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Both the R-process and the S-process of stellar nucleosynthesis rely on absorbing 1 (S) or more (R) neutrons. Where do they come from? I thought there are no free neutrons in stars. (Also the half-life of a free neutron is only 15 minutes, so the source has to be inside the star)

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I have yet to find good information regarding the s-process, so I'll just talk about the r-process here.

The key difference between the two is the conditions under which they take place. The r-process takes place during supernovae, specifically those due to gravitational collapse. In a collapsing star, many of the protons and electrons are squeezed tightly together - so tightly, in fact, that a process called neutronization begins. The protons and electrons actually combine to form neutrons (and neutrinos). This creates a large neutron flux, and so there are now lots of neutrons available for the r-process.

None of this lasts incredibly long, though, because the "explosion" doesn't last long. It soon propels much of the outer material of the star out into space, and the remains become either a neutron star or a black hole. That said, an interesting follow-up might address the relationship between the r-process, neutronization and the formation of neutrons stars in more detail.

Sources:

https://en.wikipedia.org/wiki/R-process

http://www.ucolick.org/~bolte/AY4_00/week8/SNeII.html

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    $\begingroup$ for the s-process mso.anu.edu.au/~akarakas/Kodai_Lecture4.pdf $\endgroup$
    – Aaron
    Sep 22 '14 at 23:04
  • $\begingroup$ @Aaron Great. I'm not going to put it in my answer, though, because it obviously explains the s-process well enough. I can't add anything good to it. Good find. $\endgroup$
    – HDE 226868
    Sep 22 '14 at 23:06
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The main sources of neutrons for the s-process (can be found on wikipedia and in this case it is quite reliable) are carbon 13 and neon 22. These undergo reactions with alpha particles yielding oxygen and magnesium respectively plus a free neutron. The first reaction takes place in low-mass (1-8$M_{\odot}$) stars whilst they are in the AGB phase (hydrogen and helium shell burning); the second takes place in higher mass stars at the end of carbon burning.

For the r-process, HDE226868's explanation is the consensus explanation.

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  • $\begingroup$ I timed myself at about 16.5 seconds, so the sub-minute figure is definitely correct. . . :-) $\endgroup$
    – HDE 226868
    Dec 3 '14 at 21:31

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