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Among the most important achievements of Astrophysics, the discovery that the chemical elements that make up the various celestial bodies and their relative abundance are practically the same throughout the Universe must be included.
This result has been achieved both through indirect analysis of stars and distant galaxies with spectroscopy methods, as well as through direct chemical analysis of terrestrial rocks, meteorites and lunar rocks.
From a quantitative point of view, the most abundant element is Hydrogen (H), which represents approximately 83.9 percent of all the atoms present in the Universe; in second place is Helium (He) with 15.9 percent. All other elements cover the remaining 0.2 percent.
Usually the abundance of the elements is expressed with relationships of numbers of atoms. In the analysis of the chemical composition of the Earth and meteorites, silicon is often chosen as a reference element; in that of the Sun and of the stars in general, hydrogen.
The genesis of the heaviest and rarest elements is explained by admitting the processes of nuclear transformation that occur inside the stars from the lighter elements.
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