Max Planck and Albrecht Unsöld on plasma partition functions and lowering of ionization energy

We summarize the work devoted to plasma thermodynamics and ionization theory performed by two protagonists working at Kiel University: Max Planck and Albrecht Unsöld. First we show that Planck developed, in Kiel, the basis for describing the chemical equilibria between charged particles (ions) and formulated later, in Berlin, the first complete version of the theory of ionization equilibria in plasmas with a convergent partition function. Abrecht Unsöld studied, in Kiel, the influence of plasma density on the atomic partition function, investigating the electric microfield between the ions. He showed that this effect leads to a lowering of the ionization energy going with the cubic root of the density. Finally, we discuss briefly recent theoretical and experimental work on the ionization potential depression.