We now know that supermassive black holes are common in the cores of galaxies. For some galaxies, a suspiciously high luminosity in a suspiciously small volume provides the needed smoking gun, but the actual luminosity depends heavily on whether stars and gas are available for the black hole to shear them apart. Other galaxies may have one too, in spite of an unremarkable central luminosity. These black holes may have already eaten all the surrounding stars and gas, leaving no evidence behind. But stars near the center, in close orbit to the black hole (not too close to be consumed), will have sharply increased speeds.

These speeds, when combined with the stars’ distance from the center of the galaxy, are a direct measure of the total mass contained within their orbits. Armed with these data, we can use the back of an envelope to calculate whether the attracting central mass is, indeed, concentrated enough to be a black hole.

—“Death by Black Hole,”
Neil deGrasse Tyson

Identify the two messages Tyson conveys within this passage.

a) Our universe is filled with powerful but fascinating objects.

b) Stars near the center of a galaxy move at very high speeds.

c) Despite the complexity of the universe, humans can understand how it works.

d) Some galaxies have a black hole at their center that eats stars.