What causes black holes?
Gravity, obviously, extremely strong gravity. Usually, gravity is a fairly weak force–a very weak one, on an atomic scale. It does hold Earth together, but even in the center of Earth, it is believed, atoms and nuclei are not drastically modified, just somewhat compressed. The Sun has about a million times as much mass, and if gravity could pull it together, its force could affect the sun’s atoms (and also release a lot of gravitational energy, as matter falls together; I think Helmholtz estimated 150 years ago, as much energy as the sun produces in 20-30 million years). But it won’t happen soon, since the Sun’s hydrogen is being converted to helium by nuclear processes in its core, and the heat released this way not only keeps the sun hot and shining, it also keeps the gas inside it under pressure, not allowing it to collapse. Some day the sun’s fuel will run out and it will collapse, releasing its gravitational energy and going through some strange changes (red giant, etc), but ulti
