The process of how stars change with time through stellar
nucleosynthesis is known as stellar evolution. This process
takes a very long time, so much so that it is not noticeable in
a human's lifetime. The greatest factor that effect how a star
ages is its mass. A low mass star and a high mass star
evolve in different ways. Stars are originally formed from the
gravitational pull of massive clouds of molecular matter. Once
the cloud gets hot enough nuclear reactions will start and the
process of stellar nucleosynthesis begins. For about 90% of a
stars life it the first step of hydrogen converting into helium
will occur. In this process the helium core will contract and
increase in temperature igniting the hydrogen core around the
inert core. at this point the star will expand and become a red
giant.
www.atronomycafe.net
The next step is for helium to fuse into carbon
and the core will ignite in what is called a helium core
flash. If the mass is bigger this is much gentler and
not as obvious than if the star has a smaller mass. This
whole process is also much shorter than any of the
previous stages a star goes through.
If the star is less than 8 solar masses it will not
reach a temperature hot enough to start burning the
carbon. The carbon core continues to contract until it
is supported by pressure from electrons instead of
pressure of the burning gas and the star becomes a white
dwarf. Eventually the outer layers of the star are
ejected and it forms a nebula.
If the star is greater than 8 solar masses the
temperature will get hot enough to form neon from the
carbon and continue to form heavier and heavier elements
until it forms iron. Iron cannot burn into any other
elements and does not create any more energy for the
star. This lack of energy result in the star collapsing
until the pull of its own gravity. Here is where most
stars will stop contraction of the core and create a
shock wave that will blow the star apart and cause it to
go supernova. However, if the star has a great enough
mass the star will collapse even further as it can be
held by only the pressure of the electrons, resulting in
the creation of a black hole.
www.penny4nasa.org