The Importance of Rotation in Stellar Evolution

Eddington's assessment (1) of the rate of circulation in meridian planes in the Sun, produced by its rotation, is reviewed, using a first-order perturbation theory. The resulting velocity is found to be of the order of 10^–10 cm./sec., as compared with Eddington's figure of $$2\,\times\,{10}^{-4}\,\text{cm./sec}.,$$ a reduction of importance when considering the amount of mixing of material in the Sun during its lifetime. The theory is applied to stars in general, and the equatorial rotation velocity necessary to provide a significant rate of stirring in a star is found in terms of its magnitude and effective temperature. These rotation rates are found to lie within the range actually observed for early-type stars, but are in excess of those for the later types. In view of the widely held opinion that giants are stars with non-uniform composition, the distribution of rotation rates among the stars thus assumes an added interest. The rotation rate can similarly govern the possibility of local exhaustion of hydrogen in the core, with its consequent effects on evolution.