[Morlaf]

Hydrogen? No1 cares about hydrogen. It's like saying I have rice in my risotto. Helium? yeah that's all stars do: turn H into He. Why, oh why is O the next most abundant element in the universe? Where is Be (Z=4) (following the principal of fuse 2 H--> He, fuse 2 He --> Be?) is there some magical stability regarding Z=8 over Z = 4? please help an amateur understand..

[mjc123]

Answer is yes - see attached graph of binding energy per nucleon. Higher value means more stable; peaks for ⁴He, ¹²C and ¹⁶O. ⁵⁶Fe is highest. Why that is - ask a nuclear physicist.

[Borek]

If you want to delve deeper https://en.wikipedia.org/wiki/B2FH_paper is a landmark starting point.

[Morlaf]

thanks.... :-)

[Enthalpy]

Also, the nuclides result from sets of nuclear reactions in the stars, which can only produce some of them. The temperature far to cool to produce a set of nuclides according to equilibria. It's similar to chemical reactions at a reasonable temperature: the products depend on the reactants, the possible reactions, the catalysts and so on. Only a flame, thanks to its hot temperature, can completely reorganize the atoms and reach a simple equilibrium.

The situation is a bit different in a supernova with hotter conditions. Few years ago, astronomers told supernova made the elements heavier than iron, while fusion in the main sequence stars had to stop at iron or before. But such concepts may have changed meanwhile.

[Borek]

Few years ago, astronomers told supernova made the elements heavier than iron, while fusion in the main sequence stars had to stop at iron or before. But such concepts may have changed meanwhile.

Yes, after https://en.wikipedia.org/wiki/GW170817 there is a general consensus that heavier elements are produced by https://en.wikipedia.org/wiki/R-process in https://en.wikipedia.org/wiki/Kilonova events.

[Morlaf]

thank you all, again! :-)

[Enthalpy]

I just wonder why the model of fast neutron absorption is necessary to explain the abundance of neutron-rich isotopes of the elements.

The very same process of neutron stars merging, that releases enough neutrons for multiple absorptions faster than decays, couldn't release neutron matter that decays in neutron-rich isotopes of the elements? If the merge spats and splashes some matter away, the droplets have to decay since only gravity made neutron matter stable.

[pcm81]

I just wonder why the model of fast neutron absorption is necessary to explain the abundance of neutron-rich isotopes of the elements.

The very same process of neutron stars merging, that releases enough neutrons for multiple absorptions faster than decays, couldn't release neutron matter that decays in neutron-rich isotopes of the elements? If the merge spats and splashes some matter away, the droplets have to decay since only gravity made neutron matter stable.

I may be wrong, but my gut feeling is the following:
To make heavy elements, you have to smash 2 lighter elements and hope that the product is stable nucleus. With heavier elements a larger fraction of possible nuclei is unstable. This necessitates more frequent collisions to produce the smaller fraction of stable nuclei. In turn this requires faster moving donor nuclei, to increase the collision rate. The increase in average speed of donor nuclei correlates with increase of temperature. Eventually you require such a high temperature that nuclei themselves melt apart (beyond plasma). Therefore you have a cap on probability of a successful collision production of heavy nuclei.Meanwhile a neutron absorption path allows you to build up a heavy nucleus at lower temperatures.