Stars start off doing hydrogen burning, making helium. Then they transition to helium burning. From that point onwards, most of the pathway is added helium, so things step up by helium units. There are a fair number of loose neutrons and protons so some side reactions step up by one mass unit, making the miscellaneous atoms of the universe. Eventually this ends at the area of iron and nickel as there is no more nuclear “binding energy” to release. Then the star, if big enough, explodes in a nova event, creating the heavier elements as byproducts of that energetic final collapse. That rare process is also why elements like gold and uranium are rare.
That’s the typical thumbnail sketch of stellar life cycle. But I think it skips over a couple of very important points. I’ll mostly quote from the Wiki for these, as this topic isn’t prone to political screwage.
Have you ever wondered why we have so many rocks dominated by Silicon and Oxygen? Not just quartz, but things like feldspar?
Feldspars (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8) are a group of rock-forming tectosilicate minerals that make up about 41% of the Earth’s continental crust by weight.
Feldspars crystallize from magma as veins in both intrusive and extrusive igneous rocks and are also present in many types of metamorphic rock. Rock formed almost entirely of calcic plagioclase feldspar is known as anorthosite. Feldspars are also found in many types of sedimentary rocks.
Note those elements. Why so much of them? The nickel iron core of the planet makes sense as lots of that is made in end of life stars. But oxygen? Silicon? Magnesium? Calcium? Aluminum?
Blame the stars.
Alpha Importance (not just for dogs)
Stars like to fuse atoms in units of 4 mass number, via alpha particles. A helium 4, ignoring or removing the electrons. So as soon as hydrogen burning starts piling up the helium, alpha particles become the next big thing. But Carbon is more stable than Beryllium so you get a lot of carbon:
Triple-alpha process in stars
Helium accumulates in the cores of stars as a result of the proton–proton chain reaction and the carbon–nitrogen–oxygen cycle. Further nuclear fusion reactions of helium with hydrogen or another alpha particle produce lithium-5 and beryllium-8 respectively. Both products are highly unstable and decay almost instantly back into smaller nuclei, unless a third alpha particle fuses with a beryllium-8 nucleus before that time to produce a stable carbon-12 nucleus. The half-life of 5Li is 3.7×10^−22 s and that of 8Be is 8.19×10^−17 s. When a star runs out of hydrogen to fuse in its core, it begins to contract and heat up. If the central temperature rises to 10^8 K, six times hotter than the Sun’s core, alpha particles can fuse fast enough to produce significant amounts of carbon:
The net energy release of the process is 7.275 MeV.
As a side effect of the process, some carbon nuclei fuse with additional helium to produce a stable isotope of oxygen and energy:
Fusing with additional helium nuclei can create heavier elements in a chain of stellar nucleosynthesis known as the alpha process, but these reactions are only significant at higher temperatures and pressures than in cores undergoing the triple-alpha process. This creates a situation in which stellar nucleosynthesis produces large amounts of carbon and oxygen but only a small fraction of those elements are converted into neon and heavier elements. Oxygen and carbon make up the main “ash” of helium-4 burning.
Note that before the oxygen accumulates, we have a carbon star, with helium.
Then oxygen accumulates. Carbon, and oxygen…
So what we have here is basically a CO or CO2 star. Ever wonder why planetary atmosphere has a lot of CO2? Now you know.
Helium, mass 4, beryllium mass 8 unstable, carbon mass 12, oxygen mass 16, etc.
You would expect neon at 20, Magnesium at 24, silicon at 28.
Carbon Nuclear Burning
This sequence of reactions can be understood by thinking of the two interacting carbon nuclei as coming together to form an excited state of the 24Mg nucleus, which then decays in one of the five ways listed above. The first two reactions are strongly exothermic, as indicated by the large positive energies released, and are the most frequent results of the interaction. The third reaction is strongly endothermic, as indicated by the large negative energy indicating that energy is absorbed rather than emitted. This makes it much less likely, yet still possible in the high-energy environment of carbon burning. But the production of a few neutrons by this reaction is important, since these neutrons can combine with heavy nuclei, present in tiny amounts in most stars, to form even heavier isotopes in the s-process.
