# I Chemical Elements produced inside the Sun

1. Jun 10, 2018

### Staff: Mentor

Its lifetime is still very short, $(6.7\pm1.7)\cdot 10^{-17} s$.

Edit: Minus sign

Last edited: Jun 11, 2018
2. Jun 10, 2018

### Bystander

Minus? Thanks.

3. Jun 10, 2018

### alantheastronomer

The nuclear burning cores of sun-like stars are convective, which means they have a uniform temperature. While it's true that energies of an isotropic medium lie along a gaussian curve, with few atoms along the high end of the curve, their energies are insufficient to produce the high atomic mass elements to which you are referring.

4. Jun 11, 2018

### DaTario

But the question posed in the OP is relative to the possibility of creation of these elements by any natural process inside the Sun; it is not related to the corresponding lifetime.

I would like to ask if you (alantheastronomer) agree with the following sentence, which is your sentence, quoted above, with a small change (bold):

The nuclear burning cores of sun-like stars are convective, which means they have a uniform temperature. While it's true that energies of an isotropic medium lie along a gaussian curve, with few atoms along the high end of the curve, their energies are insufficient to produce even just one of the high atomic mass elements to which you are referring.

5. Jun 11, 2018

### stefan r

I think G and F stars do not have convective cores. convection is outside of the tachocline. Many K dwarfs are convective to the core. Types A,B,O have core convection.

6. Jun 11, 2018

### phyzguy

I don't think this is true if we are talking about small quantities (like single atoms). The nuclear reactions in the sun produce free neutrons. Once free neutrons are present, you can build up heavier elements through neutron capture, which does not have a Coulomb barrier. So I think tiny quantities of all elements are built up in the sun.

7. Jun 11, 2018

### stefan r

Which fusion reaction makes free neutrons in the sun? Tritium and lithium can do it but where did they come from? Primordial T and Li should have burned before the sun reached the main sequence.

Carbon 13 + alpha could produce neutrons and will in the sun's ABG phase. The temperature dependence of triple alpha is proportional to T10 at 100 million K. The Sun has temperatures around 15.7 million degrees K and much lower density. (I did not find a link for reaction rate of 13C and 4He)
So triple alpha reactions should happen somewhere in the sun within a few orders of magnitude of 1 time per year. I am going to guess that the 13C + alpha neutron source is still less than one neutron per year. In order to create lead from an iron ion the nucleus has to catch over a hundred of them before the hydrogen or 3He gets the neutron.

That is true for the s-process elements. Does not cover Radium, or Uranium. There are also stable isotopes that cannot be explained by the s-process.

I do not know if the neutron temperature effects the s-process. Hydrogen moderating the neutron temperature might effect the odds of creating a lead ion.

8. Jun 11, 2018

### alantheastronomer

The convective zone you are referring to is part of the outer layer of the sun, but the cores of sun-like stars are also convective.

9. Jun 11, 2018

### alantheastronomer

I stand by my statement, except for the following correction, that I meant to say "isothermal" and not "isotropic" - the energies of particles along the high end of the velocity curve are still insufficient to produce EVEN ONE of the higher mass elements.

10. Jun 11, 2018

### phyzguy

The proton-proton cycle fuses protons to deuterium. D + D fuses to He3 + n or T + p with equal probability, and of course D + T gives He3 + n. So, given the size of the sun, many free neutrons are produced.

The question said one atom. There are on the order of 10^60 atoms in the sun. It didn't say anything about which isotopes, how long it takes or what the yield is. so I still think it's true that small quantities of all of the elements will be produced by neutron capture.

11. Jun 11, 2018

### stefan r

This is well stated but wikipedia says the opposite. Here it says this:
Here it says this:
Wikipedia also has gamma virginis as F0 V at 1.7 solar mass on this table. They have Eta Arietis as F5 V and 1.3 solar mass.

12. Jun 11, 2018

### Staff: Mentor

@phyzguy

I think we have partial verification of the claim "all elements". Could you please provide us with a citation that helps to clear this up fully? Thank you kindly....

13. Jun 11, 2018

### alantheastronomer

Responding to post #31 by Stefan r - the demarcation between which stellar masses have radiative and which have convective cores is not as clear cut as Wikipedia would have you believe...However, the division between upper and lower main sequence is due to what nucleosynthetic processes govern core burning, NOT which means of energy transport occurs in the core.

