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In a star, the fact that not all the hydrogen is used up, not all the helium is used up etc. is a little counterintuitive.

Shouldn't it be that a star looses the hydrogen layer passively, in other words all the hydrogen eventually fuses into helium forming a helium star which then fuses all its helium into carbon forming a carbon star etc. all the way to iron? In this way you get up to the temperature required to form iron and nickel in stars with a much lower mass and in massive stars you might even have Iron being fused to form heavier metals.

Why isn't this so?

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Why isn't this so?

If the mass of the star is about 1/4 the mass of our sun, then the core's (where fusion occurs) temperature and pressure will never be enough to fuse anything other than hydrogen into helium. The death of this star may be something like a fire dying. The flames disappear for a while, then you may get one or 2 popping back into existence.

Our sun will start to fuse helium into carbon before all the hydrogen in the core is used. In the description of a heavy star, it is described as a onion with its layers with the higher fusion products occurring in the lower layers. They use this model since the iron fusion requires temperature and pressures above a certain level. Neon has lower requirements, but still greater than carbon fusion, and so on. What is not mentioned, is that there is still hydrogen fusion occurring in the center, as well as all the other fusion, helium, carbon, neon, etc.

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  • $\begingroup$ But if hydrogen is being fused that is a little bit of mass loss and wouldn't it be that gravitational pressure would outweigh the outward pressure from the star making the star smaller until all the hydrogen has been fused and then a helium flash occuring and then same thing with helium, carbon, neon, oxygen etc. $\endgroup$
    – Caters
    Oct 15, 2014 at 15:12
  • $\begingroup$ It depends on the mass of the star if a helium flash occurs. There is a question about the death of a star that has some good references. Also, you should do your own search on a star's life and death. $\endgroup$
    – LDC3
    Oct 16, 2014 at 0:59
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The temperature in the core of the star may be sufficient to fuse hydrogen into helium but the temperature near the surface will not be high enough. During the main sequence the stellar core will be transformed into a helium core but the hydrogen in the outer layers will not have undergone fusion and will remain hydrogen.

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