Nucleosynthesis

We have already met the hydrogen burning chain. We now introduce the other nuclear reactions available to stars. The nuclear burning chain begins with hydrogen and ends with iron (Fe).

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The process starts with the simplest element hydrogen (1 proton) and produces more and more complex elements until the chain stops with iron (26 protons). Fe is the most stable nuclear structure (is the easiest nucleus to hold together). Thus, when combining elements up to Fe a little bit less energy is required to hold the product nucleus together than was required to hold the individual fusing nuclei together. This is the energy released in the fusion reaction. If you try to fuse elements more massive than Fe, then the product nucleus requires more energy to hold it together than was contained in the individual fusing nuclei. In this case, you must add energy to make the fusion process go! This is not an efficient way to generate energy.

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In each successive stage of nuclear burning, the reactions involve the ash of the previous step (that is, it involves more massive nuclei with higher charges than for the previous step). Consequently, each step has less fuel and must occur at a higher temperature. To get to the later stages of burning requires a more massive star. Also note that each successive stage of nuclear burning requires successively shorter and shorter amounts of time.

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An interesting point made later is that the dominant source of energy loss from a star after around the stage of Carbon burning is through neutrino radiation and not through photon losses.