Sjors Broersen studies supernova remnants by looking at the X-ray radiation emitted by the hot ionised gas, called plasma, of which it consists. The particle densities in these plasmas are very low: about 1 atom cm-3, compared to the 1019 atoms cm-3 in the earth atmosphere. Interactions between the different particles are therefore rare and changes in one part of the plasma take a long time to be communicated to the rest of the plasma. The plasma is therefore said to be ‘out of equilibrium’. Broersen studies these non-equilibrium effects to learn more about plasma physics at the extreme conditions present in supernova remnants. In addition, he uses it to show that two puzzling supernova remnants, RCW 86 and Kepler, exploded in an environment heavily influenced by its progenitor system.