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The distant galaxy TXS 2005+403, located in the constellation Cygnus, behaves like a lighthouse whose beam shines through fog. Light from its active nucleus, a region where a supermassive black hole heats infalling matter, crosses clouds of charged gas in our Galaxy on its way to us. These clouds are restless: they constantly churn like boiling water in a pot. When radio waves from the galaxy pass through such turbulence, they are slightly deflected and scattered, creating a flickering pattern on the sky.

Astronomers used a network of radio telescopes spread over thousands of kilometers to resolve this pattern with unprecedented clarity. Observations were made from 2010 to 2019 at frequencies from 1 to 5 gigahertz. It turned out that the scattered signal does not blur into a uniform blob, as expected, but breaks into distinct bright and faint bands. Such a pattern arises only when the plasma is turbulent, not merely uniformly "fogging" the light. Moreover, the picture remained stable for all nine years — meaning the gas clouds along this line of sight barely change.

This finding turns active galaxies into handy "flashlights" for probing the Galaxy. Previously, such studies used pulsars — rapidly rotating neutron stars with very stable emission. But pulsars are few and not visible in all directions. Active galaxies are far more numerous and brighter. Now, knowing how their light is distorted by turbulence, astronomers can correct images and better resolve, for example, the surroundings of the supermassive black hole at the center of the Milky Way.

Light from TXS 2005+403 has traveled for billions of years, but the last few hundred years of its journey were through our own Galaxy. And it is this final stretch that left its mark — a shimmering pattern that now helps us study the invisible turbulence of interstellar plasma.