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Relating to my earlier question Is it a good idea to harvest the sun to terraform Venus?

Suppose that we continue to “mine” the sun for hydrogen to be used as rocket fuel. If we continued to do so, would this begin to deplete the amount of hydrogen in the sun and start to lower it’s life expectancy, eventually causing it to explode/transform into a supergiant?

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Assuming you're taking hydrogen from the surface, you'll be lengthening its lifespan.

The sun's outer convective envelope, within which hydrogen is cycled from the surface to its interior, doesn't quite reach its actively-fusing core. That core is supplied entirely by its own internal hydrogen; by the time it's done fusing into helium naturally in ~5 billion years, the outer envelope will still be hydrogen, since it won't have been cycled in.

So the only effect of removing mass from the envelope right now is to reduce the pressure on the core, and with less pressure, the core burns more slowly.

At a certain point, if you deplete the envelope by a significant margin (difficult; we're talking thousands upon thousands of Earth-masses here), you might begin to alter which parts of the Sun are convective. A low-enough mass star is fully convective and taking more mass away will simply shrink it until it can't fuse anything more at all.

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    $\begingroup$ See also: star lifting. $\endgroup$
    – Mark
    Oct 11, 2022 at 2:14
  • $\begingroup$ So it wouldn't shorten the sun's lifespan, but it would dim the sun immediately? $\endgroup$
    – Dan Staley
    Oct 11, 2022 at 19:41
  • $\begingroup$ @DanStaley Where do you get "immediately"? $\endgroup$ Oct 11, 2022 at 21:40
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    $\begingroup$ @DanStaley It'd take many thousands of years for a change in luminosity to fully propagate from the core up to the surface, since photons take so long to escape, but if you're committing to starlifting a serious portion of the Sun then "thousands of years" is the timescale we're talking about. I neglected short-term effects since the question wasn't focused on those, but you might create more light for a while by doing major starlifting, because you're pulling glowing plasma off the surface and into space, allowing it more surface area to radiate. $\endgroup$
    – parasoup
    Oct 11, 2022 at 22:32
  • $\begingroup$ Stars operate under a "Live Fast, Die Young" principle. $\endgroup$ Oct 12, 2022 at 12:15
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As a rule of thumb, the more massive a star is, the shorter its life is.

If I remember correctly, the more massive a star is, the more densely packed are the atoms in its core and the easier it is for them to fuse, resulting in a quicker burning of the available fuel.

For a reference, you can use a calculator to estimate the life of a star based on its mass:

  • A star of 1 solar mass would have a life of 10 billion years
  • A star of 0.8 solar masses would have a life of 17.5 billion years
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I doubt it. I seriously doubt it.

A long time ago in a galaxy far, far away I lived near Coeur d'Alene, Idaho. I remember watching a news broadcast about a lady who was upset because a local golf course had been given permission to draw from Lake Coeur d'Alene to water the course. She was absolutely convinced lake water levels would drop precipitously because of it and demanded that they cease and desist immediately.

Unless you're trying to fuel billions of ships a year, your civilization could realistically draw hydrogen non-stop from its star and, unless you need it to be otherwise in your story, it won't make a measurable dent in the lifespan of the star.

and if you have interstellar travel, this is even less of an issue because you could either (a) mine another star or (b) move to another solar system when your star is mined out.

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    $\begingroup$ @vsz :-) Are we suggesting that I might have been a bit optimistic about how few ships would be needed to make a dent? I wouldn't be surprised, but I thought "trillions" might leave people wondering that I was being a bozo. $\endgroup$
    – JBH
    Oct 10, 2022 at 4:55
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    $\begingroup$ Acre-feet might be the most american unit of measurement ever. For anyone else wondering: One acre is 4046,86m² and one foot is about 30.5cm meaning one acre-foot is about 1233.5m³ $\endgroup$
    – SirHawrk
    Oct 10, 2022 at 6:02
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    $\begingroup$ Agree with the answer, but the analogy is slightly askew for me. If a golf course uses water it does go back to the water table and will re-appear in some body of water somewhere, however the hydrogen will not come back to the star. Its a closed system vs an open system. $\endgroup$
    – James T
    Oct 10, 2022 at 8:52
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    $\begingroup$ @SirHawrk, an acre-foot is sufficient water to irrigate one acre of crops with one foot of water. $\endgroup$
    – Mark
    Oct 11, 2022 at 2:15
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    $\begingroup$ @Mark And a cubic meter is sufficient water to irrigate one square meter of crops with one meter of water. Or 10 square meter with Decimeter of water $\endgroup$
    – SirHawrk
    Oct 11, 2022 at 5:59
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I did read somewhere that by controlled removal of matter from the Sun one can make it live ~12 times its predicted lifespan while maintaining constant luminosity.

Of course, this strategy implies removing a great deal more hydrogen than the amount one would need for Venus.

On the other hand, if the goal is to teraform Venus, there are much easier sources of hydrogen available in the Solar system. E.g. the moons of Jupiter and Saturn are almost reachable as a hydrogen source even with today's technology.

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  • $\begingroup$ I will just emphasize that this strategy implies removing more hydrogen from the Sun than the total mass of all other bodies within the solar system. The amount needed for spacecraft propellant and/or terraforming is rather insignificant compared to that. $\endgroup$ Oct 11, 2022 at 16:24

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