Which would be safer: removing hydrogen or adding hydrogen to our sun?

If we want to extend the lifespan of our habitable zone safely, would it be safer to institute a yearly 'tax' on our suns outer layer of hydrogen, or a yearly 'subsidy' to our suns outer layer of hydrogen?

(the feasibility of said tax/subsidy is beyond the scope of this question)

Another way to ask this question: Assuming the core of the sun remains untouched, but its outer layer of hydrogen were reduced by half, would the suns main sequence remain intact for a longer time?

To avoid ending up with a white dwarf and its associated bad weather, what would be the ideal point to reverse the tax and re-subsidize?

• I feel like this question would be clearer without the tax/subsidy metaphor. Jun 28 '17 at 19:48
• I was trying to imply slow careful changes over a long period of time. Related questions/answers like "slam Jupiter into the sun" didn't sound very safe to me, though I could be wrong. Jun 28 '17 at 20:57
• It's worth noting that the amount of energy needed to remove mass from our sun to keep it steady output, it would be far more work intensive than moving the Earth. Jun 29 '17 at 21:58

My kneejerk reaction is that your only option is to remove a chunk of mass from the outer part of the Sun.

The Sun will respond (on a Kelvin-Helmholtz timescale), by contracting and becoming less luminous because the core temperature is lower in a less massive star. This will extend its main sequence lifetime, because only the central parts of the star are involved in fuelling the nuclear burning. The core is separated from the well-mixed outer convective envelope by a stable radiative zone.

Since the default solar evolution involves it getting slowly more luminous with time, it is possible that mass could be extracted at just the right rate to keep the Sun at constant luminosity. This sounds like an interesting calculation to do.

The alternative of adding "fuel" in the form of hydrogen won't work. The Sun's luminosity will increase because of its increased mass and increased central temperature. However, because the new fuel cannot be mixed into the core (only the outer part is mixed by convection), then the main sequence lifetime would be reduced.

• Ok this confirms what I was thinking initially. Continuing on this line of questioning... it sounds like slowly removing mass from the sun could keep the habitable zone the same for a very long time, as long as the balance between the core and its outer layers prevent the luminosity from changing. There must be a limit to how long this could be done? (Chandrasekhar limit comes to mind). Jun 28 '17 at 23:01
• @KeithKnauber Correct. I will be editing. Jun 29 '17 at 8:11
• "This sounds like an interesting calculation to do." I thought so too, so I fired up a stellar evolution code just to see what order of magnitude mass loss rate would give roughly constant luminosity. Starting from a solar-mass zero-age main-sequence (ZAMS) model, I get roughly constant luminosity for about 15 Gyr using a constant mass loss rate of $3\times10^{11}\,M_\odot/\mathrm{yr}$. To answer the question fully, I'd ideally start from a current solar model and try to work out what (variable) mass loss rate would give constant luminosity. But I don't have time! Jun 29 '17 at 13:40
• Also, this neglects that in the long run, there'd be an overall cooling of the star, although it's more modest than I expected, which would presumably affect the actual habitability of the Earth. (After 15 Gyr, my star had a surface temperature of 5250 K.) Jun 29 '17 at 13:41

This has been considered before. The phrase you want is star lifting (aka stellar husbandry), which is the (theoretical) practice of removing mass from a star to extend its lifetime. Since the rate of fusion reactions in the core of star increases faster than increasing math, smaller stars have much longer lifetimes, up to several trillion years for a 0.1 M☉ red dwarf. (A 1M☉ star with 90% of its mass extracted is probably not exactly the same as a 0.1 M☉ star, but should behave similarly.) Further, the extracted matter could be used to either replenish the existing star or create new stars, extending the luminosity even further. Smaller stars do have lower luminosity, but adjusting planetary orbits is expected to be a minor side project for any civilization capable of star lifting, if they would even bother with planets at that point.

At present we have only extremely sketchy ideas of how we would even go about learning how to learn to do star lifting, it does not seem to violate any laws of physics. Stars already lose small amounts of mass over their lifespan, so it seems plausible that megascale engineering could enhance that and collect the lost mass.

• A couple of fictional treatments of star-lifting, which I can edit into the main answer if it's considered appropriate for this exchange: Palimpsest, Orion's Arm Jun 29 '17 at 19:47
• Haha this is awesome! The wiki page has tons of semi-plausible ideas how to do this. Just poking around on WolframAlpha, if every year you removed a mass equivalent to the mass of earth's oceans, it would take 1.4 billion years to remove the entire mass of the sun. wolframalpha.com/input/… Jul 6 '17 at 20:21