This passage is excerpted from material published
in 1997.
  Is there a massive black hole at the center of our
galaxy, the Milky Way? The evidence is inconclusive.
Just as the Sun's mass can be determined, given
knowledge of other variables, by the velocity at
which its planets orbit, the mass at the center of the
Milky Way can be revealed by the velocities of stars
and gas orbiting the galactic center. This dynamical
evidence, based on recently confirmed assumptions
about the stars' velocities, argues for an extremely
compact object with a mass two to three million
times the mass of our Sun. Although according to
current theory this makes the mass at the center
of the galaxy too dense to be anything but a black
hole, the relative lack of energy radiating from the
galactic center presents a serious problem. A black
hole's gravity attracts surrounding matter, which
swirls around the black hole, emitting some energy
as it is engulfed. Scientists believe that the amount of
energy that escapes the black hole should be about
10 percent of the matter's rest energy (the energy
equivalent of its mass according to the equation
E=mc2). But when the energy coming from the
galactic center is compared to widely held predictions
based on how much matter should be falling into a
theoretical central black hole, there is a discrepancy
by a factor of a few thousand.