Fully Convective Stars
Protostars collapsing to the main sequence have large luminosities. They are also
very opaque to radiation, once their temperatures are high enough to ionize hydrogen. This
combination usually ensures that energy cannot be transported fast enough out of the protostar
by radiation and its interior becomes unstable to convective motion.
Thus, we expect on general
grounds that protostars collapsing to the main sequence are convective through their entire
volumes. This has an important effect on the motion of the star in the HR diagram, as
explained in the left panel.