The early evolution of the sun

Hayashi has predicted that the early contracting sun should be highly luminous and fully convective, if one makes a correct choice of the photospheric boundary condition in constructing solar models in the contracting stages. We have constructed a sequence of models which fully confirm his predictions. The maximum radius of the protosun consistent with gravitational stability is 57 R_solar this has a luminosity of about 450 times that of the sun. As the protosun shrinks its stays fully convective until a radius of less than 3R_solar is reached; the luminosity continues to decrease until a radius of 1.7R_solar is reached. The sun requires about 2 million years to contract onto the main sequence, but this number is very uncertain because of the unsatisfactory state of convection theories. If the primitive solar material has the terrestrial ratio of deuterium to hydrogen, then a further 3 × 10⁵ years is required to burn the deuterium. These numbers are very much less than previous studies had indicated. The degree of lithium burning in the outer convection zone of the sun is highly uncertain owing also to the unsatisfactory state of convection theories.