I haven't written this Chapter yet, but what it will say is...
The first nuclear reaction in stars is the formation of deuterium from two protons. This reaction is mediated by the weak nuclear force. Its reaction rate is extremely slow - some 18 orders of magnitude slower than if it were an electro-strong reaction (like the subsequent stellar reactions). Consequently, this first reaction is the rate determining step. Specifically, for solar conditions, its rate is such that a proton can expect to survive about 10 Byrs before reacting. This contrasts with the subsequent formation of helium-3 from the deuterium, which takes just a few seconds. Since hydrogen burning accounts for the bulk of the life of a star, this first (weak) reaction determines the lifetime of stars, which, in the case of a star of solar mass, is therefore ~10 Byrs.
Anthropic considerations require stellar lifetimes in the order of billions of years. This has lead some authors to claim that the strength of the weak interaction is fine tuned to provide the correct stellar lifetime.
This claim is examined quantitatively in this Chapter. The claim is found to be spurious. The point is that stars are auto-stabilised systems. Nuclear reaction rates are extremely sensitive to temperature. Hence, even reactions which are intrinsically peculiarly fast or slow can have their rate adjusted over many orders of magnitude by relatively modest changes in temperature. The required changes in temperature will come about automatically, via stellar thermostasis. Thus, if a reaction is proceeding too slowly to maintain the pressure required to balance gravity, further gravitational collapse will occur which raises the temperature and density. This continues until the reaction rate has become high enough to achieve equilibrium.
A better guide to stellar lifetimes is provided by considering the available heat transfer mechanisms. Since the amount of fuel is known, and hence the amount of heat required to be transported over the star's lifetime is known, the lifetime can be estimated by dividing by the rate of heat transfer. From this perspective, the reaction rate is simply obliged to be whatever results in the required rate of heat production. The conditions of temperature and density follow accordingly.
This is, of course, a massive simplification. However it serves to discredit the claimed fine tuning of the weak force as regards stellar lifetimes.
