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).
