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BBC Space Black Holes

where this is the story of the power If you throw something hard enough, it might never come down. If something goes up fast enough, it can escape the Earth altogether. Our sun, the force that holds the planets in place. Billions of stars that make up our galaxy the Milky Way. But there is one object out there whose pull is so powerful you can never escape, no matter how fast you go, not even if you travel at the speed of light. This is a black hole in action. It is tearing apart a star that has strayed too close. Anything that comes near is destroyed. It's hard to believe anything is powerful enough to destroy a planet or a star, but it's true. Let's take a closer look. Put a black hole near something and immediately it starts ripping it apart. There's a star in there - it could just as easily be our sun - and it is being pulled apart by a black hole. On this scale, our Earth would be no bigger than a pebble. We wouldn't stand a chance. The shocking thing is how small the black hole is. The black hole itself is right at the centre of the disc. It's tiny. It's a million times smaller than the star. Just look what it can do. What is it about a black hole that makes it so powerful? The answer is gravity. It's the force that keeps us all stuck to the surface of our planet. If something's heavy enough, it pulls YOU towards it. And planet Earth is heavy - so heavy, in fact, that to get off it, you have to do this. All of this, just to escape from our tiny globe. And if Earth's gravity seems strong imagine the pull of the sun. Our sun is a million kilometres across. This is the real heavyweight of the solar system. But if you think our sun is big, think again. There are stars out there that are vast. Their gravity is mind-boggling. But compared to a black hole, even this star is a weakling. A black hole weighs as much as a massive star, but it's crammed into an area smaller than a pea. A black hole is gravity gone mad. Nothing can ever escape. What could create such a monster - something so heavy and yet unimaginably small? An event powerful enough to create a black hole should be visible right across the universe. And recently, we might actually have witnessed one as it happened. A team in Australia, headed by Professor Brian Boyle, spotted it. The first clue that led to his discovery came in the form of radiation - gamma rays - that are invisible to the human eye. The night sky that we can see with our own eyes is only part of the picture. Light comes to us in many different forms, from low-energy radio waves to the highest energy form of light: the gamma rays, the form of light that packs the biggest punch. Every night, in the gamma-ray sky, is fireworks night. We've been detecting violent bursts of gamma rays for decades, but we've never actually seen what causes them. It has to be a violent event, but what kind? The problem is gamma-ray bursts only last a few seconds. And to make things harder, the best way to detect them is from space. This gorgeous spacecraft... During a routine observation, the Gamma Ray Observatory detected an enormous blast of energy going off in deep space. What had triggered it? Brian Boyle's team, guided by the space observatory, turned their ground-based optical telescopes on to the blast in the hope of seeing it before it faded. The information was really down to the ground. The optical telescopes sprang into action, to try to localize where this burst of energy had come from. What we found was something we didn't expect, that this light was actually coming from a supernova. What they'd seen with their telescopes was an power star. But the power was far larger than anyone had ever witnessed before. And in the heart of that cataclysmic power, the researchers realized that something astonishing and terrifying had happened. As the massive star died, a monster had been born. We'd witnessed the birth of a black hole. What Boyle's team had seen was the death of a star so heavy that when it power, its mass collapsed inwards instead of blasting out into space. This star is absolutely huge. It's a hundred times bigger than our sun and thousands of times brighter. But it doesn't just power. As its surface layer blasts upwards, its core is smashed inwards. The centre of the star collapses in on itself, billions of tones of star stuff crushed smaller and smaller, until the whole star is squeezed to a single microscopic point. And from the remains of the dying star, a black hole is born. In our galaxy, a massive star power and creates a new black hole every 1,000 years - which may not sound like a lot, until you remember that the galaxy has been here a very long time. Speeding up its history, you can see that stars have been going off like firecrackers. And when a black hole is born, it never dies. Every hole that was ever created is still out there, so there should be around ten million of them, somewhere. The question is - where? But something as deadly as a black hole can't remain hidden forever. Like most predators, they leave a trail of destruction. And scientists are now beginning to recognize these telltale signs. One black hole hunter is Janna Levin. Even though black holes are invisible, it doesn't mean they have no effects. They're extremely strong vortices and pull matter in these swirling winds around them, a lot like a tornado. And like a tornado, you might not see it until the debris gets sucked up, like THIS tornado is now pulling the gases in it. So suddenly you can see the presence of this vortex, this strong swirling wind. It isn't the wind of a tornado you see - it's the havoc it creates. That's how we detect black holes, too - by the damage they do. Tornados are incredibly powerful, but you don't SEE them until they suck stuff into them, until you see them pulling up houses and cars, and gas and smoke and clouds. It's the same with black holes. You don't see them until they pull in the matter around them. This is what astronomers look for. Not the black hole itself, but stars caught in the black hole's incredible gravitational pull. This one is tearing apart a star that drifted too close. A feeding black hole is anything but black. The whole star is wrenched out of shape as the monster tugs at it. Some of the most spectacular black holes we've seen are SO powerful and spinning SO rapidly that they create these huge jets, these powerful funnels of material. They're thin but incredibly, incredibly long, incredibly vast. And the jets themselves can cross an entire galaxy. They're absolutely huge. The damage a black hole inflicts on a star can be seen clear across the universe. Once astronomers knew what to look for, they began to hunt for feeding black holes. Using powerful space telescopes, we've tracked down more and more of them. And these are the actual images - black holes tearing apart everything they meet. Most of them are remote. They're in distant corners of the universe. And we've only found them because they're feeding. But what about the ones that aren't feeding? Where are the millions of black holes that should be wandering through OUR galaxy? They remain hidden against the dark background of space. Luckily, there is a way of tracking even the blackest of black holes. And THIS is what gives them away - light. A black hole's powerful gravity affects everything around it. It can even bend light. So when a black hole passes in front of a star, the light from that star is distorted and the black hole gives itself away. Finding a star at the precise moment it's distorted by a black hole is a daunting task. But that didn't stop one very, very patient astronomer from trying to see the invisible. Tim Axelrod has dedicated many years of his life to the pursuit of the universe's hidden objects. For eight years, we've been looking at the same patch of sky, monitoring the brightness of 20 million stars.


