If only some stars become black holes what sets them apart from other stars, is it just size?
To be precise, rather than size, it is mass. Massive stars evolve to a supergiant star after it's main sequence. That's because of it's large mass, which ignites furthermore reactions and after the formation of iron, a supernova occurs. The leftover core may become a neutron star or a black hole, depending on it's mass.
Right, mass, should have been more specific, so giant stars at the end of their life go supernova, this is due to the fusion of Iron in the core sending out a massive shock wave. So what then is so special about Iron, I believe it was the cosmos series by Carl Sagan where I heard that all elements tend to want form Iron, that is they will gain or loose protons or undergo fission or fusion, to attain this state. So if black holes come from supernovas, then could the Iron be crushed to a single dense enough point to be considered a black hole, does Iron have the mass for this?
When a core of iron forms at the heart of the supergiant, and other elements required for fusion exhaust, no more reactions take place. (because iron is a stable element with respect to fusion or fission. The energy needed to trigger an iron fusion reaction is greater than that it gives out while fusing, thus consuming all heat and energy in the star). Now the gravity overpowers the radiation outburst (since there is no radiation outburst), and the star collapses greatly, crushing the core where the protons are fused into electrons, forming neutrons. Thus generated shock wave rips the outer layers apart, blowing the star off in a supernova. The remaining core is not of iron but neutrons which contract to form a neutron star. But, if the initial mass was even more greater (about 30 suns), the neutron star collapses further to form a black hole, a singularity in space.
For a black hole to form, the core remnant must be </= its Swarzschild radius. For a 3 solar mass remnant, the critical size is about 9 Km - re: http://hyperphysics.phy-astr.gsu.edu/hbase/astro/blkhol.html
Size is not the determining factor. The sun will become a red giant yet it lacks the mass necessary to become a black hole. So it will wind up as a white dwarf instead.
Some additional details about your question:
The first collapse into a neutron star occurs because the electron degeneracy pressure can no longer withstand the gravitational force
If it collapses again, it is now because neutron degeneracy pressure can no longer
withstand the gravitational force
What you are left with in a black hole, after a neutron star collapses, is a quark-gluon plasma.
Typically, a Red Super Giant will burn through its hydrogen, helium, carbon, oxygen, neon, magnesium, and sillicon layers, in that order, and will then collapse because no fusion reactions are keeping it from doing so.
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