Advanced Shell Burning
As more massive
stars go through advanced burning stages, shell sources for elements heavier than hydrogen can
form.
In the most complex situations a star can have both a core source and one (or more) shell
sources burning simultaneously.
The following figure illustrates several stages of shell
burning that a heavier star may go through.
The regions shown in red represent fusion regions. Note that in step 3 there is both a helium core source
and a hydrogen shell source, and in step 4 there are two shell sources (one hydrogen and one helium) and
no core source.