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What I do know is that as a star starts to run out of hydrogen it can start to collapse further under gravity. This produces new, higher pressures and temperatures. This allows for more fusion processes and then more again. Soon there are a number of them going on in the star in layers. IIRC the red giant phase of the sun will be produced as these other processes start to occur.
Let me see if I can remember more details off the top of my head.
A main sequence star "burns" hydrogen into helium. The helium is "heavier" and so displaces the hydrogen at the very core: it basically "sinks below" the hydrogen. When the star converts enough of its fuel (hydrogen) into something it can't "burn" (helium) there is an inner helium core surrounded by a hydrogen shell. Only the hydrogen shell is undergoing fusion so the nuclear furnace slows and so produces less outward pressure. With insufficient means of countering the force of gravity, the core begins to contract. This heats up the core and when the termperature gets high enough, the helium can begin undergoing fusion. This realease energy and heats up the overlying hydrogen shell. Thus, becuase of core contraction, the helium core and the hydrogen shell "burn" fuel and the outward pressure increases dramatically. This overcomes the "pull" of gravity and the star - its envelop - expands, creating a giant and eventually a red giant.
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If the star is big enough ( I think it is about 8 solar masses) the whole thing can get to the point of being able to force the fusion to produce iron. However, this doesn't release more energy, it requires more than it produces.
Up to iron (iron-56, I believe) fusion releases more energy than it requires. Above iron it takes more energy to make the element than is releases by fusion.
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From the blown off material we get all the elements heavier than helium that make up the earth and us.
Yes, and such stars require certain conditions, such as having the four fundamental forces of nature to each be within a very narrow range. If even one of the fundamental forces (or several other constants of nature) were only slightly different, then the appropriate type of star would not form, which means there would be no carbon or oxygen, which means there would be no organic compounds, which means there would be no life as we know it. In fact, without the blowing off of elements heavier than helium, there couldn't even be silicon, or aluminum, or sulfur based life.
[This message has been edited by DNAunion, 11-21-2003]
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