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Mass Absorption -V- Black Hole size

  1. Aug 24, 2012 #1
    How much mass would even the smallest of black holes have to absorb (keeping also in mind the radiation lost as it grows) to obtain its size?

    And considering the largest Black holes discovered so far, how much mass would it have had to gobble up to get to that size?

    On the average, how much mass is there in an average galaxy? or even that of galactic dust clouds?

    And lastly, given the rate of evaporation due to (I think its called Hawkings radiation) venting, how long would it take even the smallest black hole to vanish? (or would it after a point not be large enough to lose its own mass due to the venting)?

    Added preponderance: If a black hole is so dense, How could the hawkings radiation escape through the mass? I am not talking gravity, but instead Its packed density.

    Thank you.

    T Spicer
    Last edited: Aug 24, 2012
  2. jcsd
  3. Aug 24, 2012 #2


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    Staff: Mentor

    The mass of a black hole is the mass of whatever it "gobbled up". All known naturally occurring black holes have at least 3.6 solar masses, though they can theoretically be less massive than that.

    I don't know what the average galaxy is, or even if we know, but the Milky Way has a mass of about 1 - 1.5 x 1012 solar masses. (1 solar mass = the mass of our sun)

    A black hole of 1 solar mass would take 2.098 × 1067 years to evaporate, far longer than the current age of the universe. However, it would still be gaining mass as it absorbs radiation from the Cosmic Microwave Background, meaning that the actual time would be a bit longer as the energy of the CMB would need to drop further in order to allow the black hole to give off more energy than it takes in.

    The radiation is released from OUTSIDE the event horizon, so it doesn't have to get through anything. (Unless there is something orbiting the black hole, such as an accretion disk)
  4. Aug 24, 2012 #3


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    A black hole with a mass more than about 10^23 kg [slight less than 1% earth mass} would absorb more energy from the present day CMB than it radiates via Hawking radiation. The smallest possible black hole mass is the planck mass. It would only 'live' for about a planck time before it evaporated.
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