Re: minimum size of a black hole

Hi --

Black hole masses are measured by the strength of their gravity on surrounding
stars and gas, and not from the size of the event horizon.

There is a simple relation between mass and the radius of the event
horizon.  For an object of mass M, where M is in units of the solar mass, then
the radius of the event horizon is

   R ~= 3km * M

where ~= means "about equal".  So if the Sun became a black hole, the event
horizon would be 3 km in radius.  A supermassive black hole of mass 100,000
solar masses would have an event horizon of radius 300,000 km, or about  1
light second.

We do not know if there is a minimum black hole mass produced by Nature.
According to our current physics, black holes can be any mass at all, and
there have been suggestions that tiny black holes with masses like that of
protons (!) could have been made in the Big Bang.

We do expect, however, that massive stars can make black holes at the end
of their lives.  These black holes are expected to be about 5-10 solar masses.