars were circling, the harder it would be to push them away and the bigger the black hole would need to grow to produce enough energy to overcome the motion of the circling stars. Which means the size of the black hole in the end depends on how fast the stars are moving in the newly formed galaxy around it.
JOSEPH SILK: If our theory is correct there should be a simple relation between the mass of the central black hole and the speed or the sigma of the stars in the newly formed surrounding galaxy.
NARRATOR: And this is exactly what has just been found. It means that Silk and Rees's theory may be right and if it is also right that supermassive black holes helped trigger star formation, then it must mean that all giant black holes and their galaxies are connected from birth. It means the answer to the mystery of galaxy formation may lie in the creation of the supermassive black holes at their heart.
LAURA FERRARESE: The real implication of the relation is that whatever controlled the formation of the galaxy and whatever controlled the formation of the supermassive black hole is basically the same thing, there is only one thing behind everything.
NARRATOR: The discovery has caused a sensation. Suddenly supermassive black holes are big news in cosmology.
KARL GEBHARDT: I'm very excited to talk to you about some of the results that I've been working on these past couple of months.
SCIENTIST: We believe that most, and perhaps all, galaxies contain supermassive black holes at the…
LAURA FERRARESE: Really till five years ago they were just considered oddities, you know, very exotic curiosities and certainly fascinating but of really no consequence and now we know that that is not true. Supermassive black holes are really the fundamental constituent of galaxies and they have to be taken into account.
NARRATOR: Together theory and observation are leading scientists to a new view of galaxy formation. It's still just a theory and there are many details to be worked out, but if it's true, then it all would have begun like this. In the early Universe, a time of formless gas, each swirling gas cloud would have become a galaxy with one crucial event: the creation of a voracious feeding black hole. The black hole would have immediately started churning its way through the gas cloud. This would have triggered giant bursts of star formation and the galaxy would have sparked into life. Eventually the black hole and its quasar would have pushed the rest of the galaxy away. After sucking up its quasar and with nothing left to feed on, the black hole would have been left dark and silent at the heart of the galaxy. So a supermassive black hole, a force of terrible destruction, could also be fundamental in the creation of our galaxy. Nevertheless, its latent destructive power should not be underestimated. Back in Hawaii Andrea Ghez has made a new discovery. She's discovered a new source of light in the centre of our galaxy. The black hole may be starting to feed again.
ANDREA GHEZ: All of a sudden we saw something that looks like a star, but maybe isn't a star, but it's definitely a new object in our map and the interesting thing is that it's located where we think the black hole is.
NARRATOR: Ghez thinks this spot of light could be something amazing.
ANDREA GHEZ: One idea that I'm particularly intrigued by at the moment is the idea that perhaps the black hole is feeding more right now.
NARRATOR: A quiet supermassive black hole can start feeding again at any time. The light could be coming from hot gas as it's sucked into the black hole. If this light was just another star, it would be circling with the others. If it's the black hole itself, then it should stay still. So to see whether it was moving Andrea took two pictures of it, one in May and one in July.
ANDREA GHEZ: If we flash back and forth between the two, you can see the new source. You can also see the other stars that we said are high velocity stars are moving.
NARRATOR: Although the stars around it move, the new source stays still. This suggests the light is coming from the very centre of the galaxy, the supermassive black hole itself. Andrea thinks that the light she sees is coming from hot gas being sucked into the vortex of the black hole. So if our black hole has started feeding again, could this affect the Earth even though we're 24,000 light years away?
ANDREA GHEZ: We're in absolutely no danger of being eaten by the supermassive black hole and in fact if we do think the black hole is going through a slightly larger feeding at the moment, it's tiny, it's tiny compared to what other galaxy, galaxies are doing so in fact still this a very quiet black hole. In spite of the fact that there might be new emission from it it's still extremely low.
NARRATOR: Our black hole is merely having the equivalent of a small snack, feeding on a wisp of gas that's strayed too close. The black hole stopped growing billions of years ago. Only a major catastrophe could make it fire up again, something violent enough to hurl stars from the safety of our galaxy's edge into its deadly heart and we now know that one day this catastrophe could happen. In January this year, John Dubinski set out to calculate the final fate of our galaxy, the Milky Way, and that of our nearest neighbour, Andromeda.
PROF. JOHN DUBINSKI (University of Toronto): The Andromeda galaxy is actually falling towards the Milky Way which means they'll probably have some close encounter at some point in the future.
NARRATOR: At the moment Andromeda is moving towards us at 400,000 kilometres per hour and scientists think one day it will hit us. So Dubinski decided to work out what'll happen to us in 3 billion years, when the two galaxies finally collide. After a long and complex calculation the result was a vivid picture of the impending collision. A detailed prediction of how the Milky Way will end.
JOHN DUBINSKI: The clouds of gas hit each other at these huge velocities, hundreds of kilometres per second, and that basically creates great shockwaves which move through the gas and heat it to great temperature.
NARRATOR: At the heart of this maelstrom the boiling gas is hurled towards the two converging black holes. This kick-starts a violent dual feeding frenzy as the two monsters spiral towards each other.
JOHN DUBINSKI: And eventually those two independent black holes with their accretion discs will spiral together and merge themselves and form an even more massive black hole.
NARRATOR: Dubinski worked out that this violent collision would knock the Earth and its Solar System out of orbit. Two possible fates await us. If we're on one side of the galaxy when this clash happens, we could be thrown out into the emptiness of space - if we're lucky.
JOHN DUBINSKI: The second possibility is that we're on the other side of the galaxy at the time of the collision in which case we could be thrown right into the centre of this chaos.
NARRATOR: In the active centre of the merging galaxy the huge feeding black hole will trigger giant stellar explosions and supernovae. This is bad news for Earth.
JOHN DUBINSKI: There could be a horrible catastrophe. The wave of radiation from the blast wave of the supernova would hit the atmosphere and boil it off in an instant, so the atmosphere would be gone, the seas would boil off into space and the Earth would be toast.