why do some stars end up as black holes
I once heard of a Japanese cartoon/movie/show where space pirates threatened to compress the planet Jupiter into a black hole and thus destroy half the Milky Way galaxy. It sounds like an interesting idea, but. even if you could compress Jupiter into a black hole, its mass would remain the same, meaning that Jupiter (now a black hole) would still continue to move around our sun in its same orbit, and Jupiter's moons would still continue to orbit Jupiter as they did before. Many people think that once a star collapses into a black hole, its "sucking power" (its gravitational force) increases. This is simply not the case. Believe it or not, many stars are
less massive after they turn into a black hole than before, when they were shining stars. This is because, at the end of their lives, some stars shed a significant portion of their outer layer into space right before they collapse into a black hole. I've read that if you compressed the Earth to the size of a cherry, its density would be so great that it would turn into a black hole. Assuming that were true and it was actually done, the black hole of Earth would still continue orbiting the sun once every year, and Earth's moon would continue to orbit the Earth about once every 29. 5 days. (Now, the spin of the new black-hole-Earth about its axis would probably be different, but the time it would take to orbit the sun would not change. ) Surprisingly, once the Earth got compressed into a cherry-sized black hole, less space debris would fall into it than before (when the Earth was the size of. well, Earth).
This is because the newly-formed black-hole-Earth would take up much less space (volume) and asteroids and comets would be more likely to miss the cherry-sized (or slightly-larger-than-cherry-sized) volume that, if not missed, would cause the debris to be sucked into the black hole. If the debris missed the black-hole-Earth by even a kilometer (which might seem like a large distance to us, but is very miniscule in astronomical terms), it would be flung off in a different direction, possibly never to return. So, basically, a common misconception people have regarding black holes is that nothing has more gravity than a black hole, and that stars that form into black holes all of a sudden have increased gravity and therefore get more "sucking power. " This is simply not true. Black holes still have the same mass as before (sometimes less, depending on how they're formed), and how much "sucking power" they have is still dependent on how much mass they're made up of.
While it may be true that the most massive stars in the universe are indeed black holes (if you'd even call them stars at that point), there exist many stars that are more massive (and therefore have more "sucking power") than many black holes. So the fact that our galaxy's center probably contains a super-massive black hole doesn't mean that the black hole would suck up any more matter than if it were the same amount of mass which happened to not be in black-hole-form. Without going into great detail, I think you kind of answered your own question. While there are tons of variables that come into play in ultimately determining the fate (black hole vs supernova), the mass of the star is the most important. Basically, there are mass-based guidelines that approximate whether a star will become a white dwarf/supernova/blackhole, etc. For instance, the Chandrasekhar Limit says that a star above 1. 4 solar masses will collapse into a neutron star. At stellar masses greater than around 3 solar masses, neutron degeneracy pressure is not enough to overcome gravity and the star can theoretically collapse to form a black hole. So, these are the theoretical cutoff points that allow for each process to occur. Distinguishing between supernova and blackholes is dependent upon lots of variables.
However, the general idea is that the more massive a star is, the more likely it is to form a blackhole. Given this, I think it also important to understand that supernova and blackholes, while two separate phenomena, are often linked. For example, a black hole is often accompanied by a supernova explosion - once conditions allow for gravitational collapse, the core of the star may collapse to form a black hole, while the outer envelope may acquire enough energy/heating during collapse that it goes supernova and escapes. Although theoretical mass limitations may allow for a certain process to occur, other variables largely control the fate. In addition to the mass, I would imagine that the geometry, mass-distribution, and current stage of the star would play a major role in determining rate of collapse and whether a blackhole, blackhole and supernova, or just supernova will result. Similarly, supernova are often associated with the formation of neutron star cores as well, so it isnt always a case of one or the other. This is an oversimplification, but hopefully it shows you that there isnt always a black and white answer. Sorry if I have rambled let me know if this makes sense or if I can try to help any more!
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