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Question about nucleosynthesis

by manenbu
Tags: nucleosynthesis
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Feb7-12, 04:28 AM
P: 103
I've been reading lately about the origin of the elements and their production in stars and I have a few questions. Let's start by the simple stuff, and correct me if I'm wrong.

When stars form and join the main-sequence they burn hydrogen to make helium.
At a certain stage when the hydrogen in the core is exhausted, depends on their mass, they turn into red giants and begin burning helium and make the heavier elements by carbon burning, neon burning, silicon burning. Elements up to iron can be produced like that.
If the star is massive enough, it will end its life in a supernova and form heavier (actinids, etc) elements in the r-process.
So far so good? I hope so.

I've also read that certain stars can make elements up to Bi by the s-process. Is this occurring in a supernova? If not, then why is it widely cited that Fe is the heaviest element to form in stars when in fact heavier up-tp-Bi elements can form?

Thanks for clearing up my confusion.
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Feb7-12, 01:15 PM
P: 334
You are right about the s-process producing the Bi element. As for the reason concerning the statements you mention having read, I don't know.

The S-process

The net result of this cycle therefore is that 4 neutrons are converted into one alpha particle, two electrons, two anti-electron neutrinos and gamma radiation:
The process thus terminates in bismuth, the heaviest "stable" element. (Bismuth is actually slightly radioactive, but with a half-life so long—a billion times the present age of the universe—that it is effectively stable over the lifetime of any existing star.)


In the red giant stars of the asymptotic giant branch, the s-process (slow process) is ongoing to produce bismuth-209 by neutron capture as the heaviest element to be formed. All elements heavier than it [Bismuth-209] are formed in the r-process, or rapid process, which occurs during the first fifteen minutes of supernovae.[4]

Brackets mine.
Feb7-12, 03:21 PM
Sci Advisor
P: 6,104
Fe is the limit of elements produced during the lifetime of a star undergoing a series of fusion reactions, H -> He -> C, O -> ... -> Fe.

Heavier elements are produced in Supernova explosions. It doesn't stop at Bi. Everything up to and maybe past U are also formed. Supernovae impart so much energy to their constituents that high energy fusion reactions occur.

Feb7-12, 06:25 PM
Drakkith's Avatar
P: 11,997
Question about nucleosynthesis

Saying that the heaviest element that a star can produce is Fe is simply referring to the generation of elements by the fusion reactions in the core. (Which in itself is just simplifying things as isotopes of iron, nickel, zinc, and cobalt are all produced at the same time) The S-Process occurs under a different set of circumstances and depends on the initial content of iron in the star when it is initially formed, as iron is the starting material for the S-process.
Feb9-12, 03:13 PM
P: 1,261
All of the previous comments seem totally right on; but one thing I think is important, is the stability argument. My understanding is that, the primary determining factor is stability. Fe is (approximately) the most stable element (binding energy per nucleon). All elements below Fe tend to undergo fusion to grow towards Fe; elements higher than Fe tend to undergo fission to decay towards Fe.

Nucleosynthesis is a thermodynamic (i.e. stochastic) process, and thus all sorts of crazy stuff is going on. The more stable something is, the more it will be produced and survive. The 's' in 's-process' is for slow (neutron capture), which means that unstable elements/isotopes will have time to decay before capturing further neutrons. S-Process elements do exist, but in low quantities relative to iron.

The 'r' in 'r-process', on the other hand, is for rapid (neutron capture) which can cram neutrons onto nuclei fast enough to reach higher A/Z stable states. This only happens during supernovae (as said above), but is much more effective at producing heavier elements, especially nickel (which ends up being the primary source of optical emission in SN light curves).

In summary: you've got the right picture. Fe is referred to as the highest element produced in stars because its the highest element produced in significant quantities, but up to ~Bi occurs. Supernovae can produce elements up to and past U; but still Fe, and near-Fe elements are the most stable, and thus preferred.

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