ionization of helium (from He⁺⁺ to He⁺ and back to He⁺⁺).

When the star is in the swelling phase, the partial ionization zone expands, causing it to cool. Because of the decreasing temperature the degree of ionization also decreases. This makes the plasma more transparent, and thus makes it easier for the star to radiate its energy. This in turn makes the star start to contract. As the gas is thereby compressed, it is heated and the degree of ionization again increases.