Yan Gong

In cold clouds of interstellar gas, carbon hides inside carbon monoxide (CO) molecules, like sugar dissolved in tea. But near hot, massive stars, this 'tea' boils: harsh radiation tears the molecules apart, and carbon escapes as lone atoms or ions.

Weighing a cloud of hot gas the size of a million galaxies is no easy task. Astronomers observe how this gas bends light passing by, and from that bending they calculate its mass. But the gas in galaxy clusters is not static: it churns like a boiling pot, and some of the pressure goes into turbulence rather than holding the gas against collapse.