J. Comparat

A pot of boiling water sits on the stove: the higher the heat, the hotter the water and the more steam escapes. Hot gas in galaxy clusters behaves much the same way. But in small galaxy groups, this "steam" leaks out faster than you'd expect from the temperature alone.

Giant galaxy clusters are cities of thousands of star islands, held together by gravity. Inside them, hot gas churns and glows in X-rays. If a cluster has recently collided with another, it develops clumps of galaxies and distortions in the gas, like ripples on water after a stone is thrown.