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State of matter before a black hole is formed.

  1. Sep 29, 2010 #1

    Judging by the questions and answers, there are obviously many talented physics and astrophysics professionals here. I am not one of these professionals, nevertheless I am interested in this field. So please excuse my naïve questions and thanks for any help answering them.

    I have been thinking about the formation of black holes. The most commonly presented theory is that a star runs out of burnable fuel, increases in size and then collapses dramatically on itself, forming an ultra-dense core, that is so dense, that it captures light. So far so good. However, the bit I’d like to ask this forum about is the ultra-dense part.

    I mean if I have solid, like a chunk of pure iron or a block of ice, then the atoms are arranged into a fairly rigid pattern. If I apply pressure, in an attempt to make the same element denser, then it may turn into a liquid or even later to a dense gas, till at last the minimal size for the material is reached that the atoms allow. But that is surely not enough to make a black hole, the material has to be much denser. So I’m guessing, that on the subatomic scale, the nuclei of the atoms are being crushed together. But that is just normal fusion isn’t it. That goes on in our sun too, yet our sun is no black hole.

    So is there any knowledge or theories what happens to make matter so dense that a black hole can form? Assuming that black holes really do exist (as is modern thinking) then the state of matter reached must be stable. Otherwise the *crushed* matter would just be converted to energy and dissipated.

    Or put in a simpler way. “Is there an accepted model for the state of matter, just prior to tipping over into the super dense matter required for a black hole?”.

    Thanks in advance

  2. jcsd
  3. Sep 29, 2010 #2


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    It is partly a question of density, but also total mass. For example, the densest star remnants are neutron stars, consisting of neutrons all packed together. Once such a star exceeds a certain mass - without any increase in density, it is a black hole. What happens inside is unknown.

    Stars that are still shining are at a much lower density, the pressure of the photons being generated by the fusion reactions balances gravity and keeps the density in an equilibrium state.
  4. Sep 30, 2010 #3
    Formation of Black Hole is not a question of density. Having enough matter, you can make black hole of any material without compressing it: from iron, water, or even interstellar gas.
  5. Oct 5, 2010 #4
    Hi Dimitry,

    thanks for your answer. I'm still trying to understand what you wrote. What happens with the biggest stars in the universe? If density is not an issue, then these stars would automatically be black holes. What stops these stars from becoming black holes?

    Thanks for any tips.

  6. Oct 5, 2010 #5
    to have a black hole all you need is an event horizon


    The radius is directly proportional to the mass so the density can be as low as you like

    A 100 solar mass star would have to be less than 295 km in radius to form an event horizon
    Last edited: Oct 5, 2010
  7. Oct 5, 2010 #6
    Theoretically that's true, but in practice there are limits to how low you can make the density be, a 100 solar mass star with less than 295 km of radius is quite dense, how big would the radius have to be to make a black hole with interstellar gas density? I'd say really big, kind like as big as the observable universe.
  8. Oct 5, 2010 #7
    Maybe I need to re-phrase my original question. How about the following.

    Does something need to happen at the sub-atomic level for normal matter to form into a black hole?

    I mean, our sun is pretty damm big and so I imagined that amongst the plasma soup there, that the atoms are squashed in pretty close to each other.

    But to get something that is one hundred times as large squashed down to just 295km wide, my poor brain just thinks that something has to be happening at the sub-atomic level. I can't really picture matter in a normal state being that dense.

    So am I just stuck in a traditional world of solids, or is it possible for example, that a sun that is one hundred times as big as ours, can get squashed into a 300km radius, and avoid being a black hole and the atoms are just as normal as usual?

    Thanks again.

  9. Oct 5, 2010 #8

    nothing NEEDS to happen at the atomic level.
    You dont need a singularity to have a black hole.
    all you need is an event horizon.

    the radius of the sun is 700,000 km so
    a star of 1 billion solar masses would have an event horizon of 3 billion km
    this is more than 1000 times the radius of the sun and would therefore be less dense than the sun.

    if on the other hand you are just asking because you want to learn about degenerate matter
    then you have already been pointed to neutron stars and I would suggest the following wiki article:
    Last edited: Oct 5, 2010
  10. Oct 5, 2010 #9


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    Could someone please explain this equation, all the variables and such, as I am not familiar with some of them yet. (If it's not too much to ask that is).
  11. Oct 5, 2010 #10
    all you need to know is
  12. Oct 6, 2010 #11


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    For conan:

    Does something need to happen at the sub-atomic level for normal matter to form into a black hole?

    The answer to this question is "yes' for a stellar mass black hole which is more dense than a neutron star, and "no" for a supermassive black hole which can be formed from ordinary gas.

    I hope this helps.
    Jim Graber
  13. Oct 6, 2010 #12
    For a collapsing neutron star, it's not so much an issue of state of matter but state of spacetime. If we consider the time component only of the Schwarzschild interior metric-

    [tex]d\tau=\left( \frac{3}{2}\sqrt{1-\frac{2M}{r_0}}-\frac{1}{2}\sqrt{1-\frac{2Mr^{2}}{r_0^{3}}}\right)dt[/tex]

    where r0 is the radius of the star and M is the gravitational radius (M=Gm/c2).

    We can say for a regular neutron star (with no or little spin) with a mass of 2.5 sol and a radius of 12 km, the time dilation at the surface is 0.6202 and the time dilation at the centre (r=0) is 0.4303. If this neutron star is accreting from an adjacent star, it will increase in mass, as it increases in mass, it's radius reduces. Once the radius has reached r=3M/4 (2.25 times the Schwarzschild radius), the time dilation at the centre becomes zero, this is the point of no return and the neutron star will collapse into a black hole, not because the state of matter has changed (though by this time, it's most definitely quark matter or similar) but more likely because the nature of the spacetime at the very centre changes significantly.
    Last edited: Oct 6, 2010
  14. Oct 6, 2010 #13
    I think this distinction is superfluous, spacetime is just the geometrical representation of matter, in a broad sense (including mass and energy).
  15. Oct 6, 2010 #14


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    How do you calculate time dilation?
  16. Oct 8, 2010 #15
    Yes there is.....

    You can't travel faster than the speed of light. Once you crush things, the atoms in there start vibrating at near the speed of light. Once the start moving close to the speed of light, they start acting like light, and it turns out that you can't create a stable structure made of light. So if you increase the density past a critical point, there's nothing to keep the thing from collapsing.

    Now what happens *before* all this happens is rather complex, but once you have collapse, then the physics is really simple.

    It's not. What happens in a black hole is that everything gets crushed to zero volume and infinite density. What that means, no one knows.

    Curiously not really. Before you hit the tipping point, you have a lot of messy physics that people don't quite understand. Once you hit the tipping point then things get really simple, because a lot of the messy physics doesn't matter.
  17. Oct 8, 2010 #16
    Interesting things happen at the sub-atomic level in supernova that form into black holes, but you really don't need for that to happen. If you put enough feathers in a small enough space, then light won't be able to escape. The feathers will then collapse, so the weird and interesting things happen sub-atomically *after* you've forms the black hole.
    There are two separate things going on. The black hole forming, and what happens to the matter as the black hole forms.
  18. Oct 8, 2010 #17
    Thanks to all the above for trying to help me understand this. I will try to do some outside reading to make sense of the comments, then probably, come back with more questions.

    Appreciate the effort everyone.


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