14. Jun 11, 2018

Staff Emeritus
I am puzzled by this as well. Figure 6-12 in Clayton suggests virtually no convection in cores of one solar mass stars.

That's not as big a number as it looks. Uranium requires 237 nucleon additions. What does that mean for the probability of each nucleus to glom on another nucleon? Is it 0.1%? 1%? 10%? No - it has to be 71%, otherwise you don't get all the way to uranium.

That said, I believe every (natural) element is present in the sun, but for another reason: uranium fission. The tiny bit of primordial U-235 in the sun is exposed to neutrons and will fission.

15. Jun 12, 2018

### DaTario

So, due to the preexising Uraniun inside the Sun, which was there even before the star is born, other elements are then produced. This seems to provide a valid answer to the question in the OP, but in order to be complete, if those heavy elements were not present, would the natural ascending (starting basically from H) processes of nucleosynthesis provide all the elements?

16. Jun 12, 2018

### Staff: Mentor

D+D is extremely rare. For every deuterium nucleus there are ~1017 protons around for p+D -> He-3 + photon. It does happen, so some tritium is produced. But then you have the same problem again: Deuterium is extremely rare. p+T -> He-4 + photon is much more likely than D+T -> He-4 + n. I guess a few neutrons are produced via this reaction path, but the total number must be really small.
You don't have to start with hydrogen. The Sun contains all long-living isotopes already from its formation, for most elements you just have to add one more neutron and wait for a decay, or just wait for a decay.
The fraction of induced fission of U-235 should be tiny, but there is spontaneous fission. I said this many posts ago already.
If the Sun would have started with hydrogen exclusively it wouldn't contain uranium by now. Probably not even iron.

17. Jun 12, 2018

### John Collis

The short answer is no. From a fusion point of view Iron is the end of the line, to continue fusion requires energy to be input, which is why the heavier elements are formed by supernovae. Because the heavier elements are found on earth it has been known for quite some time that the sun is at least a second generation star, but it is probably 3rd or even fourth generation.

18. Jun 12, 2018

### Staff: Mentor

There is nothing fundamentally problematic with endothermic reactions.
To study if the reaction happens you need a quantitative statement - how much energy is necessary. Too much for fusion of iron to heavier elements, but the same is true much earlier in the Sun. There is nothing special about iron in the current Sun.

19. Jun 13, 2018

### phyzguy

Well, I don't have a peer-reviewed document, but below is a set of lecture notes:

http://www.uio.no/studier/emner/matnat/astro/AST1100/h06/undervisningsmateriale/lecture-7.pdf

In addition to the D+D source of neutrons I mentioned earlier, this source also talks about several other sources of neutrons in the CNO cycle, which probably have higher reaction rates in the sun. From the above lecture notes:

"The r and s processes There is an exception to this temperature rule if there is a source of neutrons present as neutrons do not feel the Coulomb force. It is possible to distinguish material synthesized in a neutron rich environment from that synthesized in a neutron poor one. s-process (‘s’ for slow) elements are those formed where β-decay is expected to occur before a neutron is absorbed, while r-process 6 (‘r’ for rapid) elements are those formed where new neutrons can be absorbed readily. Sources of neutrons are various, for examples such chains as
He4 + C13 → O16 + n
O16 + 16O → S31 + n

Free neutrons produced in this manner are a way of forming elements beyond the iron peak in binding energy. In ordinary circumstances in stellar cores it is the s-processes that dominate, in extreme situations such as in supernova r-process nucleosynthesis can occur."

I'm not claiming the above reactions are common, or that the free neutrons produced are abundant, but we are talking one atom out of the ~10^59 nucleons in the solar core.

20. Jun 21, 2018

### Ratman

And what are the expected lifetimes of these high-neutron isotopes? Especially compared to the time the nucleus needs to wait to capture another, and another, and another neutron? Getting from lead/bismuth to uranium is hard, and venturing into the realm of superheavies is not very probable even in environment with far higher neutron fluxes, like supernovae. Intermediates needed to clear the gaps in the nuclide chart have too short half-lives. Though it would require checking the numbers to tell how improbable it is for even one in 1059 nucleons throughout all 10–12 billion years of Sun's existence.