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Difference between BH's and BH's?

  1. May 2, 2005 #1
    I am wondering what the fundamental differences are between small Black Holes and more massive Black Holes, besides from one is smaller than the other :)

    Event Horizon differences? Does a smaller black hole spin quicker than a more massive black hole? If so what is the gravitational effect of this?

    Also, are densities of BH's always the same (infinite density?), just with differing radius'?
     
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  3. May 2, 2005 #2

    chroot

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    The more massive black hole has an event horizon with a larger radius. The rotation of a black hole is not related to its mass.

    The "density" of black holes is an ambiguous quantity. If you mean the mass inside the event horizon divided by the volume inside the event horizon, the density is never zero. If you mean the density of the classical singularity, it's always infinite. The singularity has zero radius; infinite density implies zero volume.

    - Warren
     
  4. May 3, 2005 #3

    SpaceTiger

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    Observationally, supermassive and stellar mass black holes seem to behave in a very similar manner, both exhibiting apparent accretion disks, winds, and jets. Obviously, the smaller and less massive black holes will be able to accrete less material per unit time, so they'll be less luminous.

    Another interesting difference between the two is the magnitude of the tidal forces. Even upon nearing the event horizon, a human being could survive the tidal forces of a supermassive black hole, while a less massive one would tear him/her to shreds.
     
  5. May 3, 2005 #4
    Can a BH ever be overwhelmed by too much matter per unit of time?
     
  6. May 3, 2005 #5

    SpaceTiger

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    Excellent question, yes it can! The more matter that falls into the black hole, the more radiation it emits. We know, however, that radiation carries momentum, so if there's too much of it, it will tend to push the matter back out away from the black hole. The limit to the luminosity at which an object can shine with before blowing away the infalling matter is known as the Eddington Limit. It applies also to stars, which will tend to blow away their outer envelopes if shining too brightly.
     
  7. May 3, 2005 #6
    How exactly do blackholes which have such a great gravitational field spit out x-rays or anything? I just can't figure out how something with such a great pull could even get something to spit out of itself? Are x-rays not affected by the gravitational pull? I thought they would be since even light gets sucked into blackholes...
     
  8. May 4, 2005 #7

    chroot

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    The x-rays are not emitted by the black hole itself (aside from Hawking radiation, black holes cannot radiate anything); the radiation is emitted from the accretion disc, an orbiting disc of infalling matter which surrounds the event horizon. The black hole's great tidal forces stretch and pull the infalling matter around, heating it until it generates thermal x-rays.

    - Warren
     
  9. May 4, 2005 #8
    Wow thats amazing! What is Hawking radiation by the way?
     
  10. May 4, 2005 #9

    chroot

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  11. May 4, 2005 #10

    Chronos

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    It looks like the original question remains so I will toss a few crumbs. Black holes are typically assigned one of three size classifications: Supermassive [the ones found in galactic cores] stellar [like Cygnus X-1] and primordial black holes [the smallest]. Supermassive black holes are thought to have formed through mergers of smaller black holes. Stellar black holes, believed to be the most common species, form when an extremely massive star goes supernova and the remnant left behind is too massive to stabilize as a white dwarf or neutron star. They can also form if a neutron star accretes too much mass to remain a neutron star. The smallest, and most exotic species is the primordial black hole. These are theorized [originally by Stephen Hawking] to have possibly formed shortly after the big bang due to localized overdense regions in the primordial soup. These are very different in the sense they were formed by extreme pressure instead of gravitational collapse. Theoretically, small such black holes [say the size of Mount Everest] might be detectable today as they evaporate due to Hawking radiation. No such evaporations have been confirmed to date - although it has been hypothesized this could be the source of certain types of gamma ray bursts.
     
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