Dylan Nelson

Every galaxy is wrapped in an invisible shell of hot gas, like steam rising from a pot of boiling water. This gas barely glows in X-rays, making it notoriously difficult to measure. Yet it holds the memory of stellar explosions and the galaxy's growth. The new method uses light from a galaxy's bright center.

Galaxies don't hang in a void: they're surrounded by tenuous gas, like invisible steam over a pot of soup. This gas contains oxygen ejected by stellar explosions. Astronomers detect it via dark lines in the spectra of distant quasars: oxygen atoms absorb ultraviolet light, creating dips in the rainbow.