Most stars, around the size of the sun, fuse helium to form carbon and oxygen, and then fusion stops. why do you think large stars, bigger than 8 times the mass of the sun, are able to fuse carbon and oxygen, forming elements up to iron?

I apologize in advance if this sounds a bit convoluted...

There are two main ways that nucleosynthesis occurs in main-sequence stars. The first, for lower-mass stars, is called the proton-to-proton chain. What happens chemically is  → . For higher-mass stars, the CNO cycle also nets a He nucleus but produces more energy.

When these MS cycles end in a star due to exhaustion of H, the star switches to the triple-alpha process, which relies on all the stored He. This is how elements up to Fe can be fused, after which point, it requires energy to fuse heavier elements (they come from supernovae generally). It produces a lot of C and O but very little of it is converted into Ne and heavier elements. 

The CNO cycle is very temperature-dependent. As I'm sure you know, larger stars are hotter stars. Thus, the biggest stars are able to synthesize the vast majority of their energy through this process and the alpha process. Core fusion increases the atomic weight of elements and reduces the number of particles, which leads to a pressure loss, except that gravitation leads to contraction, an increase of temperature and a balance of forces (this is the hydrostatic equilibrium at work). 

I can try to clarify this if you need that. I just read it over and admittedly it seems kind of messy.

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Well, a and b together would be negative. b alone would also be negative.