By measuring how fast these stars were moving, he could prove if there really was a black hole at the center of an active galaxy.

I picked a galaxy nearby which is called NGC 1068, an active galaxy, which meant that it probably had a supermassive black hole in it, at least, that's what we wanted to prove.

To be certain that the stars were moving unnaturally fast in NGC 1068, Dresslar wanted to compare them with stars in a normal galaxy without a black hole. Stars circling around a weak center of gravity would move at half the speed. So for this comparison, he chose the very ordinary galaxy next door to us, Andromeda(仙女座), with a quiet, inactive center like our own.

To measure the speed of the stars in these two very different galaxies, Dresslar used an instrument called a spectroscope. This looks at the changing pattern of light coming from stars as they rotate around the galaxy core. The spectroscope shows the center of the galaxy as a white band and the movement of stars around the core is traced by a dark, vertical line. If the stars of the galaxy's center are circling slowly, then the dark band would show hardly any change, but if they are traveling at great speed, whizzing towards and away from us either side of a supermassive black hole, then the dark band should show a sudden shift across the center of the galaxy.

I would expect to see some rather rapid change in this dark line so that there'd be a very big change in the speed from one side of the galaxy to the other, very suddenly, right over the center and that would show that the stars were moving very rapidly in the center of the galaxy because of the influence of the great mass in the center, the supermassive black hole.

Over the next a few nights, Dresslar measured the speed of the stars in NGC 1068 and in Andromeda.

spectroscope: an apparatus for producing and recording spectra for examination

whiz: move quickly through the air with a whistling or whooshing sound