Light from a distant star cuts through dust like a flashlight beam through fog. Dust holds back blue rays more than red ones, making the star appear redder than it really is. From this reddening, astronomers gauge how much dust lies in the light's path.
In the new work, they used pulsating stars of the RR Lyrae type: their brightness changes with strict periodicity, like a pendulum clock. Knowing the period, one can calculate the star's true color and brightness, then compare that with what we see. The difference reveals how much dust the light encountered. This allowed them to build three-dimensional dust maps for the Large and Small Magellanic Clouds — the two nearest galaxies to us.
The maps revealed that in the dustiest regions, the reddening of light does not follow the general rule. It seems that there the dust grains are larger or differently structured. This means that dust varies in different parts of the galaxies, and distance calculations need to account for this separately.
Now astronomers have a tool to more precisely correct measurements for dust and better understand the interstellar medium in neighboring galaxies. And when new data from the Gaia satellite arrive, the maps can be made even more detailed.
Dust in space is not dirt but tiny solid particles, about the size of smoke from a campfire. They make up only about one percent of the mass of interstellar matter, yet they strongly affect light.