What does interstellar dust really look like? A black hole reveals its composition.

Using high-resolution X-ray spectra from Chandra and XMM-Newton, astronomers dissected the dust between Earth and a black hole called GX 339-4. They measured absorption features from iron, oxygen, silicon, and magnesium simultaneously—an unprecedented combination—and determined that the dust grains are made of magnesium-rich pyroxene (a silicate mineral) plus metallic iron. This reveals what cosmic dust is actually made of along our galactic neighborhood.