What Do You Know About Black Holes？ ｜ Universe ｜ BBC Earth Science

To understand black holes, we have to start at the beginning. At the moment of birth. Black holes that are a few times the mass of the sun probably formed from giant stars that were maybe about 20 to 30 times the mass of the sun. Enormous stars burning bright blue with intense heat. But the brightest stars are the shortest lived. A star is a big ball of gas, right? There's outward gravity pushing in, right? The thing wants to collapse in on itself under its own self-gravity. But the fusion that's happening within the star's core liberates so much light that the outward radiation pressure prevents the collapse of that star. But eventually, that gives up. A star like that can burn through its nuclear fuel in just a few million years. And when its power source runs out, it collapses under its own gravitational pull. There's so much material that's collapsing during their final few moments that they create this massive dense ball of neutrons that continues to collapse. A star 20 times the mass of our sun or larger. Crushed by the force of gravity. Until the star disappears. Leaving only a ghost behind. A black hole. This transformation does not await all stars. Smaller, less massive stars like our sun eventually become burnt-out dwarves when their fusion stops. Slowly fading cinders. But it's possible that almost all the massive stars that dominated the early universe formed black holes when they died. Because black holes are simply what happens when enough matter is crushed into a small enough volume, dramatically warping the space around it. A river is a great analogy for the area right around a black hole. Here I am far upstream and the water is fairly placid. It's not moving too fast. If I were to get into the water here and swim across I'd be able to do that very easily. In the same way if you're far away from a black hole you'll be able to get around with just a normal spacecraft without too much trouble and simple propulsion. But the closer you get to the black hole the stranger things become. The collapsed massive star crushes down so small and so dense it ceases to have a physical surface at all. Becoming an infinitely small point in space exerting a profound effect on the spacetime around it. As the water gets closer to the waterfall the speed of the water increases. If I were to jump into the water right here the speed of the current would be so intense that I wouldn't be able to swim against it. And I would be gradually pulled closer to the edge of the waterfall till I reach a point of no return. And that's the same around a black hole. Just outside the black hole the fabric of space itself actually stretches inward towards the center. Not even stars, planets, people, even light cannot escape the pull of a black hole. It's like a waterfall in the fabric of the universe. The black hole's gravitational reach is not infinite. People have this idea that black holes suck in the sense that they suck everything into them. But that's not true. Black holes can only eat things that are within a certain distance away from them. If you're further away then the black hole has no way of eating you. But once in its grasp you are lost forever. And this is the key to their mystery. The black hole's interior is hidden from view, cut off from the rest of the universe by a boundary in space. The event horizon. Beyond this point there is no escape.