azhelton69 said:
So you have a black hole, it's consuming a ton of hydrogen, squeezing and condensing it, heating it up. So why doesn't fusion start and the black hole become a star again? You have enough gravity. You have I'm sure enough hydrogen, The gas has got to be heating up quite a bit being packed so densely. Is this not how stars are formed, yes it is. So what is preventing fusion from taking place in a black hole?
Thanks.
First, let's look at the fusion process in a a normal star.
A gas cloud collapses in on itself, heating up until the temperature and pressure becomes high enough for nuclear fusion to begin, using hydrogen as the fuel source. For the most massive stars this process lasts a few million to a few tens of millions of years, stopping when the hydrogen in the core runs out. Then the star begins to 'collapse' in on itself again. This further increases the pressure and temperature in the core. These massive stars have enough mass that their temperature gets high enough to ignite not only the next fusion process, helium fusion, but multiple ones after that. After each element is used for fuel the star collapses again, getting hotter and denser as time goes on.
This all ends once a star has undergone silicon fusion, whose end product is Nickel. Nickel has one of the highest binding energies per nucleon of any element, and fusing nickel with itself does NOT release energy. It actually takes energy. So when these massive stars run out of silicon to burn, their life is over. Once the silicon runs out the star again starts to collapse again, raising the temperature and density of both the core and the area around the core. This area surrounding the core eventually reaches a point where silicon fusion happens there too, which creates more nickel. All this nickel piles up in the center of the star until the pressure is so great that the repulsion of electrons can no longer hold the core up against it.
This is the absolute end for the star. A chain reaction starts, with electrons combining with protons to form neutrons, a process which removes energy, and thus outward pressure, from the core. This removal of outward pressure causes further collapse, and an extremely energetic process known as a Supernova occurs, blowing the outer layers of the star out into the rest of the universe and leaving only the small core behind.
Now, depending on how big the star was before the supernova, you can get one of two things. Smaller stars become Neutron Stars. For more massive stars the core itself has such a great density and mass that the gravitational pull it has forms an event horizon, and thus the core becomes a black hole.
The key there is the event horizon. Once this even horizon forms, we can no longer 'see inside' it. Nothing can escape it, not even light itself. If a black hole moves through a giant gas cloud, sucking up hydrogen as it goes, it is actually very likely that during the process of spiraling down into the black hole the hydrogen does indeed undergo fusion to some extent. But, remember what I said above about the neutron star. The protons and electrons that formed the elements no longer exist. They have turned into neutrons. Neutrons do not undergo fusion like protons do and they can't form nuclei by themselves. So when all this matter reaches whatever point inside the black hole where it might be able to collect and form a very very dense star, it can no longer undergo fusion. And that's assuming that there isn't a singularity at the center of the black hole. If there is one, then you cannot have a star within the black hole because no matter ever piles up on itself to form one.
However, either way, nothing gets out of the black hole.
