The stellar system genesis & supernova cycle

[swampwiz]

AIUI, the early universe, after the Great Recombination, was a fairly, but not absolute, isotropic cloud of mostly H, with some He. Eventually regions of the cloud coalesced into stars, and if the mass was big enough, the temperature & pressure in the star core was sufficient enough for H in the core to fuse into He, and then if the mass was bigger yet, once the H was used up in the core, the He fused into C & O, and if bigger yet, the C & O fused into Al, Mg & Si, and finally, if bigger yet, the Mg & Si fused into Fe (the exact elements are more diverse with near element numbers having some component, but these are the main ones); this process works because fusing small nucleons together liberates energy, but not after Fe, owing to it having the lowest binding energy per nucleon.

Once the Fe was used up, there was no energy liberation to "puff up" the star, and it underwent a gravitational collapse that resulted in a rebounding supernova explosion; in this supernova the nucleons were so violently colliding with each other, that all the rest of the elements were created at this time, leaving a nebula cloud that was mostly the H that was not in core (and hence remained as hydrogen), but a small amount of all the other elements from the core. The reason why rocky planets have so much of the elements listed above is because they happen to be at intermediate steps of the fusion cycle; everything else is more or less orders of magnitude rarer (e.g.,"rare-earth" metals). Finally the "precious" metals are the ones that chemically bind with Fe (i.e., not a true "chemical bond") such that most of it gets dragged down to the core with the Fe, and therefore are further orders of magnitude rarer. Metorites that are the result of fractured planets will have compositions in line with a typical rocky planets; the dinosaur-killing meteorite had the precious metal Ir as part of it, so it was from the core of the fractured planet.

So after a while, gravity makes regions of the nebula coalesce, with the hydrogen mainly going to the center, creating a new star, with all the other elements being outside the center, making planets; the fact that Fe is the final step in the fusion process explains why there is so much of it in rocky planets (especially the core). The new star then continues on in the same process until that cycle ends with a supernova, starting a new cycle. Of course, this only happens if the star's mass is large enough - if it's too small the stellar cycle ends with a dwarf star that is the old core simply radiating out heat from its high temperature (i.e. from the fusion) - and there still is part of the star's mass that remains as a neutron star or black hole. In all of this, since only a small part of the star's mass is the core, the non-core remains (mostly) H, and so cycles can continue on & on many times, albeit not forever.

 

[sophiecentaur]

@swampwiz That reads largely OK, I think. I can't actually find a question or a 'but' in there, though.

I have only one comment about what you say, though

So after a while, gravity makes regions of the nebula coalesce, with the hydrogen mainly going to the center, creating a new star, with all the other elements being outside the center, making planets;

You seem to be implying that no heavy elements will find their way to the core of a star that's formed from a secondary nebula. Would you say that there is a basic mechanism to cause this? Something to do with angular momentum, perhaps? I assumed that the same proportion of elements would end up at the core than in the disc around the protostar - although the elements don't seem to be uniformly distributed there (after a settling time). Light elements driven out by solar flux and temperature, for instance?