When you look up at the night sky, how do you know whether the specks of light that you see are bright and far away, or relatively faint and close by? One way to find out is to compare how much light the object actually emits with how bright it appears. The difference between its true luminosity and its apparent brightness reveals an object’s distance from the observer.
Measuring the luminosity of a celestial object is challenging, especially with black holes, which don’t emit light. But the supermassive black holes that lie at the center of