The Sun is composed primarily of the elements hydrogen and
helium. In
terms of the number of atoms, the
Sun is made up of around 91 % hydrogen,
9 % helium, and less than 1 % everything else (collectively referred to
as metals. In
terms of mass, the Sun is around 74 % hydrogen
and 25 % helium. [Note--a helium atom is around four times as massive
as a hydrogen atom.] The chemical compositions of most other
celestial bodies in the
Universe are roughly the same as the Sun.
 Sun |
 Earth |
The Terrestrial planets
are exceptions to this rule. The
most abundant elements are oxygen, iron, carbon, and other types
of more massive elements such as iron, silicon, and so on.
The fact that the
chemical composition of the Terrestrial planets (and us) is
different from the Sun and is dominated by metals is interesting
because when the Universe was born, it was composed almost exclusively
of hydrogen and helium. There were no very heavy elements present.
Question: So, where did all of the metals come from?
The metals were produced in stars. Recall that the Sun (and many other stars)
produce energy via fusion reactions in their cores. As a consequence,
stars build up heavy elements from light ones.
The process of element production is referred to as Nucleosynthesis.
All of the heavy
elements in your bodies and in Terrestrial planets were processed in
stars, that is, the significant elements in your body were at one
point parts of stars! Even more interesting is the fact that many of the
heavy elements of the Universe were produced during the
death throes of stars
more massive than around 10 times the mass of the Sun.
Because our existence (and our world's properties) relies on
the production of heavy elements in stars, it is an interesting exercise to
determine how stars work and how they produce heavy elements.
