What percentage of the Sun's mass is hydrogen?

About 73.46% of the Sun’s mass is hydrogen.

Hydrogen is the Sun’s most abundant element by mass, followed mainly by helium at about 24.85%. The remaining fraction consists of heavier elements, including oxygen, carbon, neon, iron, nitrogen, silicon, and magnesium. Astronomers often call these heavier elements “metals,” even when they are not metallic in everyday use.

This percentage describes the Sun’s observable surface composition, especially the photosphere. Composition changes with depth because nuclear fusion in the core converts hydrogen into helium. As a result, the central regions contain substantially less hydrogen than the photosphere, while the outer layers retain a composition close to the Sun’s original material.

Hydrogen fuels the Sun through the proton–proton chain. In the core, four hydrogen nuclei ultimately become one helium nucleus, with a small amount of mass converted into energy. The Sun formed about 4.6 billion years ago from a molecular cloud whose material already contained hydrogen, helium, and traces of elements made by earlier stars.