The fourth reaction might be expected to be the most common from its large energy release, but in fact it is extremely improbable because it proceeds via electromagnetic interaction, as it produces a gamma ray photon, rather than utilising the strong force between nucleons as do the first two reactions. Nucleons look a lot bigger to each other than they do to photons of this energy. However, the 24Mg produced in this reaction is the only magnesium left in the core when the carbon-burning process ends, as 23Mg is radioactive.
The last reaction is also very unlikely since it involves three reaction products, as well as being endothermic — think of the reaction proceeding in reverse, it would require the three products all to converge at the same time, which is less likely than two-body interactions.
The protons produced by the second reaction can take part in the proton-proton chain reaction, or the CNO cycle, but they can also be captured by 23Na to form 20Ne plus a 4He nucleus. In fact, a significant fraction of the 23Na produced by the second reaction gets used up this way. In stars between 9 and 11 solar masses, the oxygen (O-16) already produced by helium fusion in the previous stage of stellar evolution manages to survive the carbon-burning process pretty well, despite some of it being used up by capturing He-4 nuclei. So the end result of carbon burning is a mixture mainly of oxygen, neon, sodium and magnesium.
The fact that the mass-energy sum of the two carbon nuclei is similar to that of an excited state of the magnesium nucleus is known as ‘resonance’. Without this resonance, carbon burning would only occur at temperatures one hundred times higher. The experimental and theoretical investigation of such resonances is still a subject of research. A similar resonance increases the probability of the triple-alpha process, which is responsible for the original production of carbon.
Notice how that mix of elements is starting to look a lot like the stuff in various rocks…
I’d also note in passing that perhaps investigating similar “resonance” effects could help explain LENR effects.
But we are still short of silicon.
We need further reactions in stars to create it.
Oxygen Nuclear Burning
I’m skipping a step here, but it is basicslly just consuming the neon made. Neon gets turned into oxygen and magnesium.
Why neon, heavier than oxygen, burns first is covered in the oxygen burning wiki
Although 16O is lighter than neon, neon burning occurs before oxygen burning, because 16O is a doubly-magic nucleus and hence extremely stable. Compared to oxygen, neon is much less stable. As a result, neon burning occurs at lower temperatures than 16O + 16O. During neon burning, oxygen and magnesium accumulate in the core of the star. At the onset of oxygen burning, oxygen in the stellar core is plentiful due to the helium-burning process (4He(2α,γ)12C(α,γ)16O), carbon-burning process (12C(12C,α)20Ne, 12C(α,γ)16O), and neon-burning process (20Ne(γ,α)16O). The reaction 12C(α,γ)16O has a significant effect on the reaction rates during oxygen burning, as it produces large quantities of 16O.
Thus making neon not so common in the world.
The oxygen-burning process is a set of nuclear fusion reactions that take place in massive stars that have used up the lighter elements in their cores. Oxygen-burning is preceded by the neon-burning process and succeeded by the silicon-burning process. As the neon-burning process ends, the core of the star contracts and heats until it reaches the ignition temperature for oxygen burning. Oxygen burning reactions are similar to those of carbon burning; however, they must occur at higher temperatures and densities due to the larger Coulomb barrier of oxygen.
Overall, the major products of the oxygen-burning process are 28Si, 32,33,34S, 35,37Cl, 36,38Ar, 39,41K, and 40,42Ca. Of these, 28Si and 32S constitute 90% of the final composition.The oxygen fuel within the core of the star is exhausted after 0.01–5 years, depending on the star’s mass and other parameters. The silicon-burning process, which follows, creates iron, but this iron cannot react further to create energy to support the star.
During the oxygen-burning process, proceeding outward, there is an oxygen-burning shell, followed by a neon shell, a carbon shell, a helium shell, and a hydrogen shell. The oxygen-burning process is the last nuclear reaction in the star’s core which does not proceed via the alpha process.
Note again that we are collecting elements common in rocks, like Calcium and Potassium, along with the silicon.
At about the 1/3 point of the oxygen burn cycle, we basically have a star made of SiO2. In effect, a quartz star. Talk about crystal power!
Or, given the other elements kicking about, I suppose you could call it a feldspar star…
So how does this stuff get to make planets? Why doesn’t it all just end up iron?
Well, sometimes things blow up.
Very massive (140–260 solar masses) population III stars may become unstable during core oxygen burning due to pair production. This results in a thermonuclear explosion, which completely disrupts the star.
We are literally made of, and live on, the ashes of stars. The end result of a quartz star explosion.