The Solar
Composition

Solar Elemental Abundances

Element Number % Mass %

Hydrogen 92.0 73.4
Helium 7.8 25.0
Carbon 0.02 0.20
Nitrogen 0.008 0.09
Oxygen 0.06 0.8
Neon 0.01 0.16
Magnesium 0.003 0.06
Silicon 0.004 0.09
Sulfur 0.002 0.05
Iron 0.003 0.14
Although we cannot sample the Sun directly, we can learn a great deal about its composition from the pattern of absorption lines in its spectrum (the Frauenhofer lines). The pattern of these lines serves as a set of fingerprints for the elements that are present in the surface of the Sun, and their intensity serves as a measure of the concentration of these elements.

The Solar Abundances

Approximately 60 elements have been thus identified in the solar spectrum. The most abundant are listed in the adjacent table, both with respect to the number of atoms or ions present, and with respect to the total mass of the atoms or ions. The Sun is clearly mostly hydrogen and helium, with only a trace of heavier elements. This is also true of the Universe as a whole: most of the Universe is hydrogen, with some helium, and the remainder of the elements occur only in trace concentrations. In that sense the composition of the Earth is highly unrepresentative of the rest of the Universe.

The Discovery of Helium

The element helium is the second most abundant in both the Sun and the Universe, but it is very difficult to find on the Earth. In fact, helium was discovered in the spectrum of the Sun (the name helium derives from helios, which is the Greek name for the Sun). It was postulated that a set of spectral lines observed in the Solar emission spectrum that could not be associated with any known element belonged to a new element (the Sun is too cool to ionize helium appreciably, so absorption lines associated with helium are very weak). Only after this was helium discovered on the Earth and this hypothesis confirmed (helium occurs in certain very deep gas wells on the Earth).


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