Here's why:
* Stars are born from collapsing clouds of gas and dust. These clouds are primarily composed of hydrogen and helium, with trace amounts of heavier elements.
* Stars fuse hydrogen into helium. This process releases energy and creates a star's light and heat.
* As stars age, they fuse heavier elements. Over time, stars can fuse helium into carbon, carbon into oxygen, and so on. These heavier elements can then be incorporated into new stars or planets that form later.
Therefore, older stars have more heavier elements than younger stars because they have had more time to fuse lighter elements into heavier ones.
However, there is a caveat:
* The universe is constantly evolving. The very first stars were born in a universe almost entirely made of hydrogen and helium. These stars lived and died, leaving behind heavier elements that were then incorporated into later generations of stars. This means that the amount of heavy elements available to form stars increases over time.
So, while older stars individually have more heavy elements, the *average* amount of heavy elements in the universe has increased over time. This means that younger stars, formed from the enriched interstellar medium, will generally have more heavy elements than older stars formed earlier in the universe.
In summary:
* Individual older stars have more heavier elements than younger stars because of their longer lifetime of fusion.
* Younger stars, on average, have more heavy elements than older stars because the universe has been enriched with heavier elements over time.
