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Q & A: Sizes of Black Holes

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Most recent answer: 08/06/2009
Is there an upper limit to the size / mass of a Black Hole? How big across is it? Is there a lower limit to the minimun size to a Black Hole? Thanks!
- J. Randolph (age 56)
Greensboro, NC, USA
The "size" or "event horizon" of a black hole usually refers to its Schwarzschild radius given by the formula Rs = 2GM/c2, where G is Newton's constant, M is the mass of the object and c is the speed of light.   A photon emitted at a distance greater than Rs will eventually make it out to large distances, although perhaps red-shifted a bit.  However a photon emitted at or less than Rs won't make it out.    So, looking at the formula for Rs there doesn't seem to be any reason why there should be a lower or upper limit to size of a black hole.  Most galaxies will contain one or more black holes.  Our own Milky Way has its own in the constellation Sagittarius.

Curiously enough, you don't need general relativity to derive the formula for Rs.
It was first derived in 1783 by John Mitchell, an English parson, using classical Newtonian ideas.  Here is a web site with more information.

Other sources about black holes can be found at:

Lee H

(published on 08/06/2009)

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