Complex sentence: For higher mass stars, carbon collects in the core, displacing the helium to a surrounding shell where helium burning occurs and the pressures are lower and the mass not supported by electron degeneracy; hence it can expand and the energy production be moderated.
Split into simpler sentences: For higher mass stars, carbon collects in the core, displacing the helium to a surrounding shell where helium burning occurs. In that layer, the pressures are lower and the mass is not supported by electron degeneracy; hence it can expand and the energy production be moderated.