Why are there very few heavy elements in the matter of the oldest stars in the Galaxy, and, on the contrary

Why are there very few heavy elements in the matter of the oldest stars in the Galaxy, and, on the contrary, in the matter of the youngest stars, their increased content?

The oldest stars formed from a protogalactic gas cloud poor in heavy elements. Massive stars, rapidly evolving, exploded and enriched the gas of the protogalaxies with the heavy elements formed in them. Later generations of stars were formed from substances with a high metal content.

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