What internal structure in Sun-like stars prevents them from utilizing all their hydrogen fuel efficiently?

Answer

A large, non-convective radiative zone

Sun-like stars have a radiative zone where hydrogen fuel in the outer layers is not efficiently cycled into the core, meaning a significant portion remains unused, resulting in only about 10% fuel consumption.

What internal structure in Sun-like stars prevents them from utilizing all their hydrogen fuel efficiently?
astronomyLongevitystarlife