Re: Do black holes reach a saturation point and become unstable, what happens?

Black holes are strange objects and can't be fully understood in 
everyday terms. The popular description of a black hole is that of a
massive object which has been compressed to huge density such that its
gravitational field is so strong that even light cannot escape. The radius
from which light can just escape, is called the event horizon, and is as
close to the black-hole that we can see. This popular description may go on
to say that black-holes "rupture the fabric of space-time", and have other
bizarre properties which are often counter-intuitive.

One might ask the question, though, what if matter at some point just
won't compress any further?  At first sight it seems that this would
be a way of preventing formation of black holes, with all their strange
behaviour.  However, this doesn't seem likely, for two reasons. Firstly,
to make a black-hole, the gravitational field must be very large, but
the density is not always ludicrously high.  A black-hole of mass 100
billion times the mass of the Sun(about the size of an average galaxy),
for example, would  have an average density (within the event horizon) 
less than air.  The great mass compensates for the modest density.  The
second reason has to do with the details of the General Theory of
Relativity, which is currently our standard theory of gravity.  In GR, the
gravitational field of a black-hole does more than just attract matter in
the conventional way, it actually twists space and time around!  Thus the
fall towards the centre of the hole (the singularity) becomes as 
inexorable as the forward march of time itself.  When a particle reaches
the singularity, its timeline terminates and as far as we know, it ceases
to exist.  

Looked at this way, we can see one reason why a black-hole can't saturate
with matter, since within the hole what was the time dimension has become
a spatial dimension (and vice-versa), newly infalling matter will never
reach any kind of "surface" of pre-existing matter.  It's also true matter
falling into a black hole, causes the hole to grow in size, so in that
sense too,it won't saturate. (A black hole would be perfect for waste
disposal - but don't drop anything in you want to get out again!)

So how much faith should we have in this GR theory which makes these
strange predictions?  In fact, physicists have considerable confidence in
GR, partly because it is a very elegent theory which predicted a number
of phenomena (such as gravitational lensing) before they were ever
observed, but also because in quantitative terms the predictions of
GR are stunningly accurate.  Thus very few physicists doubt that black
holes can exist, and indeed many astronomers think that examples of black
holes have already been found. However the majority viewpoint, rather
ironically, is that GR will ultimately be found to be an imperfect
description of nature, since it doesn't incorporate quantum theory.
A great goal of modern theoretical physics is to find a quantum theory
of gravity.  Black holes are still expected to be a prediction of such
a new theory, but our ideas about the central singularity may well change.

Finally, I'll come back to your opening point, that, according to Hawking,
black holes do emit some energy.  This energy, called Hawking radiation
since he is credited with predicting it's existence in the 1970s, is in
fact a consequence of the quantum theory.  In quantum theory (which is
even more weird than GR, but also stunningly accurate in it's predictions)
even the vacuum of space is pictured as a seething sea of positive and
negative energy.  To cut a long story short, the effect of the black hole
on the vacuum is to suck in some of the negative energy, leaving a glow
of positive energy just outside the event horizon.  The negative energy
will eventually reduce the mass of the black hole, although in practice 
the net inflow of matter and radiation is likely to always be far more than
the small amount of energy emitted. (Since the luminosity goes down with
increasing mass, a very large hole, such as the 100 billion solar mass
monster we considered above, will emit a tiny amount of power, only 
about 10^-50 Watts).