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

<p>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 <em>neutronization</em> begins. The protons and electrons actually combine to form neutrons (and neutrinos). This creates a large <em>neutron flux</em>, and so there are now lots of neutrons available for the r-process.</p>

<p>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.</p>

<p>Sources:</p>

<p><a href="https://en.wikipedia.org/wiki/R-process" rel="noreferrer">https://en.wikipedia.org/wiki/R-process</a></p>

<p><a href="http://www.ucolick.org/~bolte/AY4_00/week8/SNeII.html" rel="noreferrer">http://www.ucolick.org/~bolte/AY4_00/week8/SNeII.html</a></p>

CC BY-SA 3.0
2014-09-22T23:04:14.713Z
for the s-process http://www.mso.anu.edu.au/~akarakas/Kodai_Lecture4.pdf
Aaron
CC BY-SA 3.0
2014-09-22T23:06:56.370Z
@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.
HDE 226868
CC BY-SA 3.0
2014-09-20T21:12:55.097Z
2014-09-20T21:12:55.097Z
HDE 226868
<p>The main sources of neutrons for the s-process (can be found on <a href="http://en.wikipedia.org/wiki/S-process" rel="nofollow noreferrer">wikipedia</a> 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<span class="math-container">$M_{\odot}$</span>) 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.</p>
<p>For the r-process, HDE226868's explanation is the consensus explanation.</p>

CC BY-SA 3.0
2014-12-03T21:31:08.713Z
I timed myself at about 16.5 seconds, so the sub-minute figure is definitely correct. . . :-)
HDE 226868
CC BY-SA 4.0
2014-12-03T21:12:18.260Z
2021-10-13T05:23:39.360Z
2021-10-13T05:23:39.360Z
ProfRob
<p>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)</p>

<nucleosynthesis><neutrons>
Free neutrons and stellar nucleosynthesis
stevenvh
# Free neutrons and stellar nucleosynthesis

(Asked by: username_0 on 2014-09-18)

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)

**Tags:** nucleosynthesis, neutrons


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## 2 answers

### Answer by username_1 on 2014-09-20. Score: 5

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>

> **Comments:**
> **username_2**: for the s-process http://www.mso.anu.edu.au/~akarakas/Kodai_Lecture4.pdf  
> **username_1**: @username_2 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.  

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### Answer by username_3 on 2014-12-03. Score: 4

The main sources of neutrons for the s-process (can be found on [wikipedia](http://en.wikipedia.org/wiki/S-process) 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.

> **Comments:**
> **username_1**: I timed myself at about 16.5 seconds, so the sub-minute figure is definitely correct. . . :-)  

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