Alejandro Clocchiatti

When a distant star explodes, its light travels toward us for billions of years. Astronomers measure how dim it appears and use that to calculate the distance. Usually, this involves complex models with dark energy and other mysteries.

If a supermassive black hole lives at the center of a galaxy, it heats the infalling gas to incredible temperatures. That gas glows, and we see it as a quasar — the brightest beacon in the universe. But how do you measure the size of the light source itself?