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Name: Lee
Status: student
Grade: 9-12
Location: SC
Country: N/A
Date: 5/9/2005

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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