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How to stop a black hole forming?

  1. Dec 30, 2015 #1
    Hello there! I just made this account today because I figured this place would be a good way to get some answers to things that have bugged me for years.
    I'm not particularly sure how this would even work, so bare with me here.
    There is a critical density at which a black hole forms, which should never be possible for a planet to my small knowledge. My question is, if there were a planet hundreds of timers larger than our sun, would it collapse into a black hole? If a large core of it were hollow with a structure built within to prevent it caving in, would that prevent it collapsing under it's own mass? And how would it affect the Gravity you felt standing on this behemoth? Could man even stand on something so large or would the gravity simply crush us into a puddle?
    I was thinking that the sheer distance from the centre of gravity might have some sort of effect on the forces exerted on us, but I'm probably totally wrong.
    Thanks for taking a look, and any answers I get!
     
  2. jcsd
  3. Dec 31, 2015 #2

    Drakkith

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    Staff: Mentor

    Any solid object the size of a star or larger isn't going to be a planet. There's simply no way to prevent the interior from heating up to millions of degrees and igniting fusion during the initial formation of the object.

    Also note that there's a difference between the mass of an object and its physical size. I'm not quite sure which one you mean when you say 'larger'. I'm assuming from the rest of your post that you mean the physical size of the object and not its mass.

    If we assume we are dealing with an artificial structure, then quite possibly, yes.

    It depends on the mass of the object and the radius of the inner and outer surfaces. But it's certainly a reasonable idea. After all, a Dyson Sphere, a giant spherical shell that surrounds a star, is essentially what you're talking about.

    https://en.wikipedia.org/wiki/Dyson_sphere
     
  4. Jan 12, 2016 #3
    There is a hypothetical situation in nature that can stop a black hole from forming, for a while at least. A blitzar would be a neutron star that spins fast enough so that its centrifugal force temporarily prevents it's collapse into a black hole. As far as I know none have ever been directly confirmed. I think it's an interesting notion, though. Maybe speed of rotation could figure into your theoretical megastructure, for what it's worth.
     
  5. Jan 12, 2016 #4

    mfb

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    2016 Award

    Staff: Mentor

    There are no planets larger than our sun, but we can look at artificial objects: you can slowly assemble a planet by lowering mass onto the surface to avoid fusion. If you do that, the planet will first become larger. At some point, the core pressure is so high that the matter gets compressed significantly. This allows more matter to get closer to the core, where it is compressed as well, and so on - increasing the mass lets the planet shrink and (with sufficient mass) become similar to a white dwarf, roughly the size of Earth for the mass of sun. Add more mass and it will get smaller, until you reach 1.4 times the mass of the sun, then it quickly collapses to a neutron star, with a diameter of something like 10 kilometers. Add even more mass and at roughly 2 times the solar mass you get a black hole.

    The average density of sun is ~1/3 the density of Earth, a "planet" with an Earth-like composition but the size of the Sun would collapse.

    You can reach higher masses by making large hollow structures, potentially with rotating elements to take some of the forces.
    For a spherical mass distribution, forces on the outer surface scale with M/R2. Increase the radius by a factor of 100 and you can increase mass by a factor of 10000. Note that the density goes down by a factor of 100 then, so the structure has to change (get more light-weight) to increase the radius while keeping surface gravity reasonable.
     
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