How are stars and black holes born?
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Most of the interstellar dust is hydrogen gas. The current dust also has heavy elements from the remains of ancient stars that died and created heavy elements. So that part mostly goes into planet formation, but of course it increases the forming star’s metalicity.
But most of the gas is hydrogen. That hydrogen can end up accumulating around a point which will be the gravitational center of the forming solar system. As more and more hydrogen accumulates the force of gravity gets stronger. This pulls the hydrogen towards the middle compressing it, accelerating it and heating it up. Near the center this creates a very hot and high pressure environment and in that environment hydrogen fusion can start. For smaller stars its going to be pp fusion and for large stars its a CNO cycle. You can read up on them its not that complicated. But the point is that hydrogen plasma fuses and makes helium which release energy.
When that happens this energy in the form of radiation (light) will exert an outward pressure and that pressure holds against gravity and keeps the star from collapsing. Because the energy output of the star keeps it from collapsing under its own gravity, the mass of the star also defines its temperature. More massive stars must burn hotter.
As the star is fusing its hydrogen in the core it “burns away” this fuel leaving helium. When the star runs out of hydrogen to fuse, the energy output decreases so it cant hold against gravity, the core gets compressed. Gravity accelerates the particles heating them up and the pressure is also increased creating a more extreme environment. Now helium can start to fuse. The star starts to release more energy pushing away its outer layers. For a smaller star this is the start of the red giant phase. As gravity compresses the core the star starts to fuse heavier and heavier elements, until it reaches iron. Iron fusion doesn’t release energy so gravity takes over. For a red giat the outer layers get spread out and the shrinking core remains. The iron atoms reach their maximum density and that holds against gravity. We now have a white dwarf and it will cool and turn into a black dwarf. (The material of a white dwarf can be more intricate, like carbon maybe in the for of diamond.)
For a more massive star, the death is more violent. As heavier and heavier elements fuse they produce so much energy that it blows off the outer layers, thats a supernova explosion. Once that happens that incredibly massive core starts to collapse and not even the atoms can hold gravity back this time. As protons and electrons are accelerated by gravity they turn into neutrons. Neutrons are fermions so the Pauli exclusion principle applies stating that no to fermions can occupy the same state. This is neutron degeneracy pressure which holds against gravity. The result is a stellar core that is as dense as a nucleus, made from neutrons. Its a neutron star.
For an even more massive star with a core that is at least 3 times the mass of the sun not even neutron degeneracy pressure can hold back gravity. The usual explanation is that the rapid collapse forces neutrons to occupy vastly different momentum values which means they can occupy the same positions. So the formed neuron star collapses further and there isn’t any other mechanism that can hold gravity. So all the material theoretically shrinks into a point of infinite density. But this is where General Relativity starts to break down so we dont really know whats actually going on after gravity wins against the neutrons. We are left with an event horizon. A black hole.
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