The Helmholtz-Kelvin Time Scale for Stars of Very Low Mass.

Assuming that the contracting stars in convective equilibrium evolve vertically downward in the H-R diagram, a simple expression for the Helmholtz-Kelvin time scale tHK is derived. Application of this expression to stars of mass M < 0.1M0 shows that these stars contract to a radius of about 0.1R0 in a time scale of approximately 1 billion years, while the earlier estimates, based on horizontal evolution, gave a time scale tHK greater than a hundred billion years.