About three-fourths of the baryonic matter in the universe is currently in
the form of ordinary hydrogen, the simplest of all elements. In addition, when
we look at the remainder of the baryonic matter, it is primarily in the form
of helium, the next simplest element. The Sun’s atmosphere, for instance,
contains 70% hydrogen by mass, and 28% helium; only 2% is contributed by
more massive atoms. When astronomers look at a wide range of astronomical
objects – stars and interstellar gas clouds, for instance – they find a minimum
helium mass fraction of 24%. The baryonic component of the universe can
be described, to lowest order, as a mix of three parts hydrogen to one part
helium, with only minor contamination by heavier elements.