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.

Previously, astronomers manually removed starlight and separated close spectral lines, like a careful cook separating yolk from white. This is time-consuming and sometimes leads to errors. The new tool works differently: it looks only at a few bright "color stripes"—hydrogen and oxygen lines, and immediately delivers a verdict.

Accuracy is 83%, and for quiet galaxies—96%. Moreover, the instrument doesn't need to know the distance to the galaxy or calibrate brightness. A narrow slice of the spectrum suffices, so the method works even for the most distant objects, which appear as faint dots.

This way, astronomers can quickly sort thousands of galaxies and find rare active nuclei among them, like a sorter who, with a single glance at an apple's color, determines whether it's ripe or not.