Later Stages of Solar Evolution
I have a question about the end of the sun. I know that
the energy radiated out is created by the chemical process of converting
Hydrogen into Helium, and then Helium into even heavier elements, but
exactly what happens when this process ceases? I know that the sun will
increase in size and eventually swallow the earth, but what is the
catalyst for this process? Is the energy fueling this change residual
energy from fusion?
Am I right in assuming that the sun does not add any more mass to its body
during this process? That would mean that the average density of the sun
would decrease rapidly in this process, but why does this happen? Thanks
in advance for any help.
There are two forces acting on the sun. The expansion effect due to the
outpouring of energy, and the compaction effect due to the sun's gravity.
The sun's volume is a function of these two counteracting forces.
It can be shown that the increase in mass (which controls gravity) is
about a 1% increase from H to He. It is not a very strong effect. The
greater effect is the difference in output of energy between a H to He
fusion versus a He to heavier element fusion. Thus, as the sun loses its
fuel, as it becomes comprised of heavier elements, the sun should get
smaller in volume.
However as compaction occurs, this triggers more fusion process. Imagine
all the elements that have not fused -because the initiation conditions is
not available- suddenly happening. This would then create a massive
outpouring of energy called a nova.
On the other hand, if this trigger is less sudden stretched over a long
period, then the sun will continue to produce energy at a lower and lower
rate, until presumably it all becomes iron (the most stable element
produced in the fusion process) and the whole mass becomes compressed down
to a tiny fraction of its size.
I leave it up to you to determine which process our sun will take.
Greg (Roberto Gregorious)
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Update: June 2012