If you were to observe the interstellar medium over time, you would notice an increase of heavy elements. Why does this happen?
Answer
Stars are formed out of gas and dust clouds of the interstellar medium. Stars lose a lot of their mass back into the interstellar medium when they die, either because they enter a planetary nebula phase and gently lose their outer layers, or they blow up in a supernova explosion. Stars make heavier elements from lighter ones through nuclear fusion,(nucleosynthesis) which produces the energy to make the stars shine. Generations of stars are continually being born and die, and as they do so they encorporate material from the interstellar medium, enrich it with heavier elements, and then send it back into the interstellar medium. This process is called chemical enrichment, and over time the abundance of heavier elements in the universe increases.
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