The Sun is composed primarily of the chemical elements hydrogen and helium; they account for 74.9% and 23.8% of the mass of the Sun in the photosphere, respectively. All heavier elements, called metals in astronomy, account for less than 2% of the mass. The most abundant metals are oxygen (roughly 1% of the Sun's mass), carbon (0.3%), neon (0.2%), and iron (0.2%).
The Sun inherited its chemical composition from the interstellar medium out of which it formed: the hydrogen and helium in the Sun were produced by Big Bang nucleosynthesis. The metals were produced by stellar nucleosynthesis in generations of stars which completed their stellar evolution and returned their material to the interstellar medium before the formation of the Sun. The chemical composition of the photosphere is normally considered representative of the composition of the primordial Solar System. However, since the Sun formed, the helium and heavy elements have settled out of the photosphere. Therefore, the photosphere now contains slightly less helium and only 84% of the heavy elements than the protostellar Sun did; the protostellar Sun was 71.1% hydrogen, 27.4% helium, and 1.5% metals.
In the inner portions of the Sun, nuclear fusion has modified the composition by converting hydrogen into helium, so the innermost portion of the Sun is now roughly 60% helium, with the metal abundance unchanged. Because the interior of the Sun is radiative, not convective (see Structure above), none of the fusion products from the core have risen to the photosphere.
The solar heavy-element abundances described above are typically measured both using spectroscopy of the Sun's photosphere and by measuring abundances in meteorites that have never been heated to melting temperatures. These meteorites are thought to retain the composition of the protostellar Sun and thus not affected by settling of heavy elements. The two methods generally agree well.
Singly ionized iron group elements
In the 1970s, much research focused on the abundances of iron group elements in the Sun. Although significant research was done, the abundance determination of some iron group elements (e.g., cobalt and manganese) was still difficult at least as far as 1978 because of their hyperfine structures.
The first largely complete set of oscillator strengths of singly ionized iron group elements were made available first in the 1960s, and improved oscillator strengths were computed in 1976. In 1978 the abundances of singly ionized elements of the iron group were derived.
Solar and planetary mass fractionation relationship
Various authors have considered the existence of a mass fractionation relationship between the isotopic compositions of solar and planetary noble gases, for example correlations between isotopic compositions of planetary and solar neon and xenon. Nevertheless, the belief that the whole Sun has the same composition as the solar atmosphere was still widespread, at least until 1983.
In 1983, it was claimed that it was the fractionation in the Sun itself that caused the fractionation relationship between the isotopic compositions of planetary and solar wind implanted noble gases.
