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What density does something have to have to be considered a black hole?

  1. Apr 20, 2009 #1
    "Any amount of mass at all can in principle be made to form a black hole if you compress it to a high enough density."

    What is the high enough density?
  2. jcsd
  3. Apr 20, 2009 #2


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    Science Advisor

    This high enough density is the density at which the schwarzchild radius of the object is less than the actual radius of the object.

    If you assume the object to be a uniform sphere, we can write its mass as the product of density and volume:
    [tex]m=\rho V=\frac{4/3}\rho\pi r^3[/tex]

    And substituting into the first equation,
    [tex]r=\frac{2G(\frac{4/3}\rho\pi r^3){c^2} [/tex]
    [tex]\rho=\frac{3}{8}\frac{c^2}{G\pi r^2}[/tex]

    In a completely classical universe.
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