Why, to estimate the relative abundance of chemical elements in the Universe, it is necessary to consider their abundance

Why, to estimate the relative abundance of chemical elements in the Universe, it is necessary to consider their abundance in the atmosphere of the Sun, but not in the atmosphere of the Earth?

There are two factors:
a) the gravitational attraction of the Earth is not enough to keep light gases – hydrogen and helium – in its atmosphere;
b) the chemical composition of the solar atmosphere is close to the composition of the matter of the entire Sun, which is considered as a typical representative of the stellar population of the Universe.

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