 
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This is a black hole in action
This is a black hole in action
  Travel fast enough
Travel fast enough
  A black hole sliding silently in front of a distant star
A black hole sliding silently in front of a distant star
  Far out in space, he had seen the impossible
Far out in space, he had seen the impossible
 
We're looking for micro-lending events. These events are extremely rare. They occur when a massive object passes across our line of sight to a distant star. Axelrod set about his search for what he calls "gravitational micro-leasing" - when a star's light is distorted by a massive object like a black hole. But the odds were stacked against him. At best, his chance of finding the right star was one in a million. That factor seemed impossibly large, so most people thought we would fail pretty dismally. Undaunted by the enormity of the task and the skepticism of his colleagues, Axelrod set about looking for that one telltale pinprick of light amongst 20 million stars, every night for eight entire years. Then, one night, he hit the jackpot. This is a view of the Large Magellanic Cloud. The blue square shows the field of view of our telescope. Now we've zoomed in a bit. The star we're interested in is a pretty inconspicuous fellow right in the centre of the cross here. Now we zoom in yet again. This is picking the needle out of the haystack. And what we saw when we looked at it over a period of time was this. We were, naturally, ecstatic. Everyone that saw it agreed immediately that this was gravitational micro-lensing, so we were just over the moon. What he saw was disturbing - evidence not of one or two black holes, but hundreds. But is that anything for us here on Earth to worry about? Could our own fragile planet ever encounter one of these invisible monsters? If we do ever meet a black hole, it would tear our world to shreds. But one thing is sure - our skies seem to be full of them. And our galaxy still has one last dark secret, and it took the most powerful telescope in the world to unlock it - the Keck in Hawaii. One of the astronomers using it is 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 can collect a lot of photons, so you can see very faint things and very fine detail.     The telescope's vast mirror allows Andrea Ghez to study the centre of our galaxy with more accuracy than ever before. What astronomers have seen is a black hole a million times more powerful than normal. Here's an example of an image we got just last night. We see that there are fainter stars towards the centre. These stars are very important. It's the motion of these stars that reveal the presence of the black hole. There is a black hole at the centre of our galaxy that is SO powerful that it spins whole stars around itself at impossible speeds. The fact that they're going 1,000 km per second tells us there's two million times the mass of the sun of matter there. This is no ordinary black hole. This black hole is a giant, two million times as heavy as our sun. And it's not in some far-flung region of the universe. This black hole is sitting right at the centre of OUR galaxy. Suddenly the idea that the Earth might one day fall victim to a black hole doesn't seem quite so unlikely. If we ARE ever unlucky enough to meet one, what would it be like? It begins far out in space, beyond the furthest planets. A black hole ploughs into the cloud of comets that surrounds our solar system and flings them towards Earth with incredible force. These impacts are the first warning of our fate. As the black hole comes closer, its next victim is Jupiter, the giant of our solar system. Even from so far away, the black hole's gravity makes itself felt here on Earth. Our world is being shaken apart. But the black hole hasn't finished yet. It's heading straight for the heart of the solar system, our sun. Though tiny in comparison, it tears the sun apart. Dragging the sun with it, the black hole heads towards Earth. The Earth is now unbearably close to the sun. All life has long since ceased to exist. And our planet starts to melt. Quietly, our battered world disintegrates, and is consumed. And all that is left is the black hole, drifting through space. Earth eaten by a black hole? It sounds bizarre. But we know there are millions of these monsters out there.