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Mass of a black hole

  1. Oct 16, 2015 #1
    OK, so we know that there is a supermassive black hole at the center of every galaxy. But I want to ask where is the mass of a black hole actually. When the black hole is formed and if there is a supernova or hypernova then much of the mass is ejected out then how can we says black hole has a significant mass. And if the mass is actually present is at the singularity and if yes than how is it possible for such a small point to possess so much mass.
    Please do correct me where I am wrong and thnx in advance for any reply.
  2. jcsd
  3. Oct 16, 2015 #2


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    We don't know.
    A straightforward application of known physics tells us that the mass is concentrated in a very small region in the center - that makes sense and it's also implied by the same equations that predict that the black hole exists at all. However, we don't know just how small.

    It's not likely that the mass really concentrates into a single point of zero size - your intuition that that is impossible is pretty convincing. By far the most likely answer is that some as yet unknown physics comes into play and stops the collapse at some very small but non-zero size. The equations of general relativity predict the same results for all space outside of this small but non-zero volume, so there is no contradiction with what we already do know about black holes.
  4. Oct 17, 2015 #3
    Sometimes the gravitational field of a black hole is referred to as a fossil field, it's the memory of the matter that has fallen into the black hole. See this link

    Regarding the singularity, you could consider the Hayward metric which removes the singularity at around the Planck scale which could be considered a step in the right direction though still hypothetical (which is featured in the following papers)-

    On the Effective Metric of a Planck Star

    Formation and evaporation of non-singular black holes

    Any rotation might remove the singularity as Kerr metric is supposed to reintroduce timelike geodesics within the BH (though this second inner horizon is predicted to be unstable). Virtual particles would also play some part as they become highly energetic the closer the mass collapsed to the Planck scale.
    Last edited: Oct 17, 2015
  5. Oct 17, 2015 #4
    So can I consider that all the mass of the black hole is at singularity or is it present in the entire region inside event horizon of the BH. (I read the second one in a post..)
  6. Oct 17, 2015 #5


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    Most of the region inside the event horizon is vacuum. Whether in a singularity or a small dense area that isn't quite a singularity, the mass is concentrated at the center.
  7. Oct 17, 2015 #6


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    Certainly at any point outside the Event Horizon of a BH, the mass acts like a single point at the center, exactly as it does for ANY celestial object such as the Earth or the Sun. Beyond their surfaces, the mass acts like a point gravity source.
  8. Oct 17, 2015 #7


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    For all practical purposes the event horizon defines the size of a black hole. Since that is the limit to what we can observe, its interior structure is necessarily speculation. The best we can achieve at present is to mathematically limit the parameter space. The Planck density is currently a reasonable alternative to a dimensionless point.
  9. Oct 17, 2015 #8


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    The mass which is ejected from a core-collapse supernova is the envelope of the star. The core of the star before collapse is composed of much denser material than the rest of the star. It's the core mass which winds up inside the event horizon of the black hole after it forms. The core must be greater than about 1.4 solar masses for it to collapse due to gravity, which is also known as the Chandrasekhar Limit.

    If the original star was below about 20 solar masses, a neutron star will form after the collapse of the core, while the rest of the mass of the star is ejected in the supernova event.

    If the original star has a mass between 40 and 50 solar masses, a black hole will form.


    The most massive neutron star which has been observed is about 2 solar masses. The smallest black hole observed is about 5 solar masses.

  10. Oct 18, 2015 #9
    Thanx everyone for the replies.
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