Complex sentence: The second dredge-up occurs in stars with 4--8 solar masses of material, when helium fusion comes to an end at the core, convection mixes the products of the CNO cycle.
Split into simpler sentences: The second dredge-up occurs in stars with 4--8 solar masses of material. When helium fusion comes to an end at the core, convection mixes the products of the CNO cycle.