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.
The “widely held predictions” mentioned in line 25 are predictions about the
compactness of objects whose mass is millions of times the mass of our Sun
velocities of stars orbiting the galactic center
amount of matter swirling around the object at the center of the Milky Way
amount of matter falling into a theoretical central black hole
amount of energy that should be coming from a black hole at the center of the Milky Way