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What a black hole is
A black hole is a region of space where matter has been squeezed so tightly that its gravity is strong enough to keep everything nearby from escaping, including light.
5 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1squeezed mass
You already know gravity pulls things down: drop a ball and it falls. Gravity comes from mass, and the more mass you have, the stronger the pull. A black hole is what you get when an enormous amount of mass is crammed into a very small space, so gravity near it becomes extreme.
A black hole is mass squeezed so tightly that gravity becomes extreme.
If you keep the same mass but squeeze it into a smaller space, what happens to the pull at its surface?
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It gets stronger, because you can get closer to the same mass.
2escape or not
Normally, anything can leave a gravitational pull if it moves fast enough. A thrown ball falls back, but a rocket launched hard enough escapes Earth entirely. Near a black hole, the pull is so strong that even light, the fastest thing there is, cannot get away. Light that heads outward from within the black hole region is dragged back, never reaching the outside.
Near a black hole, even light cannot escape.
a common mix-upLight is too fast or too weightless to be affected by gravity.
3escape or not
Normally a rocket escapes gravity by going fast enough. Near a black hole the pull is so strong that even light cannot get away and is dragged back.
4nothing to see
This is why it is called 'black'. We see objects because light reaches our eyes, either from the object or bouncing off it. A black hole gives off no light that escapes, and any light that falls in is gone, so the region itself looks completely dark against the sky.
It is 'black' because no light gets out to reach our eyes.
5not a drain
It is also worth fixing the name. A black hole is not a hole in space, like a drain or a tunnel through something. It is a region of space where a great deal of mass is concentrated, so strongly that gravity there traps light. The mass is real; the 'hole' is only a way of describing what disappears from view.
Not a hole in space: a region where concentrated mass traps light.
a common mix-upA black hole is a hole or tunnel cut into space.
Is a black hole better pictured as a drain in space or as a region with concentrated mass?
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A region with concentrated mass; it is not a hole at all.
So a black hole is gravity winning completely: mass squeezed so tight that nothing nearby, not even light, can get back out.