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I am currently brainstorming a hard (like the hardest possible; everything that occurs in the story must at least be theoretically possible) sci-fi story which will take place far in the future. I don't have a specific year in mind yet but it has to be far in to the future enough that humans theoretically have the capacity to create a machine which could prevent the sun's rapid loss of hydrogen. Many articles (including this one: https://www.google.com/amp/s/phys.org/news/2015-02-sun-wont-die-billion-years.amp) I have read estimate that the Earth has roughly a billion years left until the sun increases in luminosity such that the Earth's oceans evaporate and all life dies.

Is it theoretically possible that humans could build a machine that will prevent the sun from increasing in luminosity to the point where it kills all life? The sun emits more energy the more hydrogen is burned but is it possible to lower the amount of energy the sun emits? To be clear, I am not asking for people to provide ideas of how humanity could prevent the sun from increasing in luminosity. I am asking if it would ever be theoretically possible to do so. If it is, why? If not, why not? In my story, human beings are flourishing and would have access to anything that will or theoretically will exist.

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    $\begingroup$ Hard science tag asks for numbers, for math -- and the math doesn't exist to prove something is impossible unless it directly contradicts the laws of physics (as we know them, which gives an out for folks who don't believe your math). We can't really answer why it is or (IMO) isn't possible and stay within a hard science tag. $\endgroup$
    – Zeiss Ikon
    Feb 14, 2020 at 19:36
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    $\begingroup$ Star evolution is caused by exhaustion of hydrogen and accumulation of helium. The only scientific way to change that is either lifting or adding material to the star. While the science of this process can be discussed today, there are no foreseeable realistic means of doing it. $\endgroup$
    – Alexander
    Feb 14, 2020 at 19:37
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    $\begingroup$ FWIW, it's actually significantly easier to move a planet than to change the evolution of a star... $\endgroup$
    – Zeiss Ikon
    Feb 14, 2020 at 19:37
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    $\begingroup$ A caplan thruster might do it, with the side benefit of turning the solar system into its own spaceship. sites.google.com/view/m-caplan-stellar-engines/startseite and youtube.com/watch?v=v3y8AIEX_dU $\endgroup$
    – John
    Feb 14, 2020 at 19:40
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    $\begingroup$ "Portal" technology might make it feasible to engineer stars in this manner. First, depending on its details, it might allow for the sort of FTL travel to where we could experiment on uninhabited stars (don't want to try this without testing). Second it might allow for us to remove large amounts of mass from our sun, possibly even selectively (helium). Third, it should allow us to add large amounts of mass (probably hydrogen, but I'm not an astrophysicist). There are other potential technologies too, but that's the best bet. I still want weak force manipulation though. $\endgroup$
    – John O
    Feb 14, 2020 at 20:35

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The technology is known as "star lifting", and works through the "simple" expedient of removing enough mass from the star to slow down the rate of fusion reactions. If the people who do this are clever enough, they can store the excess hydrogen from their star lifting operations in artificial gas giant planets (probably the size of Uranus) and feed the Hydrogen back into the star eons later as the nuclear fuel is used up in the star's core.

enter image description here

A mechanism for "harvesting" solar wind (RC = ring current, MN = magnetic nozzles, J = plasma jet).

This is obviously a very energy intensive and labourious process, so only a KII level civilization has the energy, resource budget and time horizon to undertake such a project. Given the project could take from tens of thousands to millions of years, there is a strong probability of the underlying civilization collapsing or radically changing its goals, leaving the apparatus and possibly dozens of artificial planets orbiting the Sun with no indication to the successor civilization how this happened or the meaning of the equipment and extra planets.

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Defensive Dyson sphere.

https://en.wikipedia.org/wiki/Dyson_sphere

A Dyson sphere is a hypothetical megastructure that completely encompasses a star and captures a large percentage of its power output. The concept is a thought experiment that attempts to explain how a spacefaring civilization would meet its energy requirements once those requirements exceed what can be generated from the home planet's resources alone. Only a tiny fraction of a star's energy emissions reach the surface of any orbiting planet.

The star increases in luminosity along its usual evolutionary path. The Dyson sphere captures this luminosity, allowing enough through to the planet to light it as they are accustomed. What exactly the civilization does with all the energy their sphere captures is an exercise for the author.

This answer was borrowed from my answer here What reason would an alien civilization have for building a Dyson Sphere (or Swarm) if cheap Nuclear fusion is available?

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Volcanic ash can do this naturally. see https://en.wikipedia.org/wiki/Volcanic_winter

So you aren't decreasing the luminosity of the sun but you would decrease the light penetrating the atmosphere.

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  • $\begingroup$ Welcome to WorldBuilding@SE. For answers bearing science-based, its considered good form to include at least a brief description of the mechanisms or principles being put forth, as well as citations themselves. $\endgroup$
    – EDL
    Feb 14, 2020 at 20:56
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    $\begingroup$ The degree to which the sun is expected to increase in luminosity is such that it will melt the volcanic ash. $\endgroup$
    – puppetsock
    Feb 14, 2020 at 22:19

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