Microturbulence as a third dimension in the G-K giant region of the HR diagram

The structure of the HR diagram in the G-K giant region is complex, because of the funneling effect and of the intersection of evolutionary tracks for different stellar masses. Therefore it is impossible to derive the age of a cool giant from its location in the HR diagram alone. To remove this indeterminacy a third dimension is needed, and the author suggests microturbulence as the appropriate observational parameter. (Auth.)