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Now, if stellar core runs on higher temperature, its power output is also higher and so is radiation pressure. To get back into equilibrium, stellar radius (and surface) must increase, to be able to radiate all this power.
The new equilibrium condition is Red Giant.
Because such star is older, and has higher content of heavier elements in its outer core and mantle, complicated ionization - recombination processes take place, resulting in variability (energy is absorbed by ionization, this reduces surface temperature and power output and stars shrinks, which in turn provokes recombination process which releases energy, temperature rises and star expands.....
Variable star condition is temporary and happens only for certain range of stellar masses and composition.
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