Catherine E. Grant

A giant city of thousands of galaxies is shrouded in an invisible cloud of scorching gas. This gas is like the air in a room: it can be calm, or it can churn with drafts. Astronomers aimed the latest X-ray "thermometer" at such a city — the cluster Abell 1795—and measured how fast the gas moves there.

A globe sits on a table, and a ruler lies beside it: one is curved, the other straight. X-ray telescopes have lived with this mismatch for years. Their mirrors gather light onto a curved surface, like the globe, while the detectors that catch that light have been flat, like the ruler.