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).
Solar Abundances
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.
At least sixty-seven 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.
Because of the way that the Sun formed and how it evolved, we believe that the present
surface abundance of elements corresponds to the original compostion of the entire Sun.
Composition of the Universe
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.