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Star to black hole same mass/gravity?

  1. Oct 27, 2011 #1
    Basic question, I know... but..

    When a star collapses and becomes a black hole, is it the same mass and does it have the same gravitational force of the original star? I heard something about it throwing off some of that mass in the process. And perhaps some aspect at the quantum level affects the gravity or mass.

    As an extension of the question, once a BH consumes other masses, does the net mass equal the sum of the individual masses and is the gravitational force the sum of the gravitational forces of the original objects?
  2. jcsd
  3. Oct 27, 2011 #2


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    I don't think there are any BHs that form by simple gravitational collapse (but I could be totally wrong about that) but if they DID then yes, they would have the same mass as a BH as they had as a star.

    Any conglomeration of matter has a gravitational effect. Aside from places inside the radius of the conglomeration, the gravitational effect is indifferent to the radius of the conglomeration.
  4. Oct 28, 2011 #3


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    It is believed possible some stellar mass black holes can form without a core collapse supernova event.
  5. Oct 28, 2011 #4
    Remember that E=mc^2

    When a star collapses to form a black hole a great deal of energy is released, often in the form of a supernova and probably gamma ray bursts as well. Therefore the resulting black hole will have a lower mass than the original collapsar.

    However, its mass will be concentrated in a tiny volume and so its gravitational well will have much steeper sides than that of the original star.

    Similarly, when matter associated with spacetime that flows into a black hole gets strongly accelerated and ripped apart it also emits large amounts of energy and so not all of its mass/energy is added to that of the black hole.
  6. Oct 28, 2011 #5
    Many thanks Blibbler, that's how I imagined it. Would you have any idea how much less net mass and how much less net gravitational force? Again, I imagine it would depend on the mass involved, but given how little mass is involved in releasing nuclear energy, I'm guessing the net result would be perhaps something like 99% of the original.
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