Natalie Hell

About 25,000 light-years from Earth, in the constellation Sagittarius, a dense veil wraps around the neutron star GX 13+1. It scatters X-ray photons the way fog scatters streetlight, and for a long time it hid the source's true power from astronomers.

A flashlight shines through fog, and its light changes color slightly. From that change, you can tell what the fog is made of. That's how astronomers study cold gas around X-ray sources: iron atoms in the gas absorb and re-emit X-rays at a very specific energy, like a fingerprint. But there's a catch: if you strip a few electrons from the atom, the line shifts.

Deep in the constellation Cygnus, two stars circle each other. One is a giant, the other a black hole. The giant continuously ejects a stream of gas into space — a stellar wind. When this wind falls under the black hole's X-rays, it lights up like a neon sign. Astronomers captured this light and split it into colors to learn what the wind is made of and how it moves.

The Perseus galaxy cluster is a giant metropolis of thousands of star islands, bathed in an invisible ocean of scorching gas. Like urban smog, this gas preserves traces of all the chemical factories that have operated in galaxies for billions of years: supernova explosions that scatter freshly forged atoms across the surroundings.