<p>TL;DR: the main sequence lifespan of the sun can be increased by a factor of 12.2.</p>
<p>Perhaps the most complete astrophysical analysis of stellar engineering for extending Earth habitability is Martin Beech's book <em><a href="https://www.springer.com/gp/book/9780387681283" rel="nofollow noreferrer">Rejuvenating the Sun and Avoiding Other Global Catastrophes</a></em> (2008).</p>
<p>In order to maintain the biosphere the sun's interior need to be mixed (in order to avoid the red giant phase) and material needs to be removed (in order to avoid a too great luminosity).</p>
<p><strong>Material removal</strong></p>
<p>Removing material is tricky because the potential energy at the Sun's surface is <span class="math-container">$U_\odot = GM_\odot/R_\odot=$</span> 1.9e11 J/kg. While just 2e-6 of the mass-energy embodied in the matter and 0.3% of the potential fusion energy it is still cumbersome. If a Dyson sphere was used at perfect efficiency the maximal mass extraction rate would be <span class="math-container">\begin{equation}
\dot{m}=\frac{L_\odot}{U_\odot} = \frac{L_\odot R_\odot}{GM_\odot} \approx 2.0061\cdot 10^{15} \text{kg/s},
\end{equation}</span>
a typical mass for a minor asteroid per second. The timescale to disassemble the sun using just solar luminosity is <span class="math-container">\begin{equation}\tau_{disassemble}=\frac{3GM_\odot^2}{5R_\odot L_\odot} = 18.852\text{Myr}.\end{equation}</span> Long before this time changes in pressure would reduce luminosity, making this estimate an underestimate by about an order of magnitude. I get 556.93 Myr, a bit more than Criswell's 300 Myr.</p>
<p>Another idealized model is that the lifting process fuses hydrogen (using the <span class="math-container">$p-p$</span> process) to power further lifting. In this case each kilogram of lifted mass produces <span class="math-container">$6.45\cdot 10^{14}$</span> J, allowing lifting of 3380 kg more fuel. This could of course speed up arbitrarily if there was technology for it.</p>
<p>I am leaving out <em>how</em> to do it. Beside Criswell's classical proposals (speeding up rotation, sucking off plasma using magnetic fields etc.) Beech suggests a stream of ramscoops or a stellar wrap could do it. This is not really key to the question.</p>
<p><strong>Luminosity decrease</strong></p>
<p>Luminosity can also be decreased by increasing non-thermal core pressure, perhaps through strong magnetic fields or rapid internal rotation. This would extend the main sequence lifetime as <span class="math-container">\begin{equation}\tau_{NTPS}\sim \tau_{MS} \left(1-\frac{p_{NT}}{p_T}\right )^{-7.5}.\end{equation}</span> A 10% increase over current pressure would double the sun's main sequence lifetime and about halve the luminosity at hydrogen exhaustion. Achieving this is nontrivial given the current solar structure (the magnetic fields are generated in the convective outer envelope), but Beech notes that were the mass reduced below about half the current mass the modified sun would be full convective and magnetic fields would also produce a higher <span class="math-container">$p_{NT}$</span> in the core.</p>
<p>Another way of reducing luminosity is increasing the stellar opacity <span class="math-container">$\kappa$</span>. In order to reduce the luminosity by a factor of 100 (and the surface temperature by a factor of 10) the opacity needs to be increased by a factor of 100. However, Beech notes that merely adding heavy elements will not help since they increase mass and hence luminosity. While one can envision &quot;dialysis&quot; where helium is removed and replaced by larger <span class="math-container">$Z$</span> elements this appears complex, and there are no local readily available sources of enough high <span class="math-container">$Z$</span> material in the solar system.</p>
<p><strong>Mixing</strong></p>
<p>Engineering enhanced mixing before the mass is removed is nontrivial. Impactors and lasers simply do not penetrate deep enough to matter. Beech suggests employing a sub-stellar mass black hole oscillating through the sun to mix it. If the hole is small, the time until absorption is long.</p>
<p><strong>Result</strong></p>
<p>With mass-loss and complete mixing Beech finds the main sequence lifetime increased up to a factor of 12.2, with reasonable factors between 4 and 6.</p>
<p>Beech's scenario has the problem that the late stage sun while having an acceptable luminosity at Earth's orbit will be far hotter, releasing significant UV radiation. The end result is in many ways similar to a blue straggler star.</p>

CC BY-SA 4.0
2020-02-13T18:20:50.340Z
2022-11-03T23:51:46.000Z
2022-11-03T23:51:46.000Z
Anders Sandberg
<p>It has been proposed before that we could use this technique to remove hydrogen from the Sun to lower it's rate of fusion and extend it's life, so it doesn't fry our planet.</p>

