Dominique Eckert

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.

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.

An orchestra of hundreds of galaxies performs a symphony in X-rays, but until recently we heard only a general hum. The Coma Cluster is about a thousand galaxies immersed in a cloud of scorching gas at a temperature of one hundred million degrees. This gas glows in X-rays, and its spectrum tells us how it moves.