The composition of the sun by mass of hydrogen drops, but the amount of helium grows because helium is heavier than hydrogen.
The percentage by mass of an element in a compound is the percentage the mass of that element contributes to the total mass of the substance.
The percentage by mass of an element depends on the mass of the element.
Heavier elements in equal amounts with less dense elements have a greater percentage mass.
Since the mass of the helium atom is greater than the mass of the hydrogen atom, the Sun's composition by mass of hydrogen drops while that of helium increases.
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