<p>I am wondering how much more life could we possibly get out of the Sun doing this? I am thinking we will want to keep the Suns luminosity constant, perhaps lower than it is now even, it used to be only 70% but perhaps that is too low without us having to have too much carbon dioxide in the atmosphere for comfort (1000 ppm is the limit before we start getting affected iirc). </p>

<p>Anyway, in order to to that I believe the star lifting would have to be done regularly over the future lifespan of the sun so a very long term project. It has to be regularly tuned so to speak as the core depletes it's fuel supply and collapses a little, we will have to remove a little more material to restore balance to keep the same power output.</p>

<p>Does anyone know how to figure out how long we could keep doing this? </p>

<p>There will then of course, come a time when we can't remove more fuel from the sun because the core will have exhausted all accessible hydrogen by then and will have to resort to helium fusion. </p>

<p>Or can we keep the suns output constant even with helium fusion? Or do we by then have to consider radically different methods, like somehow removing the spent fuel from the core and replacing it with the hydrogen we removed the previous billion(s?) of years prior?</p>

CC BY-SA 4.0
2020-02-12T08:38:24.707Z
Nobody can do this. This is science-fiction. Maybe your question is better suited for world-building.
AtmosphericPrisonEscape
CC BY-SA 4.0
2020-02-12T10:00:13.193Z
Beech's book walks through the astrophysics of it and gives numbers.
Anders Sandberg
CC BY-SA 4.0
2020-02-13T05:14:25.433Z
It'd be honestly far more energy efficient to just send humans to other star systems, and not worry about the fate of Earth over such long timescales. Given we have about 1 _billion_ years or so until the Sun becomes too hot for Earth to be safe anymore, even a "slow boat" mission to another system, at, say, 10,000 years of travel time, is nothing, and very much physically doable even if not exactly with _today's_ technology.
The_Sympathizer
CC BY-SA 4.0
2020-02-13T07:42:01.537Z
I'll be voting to re-open if it closes. This is equivalent to saying what mass loss rate from a star like the Sun would result in it having constant luminosity.
ProfRob
CC BY-SA 4.0
2020-02-17T04:29:38.657Z
An occasional, thoughtful [gedankenexperiment](https://en.wikipedia.org/wiki/Thought_experiment) never hurt science, broke the internet, or Stack Exchange for that matter. Since it can be (and has been quite well) answered, I agree, ***leave it open!***
uhoh
CC BY-SA 4.0
2020-11-23T05:51:29.237Z
It's a very good question about stellar astrophysics. To me, it's not about practicality or engineering solution, it's about understanding how the starts work. How to tweak a star in an imaginary experiment to get rid of radiative, non-convective zone? And--do such stars exist? etc.
kkm mistrusts SE
CC BY-SA 4.0
2020-11-23T05:57:18.170Z
And at the 70% the luminocity, AFAICR, about 3 bya, the Earth was able to keep liquid water only because of its atmosphere mainly consisting of CO2 and water vapor.
kkm mistrusts SE
<star><the-sun><stellar-astrophysics>
How much more life could the Sun acquire via star lifting?
DennisCA
# How much more life could the Sun acquire via star lifting?

(Asked by: username_0 on 2020-02-12)

It has been proposed before that we could use this technique to remove hydrogen from the Sun to lower it's rate of fusion and extend it's life, so it doesn't fry our planet.

I am wondering how much more life could we possibly get out of the Sun doing this? I am thinking we will want to keep the Suns luminosity constant, perhaps lower than it is now even, it used to be only 70% but perhaps that is too low without us having to have too much carbon dioxide in the atmosphere for comfort (1000 ppm is the limit before we start getting affected iirc).

Anyway, in order to to that I believe the star lifting would have to be done regularly over the future lifespan of the sun so a very long term project. It has to be regularly tuned so to speak as the core depletes it's fuel supply and collapses a little, we will have to remove a little more material to restore balance to keep the same power output.

Does anyone know how to figure out how long we could keep doing this?

There will then of course, come a time when we can't remove more fuel from the sun because the core will have exhausted all accessible hydrogen by then and will have to resort to helium fusion.

Or can we keep the suns output constant even with helium fusion? Or do we by then have to consider radically different methods, like somehow removing the spent fuel from the core and replacing it with the hydrogen we removed the previous billion(s?) of years prior?

