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.

According to the passage, the dynamical evidence referred to in lines 9–10 supports which of the following?

Recent assumptions about the velocities of stars

Widely held predictions about the amount of matter a black hole will engulf

The existence of an extremely dense object at the center of the Milky Way

The contention that too much energy is coming from the mass at the Milky Way’s galactic center for that mass to be a black hole

The conclusion that a compact object of two to three million times the mass of our Sun is too dense to be anything but a black hole


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