Final answer:
In stars with higher masses, nucleosynthesis occurs through nuclear reactions involving carbon, oxygen, and heavier elements, producing elements as heavy as iron and enriching the interstellar medium. This explains why later generations of stars contain more heavy elements.
Step-by-step explanation:
In stars with masses higher than about 8 solar masses, nuclear reactions involving carbon, oxygen, and still heavier elements can build up nuclei as heavy as iron. This process is called nucleosynthesis and it creates new chemical elements. As stars use up their available energy supplies and enter the late stages of their evolution, they eject some matter enriched in heavy elements into interstellar space where it can form new stars. This progressive enrichment explains why later generations of stars contain a larger proportion of elements heavier than hydrogen and helium.