Author: Joseph Lazio <jlazio@patriot.net>

A black hole is any object for which its entire mass M is contained
within a radius 
                        2GM
                    R = ---
                        c^2
where G is the universal gravitation constant (G = 6.67 x 10^-11
m^3/kg/s^2) and c is the speed of light.  An object this compact will
have an escape velocity larger than light so nothing can escape from
it.  (For an object with the mass of the Sun, this radius is 3 km.)

BHs can be divided into (at least) three classes: primordial,
stellar-mass, and supermassive.  Primordial BHs, if they exist, were
formed during the initial instants of the Big Bang.  The initial
Universe was not perfectly smooth, there were slight fluctuations in
its density.  Some of these density fluctuations could have satisfied
the above criterion.  In that case, BHs would have formed.  These
primordial BHs could have a range of masses, anywhere from milligrams
to 10^17 times the mass of the Sun.  Currently, however, there is
little evidence to suggest that any primordial BHs did form.  (In
fact, the available evidence suggests that no primordial BHs formed.)

Stellar-mass BHs are those with masses of roughly 10 times the mass of
the Sun.  These are formed from processes involving one or a few
stars.  For instance, a star more massive than 50 solar masses will
also start to form a iron core.  Unlike a less massive star that forms
an NS during the supernova, though, the iron core becomes so massive
that it collapses to form a BH.  Another possibility for the formation
of a stellar-mass BH is the collision of two stars, such as might
happen in the center of dense globular cluster of stars or two
orbiting NSs.  A Stellar-mass BH is identified typically when it is
orbited by a lower mass star.  Some of the material from the companion
star may be stripped away from it and fall into the BH, producing
copious amounts of radio and X-ray emission in the process.

Supermassive BHs are those with masses exceeding roughly 1 million
times that of the Sun.  These are found at the center of galaxies.  It
is not clear how these form, but it probably involves the accumulation
of many smaller mass BHs, NSs, and perhaps interstellar gas during the
formation of galaxies.  Recent work shows a correlation between the
mass of the central parts of galaxies and the mass of the central BH.
This has led to some speculation at to whether the central BHs form
first and "seed" the formation of galaxies or if there is a symbotic
process in which the central BH and the galaxy are created
simultaneously.

There have also been suggestions of "intermediate mass" BHs.  These
would be objects whose mass is roughly 100--1000 times that of the
Sun.  The suggestions that such intermediate mass BHs might exist
arise from X-ray observations of other galaxies showing strong X-ray
sources not associated with the centers of the galaxies.  Certain
assumptions must be used in relating the X-ray brightness of the
objects to their mass, though, so whether such intermediate mass BHs
actually exist is still somewhat controversial.