I just learned that the Fraunhofer lines of the Sun's spectrum indicate that the Sun contains various elements other than just hydrogen and helium (for example, Na and Fe) but don't the Sun's p-p chain reactions only involve hydrogen and helium? So where do these other elements came from?
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1$\begingroup$ See astronomy.stackexchange.com/questions/16311/… astronomy.stackexchange.com/questions/32086/… physics.stackexchange.com/questions/7131/… $\endgroup$– ProfRobCommented Mar 21, 2021 at 8:03
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3 Answers
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The Sun is currently turning hydrogen into helium. There are no other nuclear reactions taking place at any significant rate in the Sun. The Sun will not start to make heavier elements until it reaches the tip of the red giant branch in about 7 billion years time.
The elements heavier than helium that are present in the Sun were almost all made inside other stars. These stars lived and died before the Sun was born 4.5 billion years ago. The nucleosynthesis products from these previous stars were recycled in stellar winds, supernovae, kilonovae and novae, and then mixed into the turbulent interstellar medium.
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20$\begingroup$ And of course some of that very same debris never quite collapsed into the sun and was left instead rotating around it, where it ended up... well, look at a mirror. $\endgroup$ Commented Mar 21, 2021 at 12:33
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1$\begingroup$ I guess some minimal amount of material might also be from stars which died after the sun was born. Solar wind probably keeps the small particles at bay, but in billions of years, there will be the occasional unfortunate interstellar asteroid of younger age scoring a direct hit while passing through the system. $\endgroup$– mlkCommented Mar 22, 2021 at 11:59
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Pretty sure from my stellar nucleosynthesis days that the p-p I chain is the dominant form of nucleosynthesis in the sun, but the p-p II, p-p III, and p-p IV chains also occur, just to a much lesser extent. Those will make Be, B, Li.
But, Na and Fe - and other heavier elements - mostly come from the sun not being a first-generation star: Previous supernovae formed those elements and they were incorporated into the sun, just as they were incorporated into Earth.
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4$\begingroup$ Be, B and Li are all destroyed in the solar interior. $\endgroup$– ProfRobCommented Mar 21, 2021 at 15:21
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3$\begingroup$ @PM2Ring - which means there is a small concentration of them at any given time as they are created and then further reacted. $\endgroup$ Commented Mar 22, 2021 at 13:23
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2$\begingroup$ if you're looking at minority fusion cycles in the sun and counting intermediate nucleus that are consumed in later steps: CNO is responsible for ~1.7% of the He produced (more than the p-p III branch), and produces carbon, nitrogen, and oxygen as reaction intermediates. $\endgroup$ Commented Mar 22, 2021 at 14:53
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2$\begingroup$ @JonCuster True, but material in the core isn't visible in the Sun's spectrum, and at this stage of the Sun's life, virtually everything produced in the core stays in the core. When the Sun enters the red giant branch, there will be dredge-up. $\endgroup$– PM 2RingCommented Mar 22, 2021 at 16:39
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2$\begingroup$ OTOH, a small amount of nucleosynthesis does occur in the Sun's outer atmosphere (and throughout the Solar System), due to cosmic ray spallation. $\endgroup$– PM 2RingCommented Mar 22, 2021 at 16:41
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The act of fusion transfers part of atom into energy and the part left over is fused together with Hydrogen to make heavier and heavier elements. Heavier elements create more pressure in the middle of the star which creates more and more heavier elements fusing the many nuclei together. Once the sun starts making Iron(Fe), it dies and collapses which creates more elements. Or explodes if a supernova and makes gold and such. Don't know what a blackhole makes? Maybe fuses time and matter together so it can't be separated from each other.
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4$\begingroup$ Our sun is not massive enough for there. It will fuse hydrogen. At a later stage it can fuse helium. After that fusion will stop in our sun. The processes you mention can happen in other -more massive suns-, but not in our sun and not at the moment. And as such do not explain why they are currently present. Both existing answers already refer to nova and other more massive suns, and they their earlier existance has contributed to the presence of heavier material in our sun. $\endgroup$– HennesCommented Mar 21, 2021 at 19:56
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$\begingroup$ Nor will it ever explode as a supernova. Once nucleosynthesis finishes, it will collapse into an extremely dense, white-hot stellar remnant, and all the gold it currently contains (which is more than all the water in Earth's oceans) will remain trapped there forever. $\endgroup$ Commented Mar 23, 2021 at 0:19