Eli5: How do blackholes form and exist?
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Most black holes form from the collapse of very large stars.
Stars exist in a state of balance. The energy they’re producing in their core is streaming outward, which tends to push the star apart, while the pull of gravity tends to push the star’s material together into a more compact form.
At the end of a star’s life, this energy production drops low enough that gravity can start compressing the star into a smaller and smaller area. In some cases, the increased pressure at the center ignites new forms of nuclear reaction, which produce more energy and stop the collapse, but this can only go on for so long.
When the star’s energy production finally fails completely, there are three (well, technically a bit more than this, but I’m simplifying a bit for ELI5 purposes) possibilities:
* If the star is small enough, then the core of the star is compressed until (to simplify a bit) the very electrons and protons are pushed up right against one another. Compressing the star further requires a lot of energy, which the gravity of a small star can’t provide. In this case, the star stops collapsing, the core becomes very rigid, and the outer layers of the star “bounce off” the core and puff off into space, leaving a white dwarf.
* If the star is a bit larger, this compression can go further. The protons and electrons are squeezed so much that (unlike under normal circumstances) it actually produces energy for them to combine into neutrons to “save space”. But eventually the neutrons themselves are pushed up against one another, and if the star isn’t too large, no further compression is possible. As in the previous case, the outer layers bounce off (more violently this time – this is a supernova) and a neutron star (a ball of almost pure neutrons) is left from the core.
* But if the star is big enough, even neutrons being pressed right up against one another isn’t enough to resist the force of its gravity. Instead, the force continues to force the core into ever-more-exotic states of matter (the details are not well understood) and, as far as we know, none of those states is rigid enough to stop the collapse. Gravity wins out, and the core compresses down to a single point whose properties are not known. The compressed core becomes so small and dense that its gravity prevents anything from escaping from the area around it – in other words, it becomes a black hole.
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