Size of Hydrogen-Burning Core
The size of the core hydrogen-burning region depends on
whether the star produces energy by the PP or CNO process. Because the CNO process is so
sensitive to temperature, it tends to be confined to a much smaller region than the PP process,
which has strong temperature dependence but much less so than the CNO cycle.
Since the CNO cycle is dominant for stars heavier than the Sun and the PP process dominates for stars
less massive than the Sun, the hydrogen fusion cores tend to be much more compact for massive stars.