Final answer:
In stars, hydrogen diminishes over time due to nuclear reactions, leading to the accumulation of helium and changes in the star's characteristics.
Explanation:
Over time, in a star, the amount of hydrogen decreases as it is gradually depleted due to nuclear reactions occurring in the star's core. This depletion leads to the accumulation of helium, altering the star's chemical composition and affecting its luminosity, temperature, size, and interior structure. As hydrogen fusion decreases and the star evolves, it moves away from the zero-age main sequence on the H-R diagram.
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