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Why don't black holes become stars again?

  1. Apr 9, 2013 #1
    1st post here - new to the forums - sorry if this is a stupid question.

    I was thinking, if the universe is approximately 98% hydrogen, you would think the matter a black hole consumes would be mostly hydrogen. 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?

  2. jcsd
  3. Apr 9, 2013 #2
    Well, fusion does take place within the event horizon of a black hole. Material falls into it, and fuses under the gravitational pressure.

    You seem to be asking whether there is fusion at the singularity that is at the center of the black hole. We have very little idea of what is going on at the singularity. So let's look at the next best thing, which is a neutron star. The core of a neutron star is mostly neutrons packed cheek to jowl. That means that there are no atomic nuclei, or you could see the entire core as one gargantuan atomic nucleus. Either way, there can't be any fusion. There are no small nuclei, so there is no fusion.

    It is safe to say that there aren't any small nuclei in a black hole singularity. So no fusion.
    Last edited: Apr 10, 2013
  4. Apr 10, 2013 #3


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    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.
    Last edited: Apr 10, 2013
  5. Apr 18, 2013 #4
    I am not certain about this, but I believe (ignoring the idea that we don't actually know what happens inside a black hole past the event horizon) it is because of the pressure inside a black hole that surpasses that with a neutron-degenerate pressure. So any matter falling in probably helps contributes to this mass which gives rise to an even higher pressure. A lot of fusion/interactions between matter does take place in the accretion disk as well, if that means anything.
  6. Apr 18, 2013 #5
    How do you know it doesn't ?-)

    Nothing information can escape from a black hole because of gravity. Light can't escape from a black hole, nuclei can't escape from a black hole, electrons can't escape from a black hole. Not even dark matter can escape from a black hole, because even dark matter is affected by gravity.

    Anything that happens inside the event horizon of a black hole stays inside the event horizon of a black hole. So it doesn't matter whether or not there is fusion inside. The black hole will remain black.

    Energy can "leave" a small black hole by a quantum mechanical effect. Look up "Hawking hole." However, there is no information associated with that entropy. Technically, the energy doesn't really escape from the black hole. There is energy created at the surface of the black hole by some weird process.

    Energy≠Information. If you burn a letter, then the letter releases energy. However, the information inside the letter is destroyed. Hence, even the destruction of the black hole by quantum mechanical effects does not tell you what may still be going on in the universe that was inside the black hole.

    The form of energy has nothing to do with any process that goes on inside the black hole. Hence, even the release of energy at the surface doesn't tell you whether there is a star inside the black hole. So there could be a star, or there could be another black hole, inside the black hole !

    Current physics doesn't really tell us what goes on inside the black hole. There could be a complete universe in there! There could be nothing in there. At present, there is no way to even speculate what happens inside the black hole.
  7. Sep 24, 2015 #6
    Wait i can prove wrong for two statements. The final reaction for a star is into IRON and following the law that no matter can be destroyed, the things that got in to a black hole must be changed to another thing... which is radiation, Hawking radiation, named after Stephen Hawking. I am not sured if puslars are radiation but they are damn bright.
  8. Sep 24, 2015 #7


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    That's partially correct. The actual scenario is that there are many different types of fusion reactions happening in the core of the star at the very end, including some that produce iron. But the silicon burning process that starts with Silicon-28 ends with Nickel-56, not iron. See here: https://en.wikipedia.org/wiki/Silicon-burning_process

    There is no such law. Matter can be, and is, destroyed and created all the time.

    Not true. Matter can easily fall into a black hole and remain matter the entire time.

    A pulsar is a specific type of neutron star: https://en.wikipedia.org/wiki/Pulsar
  9. Sep 24, 2015 #8
    Ok the last one . The spelling is similar but it is the 'light' emmitted from a black hole many times brighter than galaxies
  10. Sep 24, 2015 #9


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    Are you talking about a quasar: https://en.wikipedia.org/wiki/Quasar
  11. Sep 24, 2015 #10
    YES omg how long have i mistaken the two idk
  12. Sep 24, 2015 #11


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    Taking the OP's question at face value: regardless of what is happening inside a black hole, it cannot turn back into a star because the gravity is so intense no matter or radiation can ever escape. The escape velocity is the speed of light, so no expulsion of energy, including a supernova, is ever going to be energetic enough to escape.
  13. Sep 24, 2015 #12
    Thermodynamics would preclude such an event also. The decrease in entropy would be, well, very very, large.
    (Something is tickling at my brain; there is a connection here. I don't expect any interest in my brain tickles.)
  14. Sep 24, 2015 #13


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    Inside the even horizon all possibl particle trajectories lead to the singularity. A collapsing gas cloud that entered the EH would be disrupted and its atomic nuclei would be disassembled. Fusion cannot occur without atomic nuclei.
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