**Tags:** star, the-sun, stellar-astrophysics
> **Comments:**
> **username_1**: Nobody can do this. This is science-fiction. Maybe your question is better suited for world-building.  
> **username_2**: Beech's book walks through the astrophysics of it and gives numbers.  
> **username_3**: It'd be honestly far more energy efficient to just send humans to other star systems, and not worry about the fate of Earth over such long timescales. Given we have about 1 _billion_ years or so until the Sun becomes too hot for Earth to be safe anymore, even a "slow boat" mission to another system, at, say, 10,000 years of travel time, is nothing, and very much physically doable even if not exactly with _today's_ technology.  
> **username_4**: I'll be voting to re-open if it closes. This is equivalent to saying what mass loss rate from a star like the Sun would result in it having constant luminosity.  
> **username_5**: An occasional, thoughtful [gedankenexperiment](https://en.wikipedia.org/wiki/Thought_experiment) never hurt science, broke the internet, or Stack Exchange for that matter. Since it can be (and has been quite well) answered, I agree, ***leave it open!***  
> **username_6**: It's a very good question about stellar astrophysics. To me, it's not about practicality or engineering solution, it's about understanding how the starts work. How to tweak a star in an imaginary experiment to get rid of radiative, non-convective zone? And--do such stars exist? etc.  
> **username_6**: And at the 70% the luminocity, AFAICR, about 3 bya, the Earth was able to keep liquid water only because of its atmosphere mainly consisting of CO2 and water vapor.  

---

## 1 answers

### Answer by username_2 on 2020-02-13. Score: 11

TL;DR: the main sequence lifespan of the sun can be increased by a factor of 12.2.

Perhaps the most complete astrophysical analysis of stellar engineering for extending Earth habitability is Martin Beech's book *[Rejuvenating the Sun and Avoiding Other Global Catastrophes](https://www.springer.com/gp/book/9780387681283)* (2008).

In order to maintain the biosphere the sun's interior need to be mixed (in order to avoid the red giant phase) and material needs to be removed (in order to avoid a too great luminosity).

**Material removal**

Removing material is tricky because the potential energy at the Sun's surface is $U\_\odot = GM\_\odot/R\_\odot=$ 1.9e11 J/kg. While just 2e-6 of the mass-energy embodied in the matter and 0.3% of the potential fusion energy it is still cumbersome. If a Dyson sphere was used at perfect efficiency the maximal mass extraction rate would be \begin{equation}
\dot{m}=\frac{L\_\odot}{U\_\odot} = \frac{L\_\odot R\_\odot}{GM\_\odot} \approx 2.0061\cdot 10^{15} \text{kg/s},
\end{equation}
a typical mass for a minor asteroid per second. The timescale to disassemble the sun using just solar luminosity is \begin{equation}\tau\_{disassemble}=\frac{3GM\_\odot^2}{5R\_\odot L\_\odot} = 18.852\text{Myr}.\end{equation} Long before this time changes in pressure would reduce luminosity, making this estimate an underestimate by about an order of magnitude. I get 556.93 Myr, a bit more than Criswell's 300 Myr.

Another idealized model is that the lifting process fuses hydrogen (using the $p-p$ process) to power further lifting. In this case each kilogram of lifted mass produces $6.45\cdot 10^{14}$ J, allowing lifting of 3380 kg more fuel. This could of course speed up arbitrarily if there was technology for it.

I am leaving out *how* to do it. Beside Criswell's classical proposals (speeding up rotation, sucking off plasma using magnetic fields etc.) Beech suggests a stream of ramscoops or a stellar wrap could do it. This is not really key to the question.

**Luminosity decrease**

Luminosity can also be decreased by increasing non-thermal core pressure, perhaps through strong magnetic fields or rapid internal rotation. This would extend the main sequence lifetime as \begin{equation}\tau\_{NTPS}\sim \tau\_{MS} \left(1-\frac{p\_{NT}}{p\_T}\right )^{-7.5}.\end{equation} A 10% increase over current pressure would double the sun's main sequence lifetime and about halve the luminosity at hydrogen exhaustion. Achieving this is nontrivial given the current solar structure (the magnetic fields are generated in the convective outer envelope), but Beech notes that were the mass reduced below about half the current mass the modified sun would be full convective and magnetic fields would also produce a higher $p\_{NT}$ in the core.

Another way of reducing luminosity is increasing the stellar opacity $\kappa$. In order to reduce the luminosity by a factor of 100 (and the surface temperature by a factor of 10) the opacity needs to be increased by a factor of 100. However, Beech notes that merely adding heavy elements will not help since they increase mass and hence luminosity. While one can envision "dialysis" where helium is removed and replaced by larger $Z$ elements this appears complex, and there are no local readily available sources of enough high $Z$ material in the solar system.

**Mixing**

Engineering enhanced mixing before the mass is removed is nontrivial. Impactors and lasers simply do not penetrate deep enough to matter. Beech suggests employing a sub-stellar mass black hole oscillating through the sun to mix it. If the hole is small, the time until absorption is long.

**Result**

With mass-loss and complete mixing Beech finds the main sequence lifetime increased up to a factor of 12.2, with reasonable factors between 4 and 6.

Beech's scenario has the problem that the late stage sun while having an acceptable luminosity at Earth's orbit will be far hotter, releasing significant UV radiation. The end result is in many ways similar to a blue straggler star.


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