E. Kyritsis

A galaxy is not just a sprinkle of stars. At the center of many, something shines brighter than billions of suns: hot gas falling into a black hole. The color of this glow reveals what kind of life the galaxy leads — quietly birthing stars or violently consuming matter.

The night forest is full of fireflies, but before, we only saw those that happened to be in the beam of a flashlight. That was the case with ultraluminous X-ray sources: points in other galaxies that shine in X-rays more brightly than a stellar-mass black hole can steadily emit. Telescopes looked at galaxies chosen for other purposes, missing most of the fireflies.