Stars Creating Elements Heavier than Iron

[SYahoo]

I am curious how stars form elements heavier than iron. I have read that it generally occurs in the supernova death of a star or when neutron stars collide but have not read anything much more specific. I have read the wiki link on Nucleosynthesis but it doesn't have much in the way of specifics.

Link to wiki https://en.wikipedia.org/wiki/Nucleosynthesis

 

[Matterwave]

There's not too many specifics because the process is still poorly understood in current day physics. The basic idea is split into two possibilities, the r-process and the s-process. The r-process stands for "rapid" process. Basically, when a star goes supernova, it creates a neutron star core, and during the post-bounce a lot of neutrons are being emitted from this core. The neutron flux is extremely high, and these neutrons bombard the iron atoms or other heavy elements that were present pre-supernova. The "rapid" process says that the bombardment is so rapid that the nuclei absorb many tens to even 100 neutrons in a short period of time. The nuclei are then extremely neutron rich and goes through many nuclear decays (details of which I'm very hazy on) to their final stable nuclei states.

The s-process stands for "slow-process" and this basically says there's a small neutron flux (during when, I'm also hazy about, perhaps after the supernova has occurred. Hopefully someone more knowledgeable can fill us in). Basically here the nuclei absorb one neutron at a time, sporadically beta-decaying into stable nuclei. In this way the nuclei go up the periodic table in a chain of stable nuclei slowly.

I'm not sure of the specific models and their agreement with the current elemental abundances though.

 

[Mordred]

Yeah I used to have an article showing what you described. Wish I kept a copy

 

[Hornbein]

The latest craze is that elements heavier than iron are emitted when a neutron star collides with a black hole or another neutron star. The star's crust is made of heavy metal nuclei. When the stars collide the crust cracks into chunks, and some escape the resulting black hole. That's all I know.

 

[Benit13]

Matterwave basically has it correct; the main sources of elements in the Universe heavier than iron is from the r-process and the s-process. They are split more or less 50:50 between these two processes.

The r-process is still being researched, but observations provide fairly convincing evidence that massive stars are the source. Note that although the r-process is poorly understood, models exist that model high-entropy winds and/or neutrino winds that predict interesting r-process abundance signatures.

The s-process comes from a variety of sources, namely AGB stars and massive stars during helium burning.

For AGB stars, a specific a scenario can occur where a $^{13}$C pocket is formed, which can undergo the reaction $^{13}$C($\alpha$,n)$^{16}$O during a thermal pulse. The neutrons are then absorbed by everything around, but mainly Fe-group nuclei since they are so abundant. The abundance signature created by this mechanism, averaged over all AGB stars of slightly different masses, gives rise to the 'main-component' of the Solar system abundances.

For massive stars, the reaction chain $^{14}$N($\alpha,\gamma$)$^{18}$O($\beta^+$)$^{18}$F($\alpha,\gamma$)$^{22}$Ne creates a fair amount of $^{22}$Ne during helium burning. This $^{22}$Ne then acts as a neutron source due to the $^{22}$Ne($\alpha$,n)$^{25}$Mg reaction. This requires hotter temperatures than the $^{13}$C neutron source to activate, so it doesn't really activate until carbon shell burning. This process gives rise to the 'weak-component', which is effectively a correction to the main component.

Note that both of these processes depend on the metallicity of the star.

There are many other nucleosynthesis processes too, such as synthesis of $^{208}$Pb in lower mass stars (s-process giving rise to the 'Strong-component'), the p-process (proton-captures), $\nu$p-process, weak r-process (low/zero metallicity equivalent of the r-process)... the list goes on! Some of these are very recent developments.