Sun composition
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Solar Chemical Composition: Hydrogen, Helium, and Metals
The Sun is primarily composed of hydrogen and helium, with a small fraction of heavier elements known as "metals" in astronomical terms. The most recent and comprehensive analyses show that the present-day solar photosphere consists of about 74% hydrogen (X), 24% helium (Y), and 1–2% metals (Z) by mass, with the exact values depending on the specific study and methodology used Bergemann2025Asplund2021Magg2022+1 MORE. These proportions are derived from a combination of spectroscopic analysis, helioseismology, solar wind measurements, and meteorite data Bergemann2025Asplund2021Magg2022+1 MORE.
Elemental Abundances: Detailed Measurements and Methods
Hydrogen and Helium
Hydrogen is the most abundant element in the Sun, followed by helium. The mass fractions are typically reported as X ≈ 0.74 for hydrogen and Y ≈ 0.24 for helium in the solar photosphere Bergemann2025Asplund2021Magg2022. These values are supported by both spectroscopic measurements and helioseismic data, which probe the Sun’s internal structure Bergemann2025Asplund2021Villante2013+1 MORE.
Metals: Heavier Elements
Metals (all elements heavier than helium) make up about 1–2% of the Sun’s mass (Z) Bergemann2025Asplund2021Magg2022+1 MORE. The most abundant metals are oxygen, carbon, neon, and iron. Recent studies using advanced 3D hydrodynamic models and non-local thermodynamic equilibrium (non-LTE) corrections have refined the abundances of these elements, confirming relatively low values for carbon, nitrogen, and oxygen compared to earlier estimates Asplund2021Scott2014Magg2022+1 MORE. For example, the logarithmic abundances (on a scale where hydrogen is 12) are approximately: log C = 8.46, log N = 7.83, and log O = 8.69 .
Heavy Elements and Isotopes
The Sun contains trace amounts of elements from sodium (Na) to thorium (Th), with their abundances determined through careful analysis of solar spectra and comparison with meteorite data Grevesse2014Scott2014. The agreement between solar and meteoritic abundances is generally good, though some differences exist, especially for volatile and refractory elements Grevesse2014Asplund2021Magg2022. Isotopic measurements, such as those for oxygen, show that the Sun is enriched in the most common isotope, oxygen-16, compared to planetary materials .
Variations and Uncertainties in Solar Composition
Photosphere vs. Solar Interior
The composition of the solar photosphere (the visible surface) is often used as the standard reference for the Sun’s composition, but it is not identical to the interior due to processes like diffusion and convection Bergemann2025Lyons2018. Over time, heavier elements tend to settle toward the Sun’s center, slightly altering the surface composition Bergemann2025Lyons2018.
Measurement Challenges
Determining the Sun’s composition is complex and subject to uncertainties, especially for elements with weak or blended spectral lines . Different models and observational techniques can yield slightly different results, and uncertainties for some elements can exceed 25% . Recent advances in 3D modeling and improved atomic data have reduced these uncertainties for many elements Asplund2021Scott2014Magg2022.
Solar Composition and the Solar System
The Sun’s composition is used as a reference for the entire solar system. By combining solar and meteoritic data, scientists estimate the primordial composition of the solar system, which is crucial for understanding its origin and evolution Bergemann2025Asplund2021Prieto2020. The Sun’s chemical makeup closely matches that of the most primitive meteorites, with some differences likely due to processes during planet formation Asplund2021Magg2022McKeegan2011.
Conclusion
The Sun is composed mainly of hydrogen and helium, with a small but important fraction of heavier elements. Modern analyses using advanced models and multiple observational techniques have provided precise measurements of these abundances, though some uncertainties and discrepancies remain. The Sun’s composition serves as a fundamental benchmark for astronomy and planetary science, helping us understand both our star and the broader cosmos Bergemann2025Asplund2021Magg2022+2 MORE.
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