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Astronomer107 and the surface gravity of a black hole

  1. May 18, 2003 #1

    marcus

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    Astronomer107 asked:

    "How can I figure out the g of a 10 solar mass black hole? If the sun is a certain "g" then can I multiply it by 10 to get the g of a 10 solar mass black hole?"

    Many people offered comment and suggestion but no one addressed this question.

    The radius of the event horizon of an ordinary (nonrotating uncharged) black hole is given by Schwarzschild's formula

    R = 2GM/c2

    The acceleration of gravity at that radius is given by the classical formula GM/R2.

    This simplifies to

    c4/4GM.

    If you plug in values for c, G, and M, this will allow you to find out.
     
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  3. May 18, 2003 #2

    marcus

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    Astronomer 107, you were also asking about the surface gravity of the sun. It is NOT true that you can multiply the surface gravity of the sun by 10 and get the surface gravity of a 10-solar-mass black hole.

    The way to find the survace gravity of the black hole is the formula in the preceding post:

    c4/4GM.

    The way to find the surface gravity of the sun is to evaluate
    the classical formula

    GM/R2

    where M is the mass of the sun and R is the radius.
    The radius is 7E8 meters.
    You must square this which gives 49E16 square meters.
    The figure of 7E8 meters which you say you got from some
    book is just fine! It is sufficiently accurate. Go ahead and calculate. Before, you simply forgot to square the radius.

    You are to be congratulated for wanting to know the surface
    gravity g of a black hole. It is a famous result of Stephen Hawking that the temperature of a black hole is proportional to its surface gravity. In fact in the units people like Hawking use the temp is simply (and amazingly enough)
    g/2pi

    So you are right on the threshhold of finding out for yourself the temperature of the socalled "Hawking radiation" of a 10 solar mass black hole (however do not get excited the large mass holes have low temp and it is the very little ones that are really hot :smile: so the temp of a 10 solar mass one is maybe not all that interesting)

     
    Last edited: May 18, 2003
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