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An X-ray telescope captures light from hot gas around stars, but that light arrives 'blurred'—like a voice over a bad phone line. To make out the words, you need a dictionary of the exact frequencies at which atoms glow. Such a dictionary is written not only with calculations but also with direct experiments.

In a Japanese lab, researchers assembled a device — a trap where an electron beam holds iron and oxygen ions like an invisible cage. When an ion captures energy, it emits an X-ray of a very specific color. The scientists measured these colors with high precision by illuminating the trap with synchrotron radiation — a very bright X-ray flashlight.

It turned out that one of the transitions in the iron ion is slightly weaker than previously thought. It's like adjusting the volume of a single note in a sheet of music. Now astronomers will be able to determine more precisely how much iron and oxygen are in distant gas, and better understand how stars live and die.