<p>There is very little mixing in the core of the Sun, where the stratification is fixed by radiative (rather than convective) heat transfer. The heavier helium does "fill the core", but takes about 12 billion years to do so, during which time, the concentration of helium gradually increases.</p>

<p>During its main sequence lifetime, most of the energy generation takes place in the central 20% of the Sun, where approximately 600 million tonnes of H is turned into He every second. (You get this by dividing the solar luminosity by <span class="math-container">$c^2$</span> and note that the pp H-burning chain is 0.7% efficient.) If we take this as a good average value over the main sequence lifetime (the Sun was fainter in the past, but will be brighter in the future), we calculate that <span class="math-container">$2\times 10^{29}$</span> kg of H is turned into He, which is 10% of the solar mass, and about 13% of its initial H.</p>

<p>Once the Sun evolves away from the main sequence, it burns more hydrogen in a shell around the core, and the convective envelope can mix fresh material into this shell. Given that the Sun is expected to leave behind a <span class="math-container">$\sim 0.5 M_{\odot}$</span> C/O white dwarf at the end of its life, then at least half of its mass will have been processed by nuclear reactions (and probably more, since some processed material is lost in the solar wind during the red giant phase).</p>

CC BY-SA 4.0
2020-04-02T23:39:44.800Z
Thanks. I only need to know how the temperature just outside radiative zone gets high enough. I thought that could only happen in the core.
Bingohank
CC BY-SA 4.0
2020-03-30T07:24:13.980Z
2020-03-30T07:24:13.980Z
ProfRob
<p>How is hydrogen supplied to the core of the sun? Shouldn't the radiative zone prevent this? Shouldn't heavier helium fill the core? How much of the hydrogen can the sun fuse?</p>

CC BY-SA 4.0
2020-03-30T00:17:05.060Z
It would be best to edit this down to one question.
Bob516
CC BY-SA 4.0
2020-03-30T07:26:22.147Z
I disagree, the 4 questions are intimately connected.
ProfRob
CC BY-SA 4.0
2020-03-30T18:51:09.613Z
@RobJeffries  the very last one is kinda separate, I think
Carl Witthoft
<the-sun><core><hydrogen>
Replenishing hydrogen in the core of the sun
Bingohank
# Replenishing hydrogen in the core of the sun

(Asked by: username_0 on 2020-03-29)

How is hydrogen supplied to the core of the sun? Shouldn't the radiative zone prevent this? Shouldn't heavier helium fill the core? How much of the hydrogen can the sun fuse?

**Tags:** the-sun, core, hydrogen

> **Comments:**
> **username_1**: It would be best to edit this down to one question.  
> **username_2**: I disagree, the 4 questions are intimately connected.  
> **username_3**: @RobJeffries  the very last one is kinda separate, I think  

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## 1 answers

### Answer by username_2 on 2020-03-30. Score: 22 | Accepted answer

There is very little mixing in the core of the Sun, where the stratification is fixed by radiative (rather than convective) heat transfer. The heavier helium does "fill the core", but takes about 12 billion years to do so, during which time, the concentration of helium gradually increases.

During its main sequence lifetime, most of the energy generation takes place in the central 20% of the Sun, where approximately 600 million tonnes of H is turned into He every second. (You get this by dividing the solar luminosity by $c^2$ and note that the pp H-burning chain is 0.7% efficient.) If we take this as a good average value over the main sequence lifetime (the Sun was fainter in the past, but will be brighter in the future), we calculate that $2\times 10^{29}$ kg of H is turned into He, which is 10% of the solar mass, and about 13% of its initial H.

Once the Sun evolves away from the main sequence, it burns more hydrogen in a shell around the core, and the convective envelope can mix fresh material into this shell. Given that the Sun is expected to leave behind a $\sim 0.5 M\_{\odot}$ C/O white dwarf at the end of its life, then at least half of its mass will have been processed by nuclear reactions (and probably more, since some processed material is lost in the solar wind during the red giant phase).

> **Comments:**
> **username_0**: Thanks. I only need to know how the temperature just outside radiative zone gets high enough. I thought that could only happen in the core.  

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