The nuclear burning in the cores of stars leads to the production of heavier elements, thereby increasing the number of nuclei like He, C, O, etc. Which nuclei get produced depends on the mass of the star. Lower mass stars (like the Sun) make He, C and O, but carbon burning is never ignited. In higher mass stars, further fusion reactions do take place, leading to the production of heavier and heavier elements.
In the final stages of the star's life cycle, when it ejects its envelope as a planetary nebula or supernova, some of the processed material is carried off into the expanding remnant. Over time, the interstellar medium is thereby enriched in heavy elements.
Sometimes the expanding remnant triggers star formation in a nearby molecular cloud, thereby linking stellar death to stellar birth.
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