Chris Byrohl

Every galaxy is wrapped in an invisible shell of hot gas, like steam rising from a pot of boiling water. This gas barely glows in X-rays, making it notoriously difficult to measure. Yet it holds the memory of stellar explosions and the galaxy's growth. The new method uses light from a galaxy's bright center.

A campfire that always blazes up to the same temperature — whether you toss a match into the center of a log or light several kindling sticks at the edges. That's roughly how Type Ia supernovas behave: cosmic explosions that astronomers use as rulers to measure distances across the universe.