Stars begin
with the
condensation of interstellar medium under the force of gravity until
the core
has sufficient mass and heat to start fusion reactions.
The primary elements in these young stars are
hydrogen and helium, and the star uses hydrogen fusion reactions to
produce
energy.
Young stars
are generally
found on the Main Sequence of the H-R
diagram, where
they spend about 90% of
their lifetime. Once they begin to run
out of fuel their cores begin to collapse and get hotter while their
surface
expands and cools. Stars the size of our
sun become red giants, and in this stage of their life begin to fuse
helium as
their primary energy source. At the end
of this period (about 8-10% of the star’s lifetime) the fuel for the
star is
spent, and its core rapidly collapses into a white dwarf, with the
external
surface material blowing away into space.
The white dwarf is sustained by heat generated from gravity
until it
fades away.
If a main
sequence star
has sufficient mass (about 10x’s that of our sun) the loss of hydrogen
fuel
results in the formation of a red super giant.
This star has enough heat and pressure to not only fuse helium,
but carbon
and oxygen, and it is here that elements up to iron are formed. When this size star approaches the end of its
fuel it rapidly collapses and explodes, resulting in a supernova. The area left where the star was now contains
a pulsar or black hole.
In the shock
wave
generated by the supernova conditions exist that allow the formation of
the
other elements up to uranium. These are
dispersed through the interstellar medium, ready to begin the cycle of
star
(and planet) formation over again. The
presence of these other elements tells us that our solar system is at
least a
second generation star.