What tech would aliens use to harvest/alter our star and why?
I'm trying to create a scenario where the local star is "quickly" (suddenly up to thousands of years) removed, dimmed with reduced output, destroyed (maybe), or otherwise fundamentally changed as to have dramatic ramifications on all life. Keeping the local system in orbit would be ideal, so complete destruction scenarios may not be the best fits.
The biggest challenge I'm having is that the only star-harvesting ideas I know of are variants Dyson concepts, which seem to commonly seek to harvest the energy of a star throughout it's entire/most natural life (implying that this is the most efficient method, which I am guessing makes sense but I don't honestly know). Dyson shells are suboptimal because they can be interrupted by an intelligent outsider leading to the potential to simply "rescue" the star, but more importantly my understanding is that a shell variant would generally be ~1AU in scale for efficiency-sake, making it disruptive to the type of story-telling I'm looking for -- it would either enclose or be obscenely close to most earth-like planets unless I'm mistaken.
Because of this, I'm struggling to think of ways and motivations for an alien or other being to quickly disrupt the local star. I'm looking to avoid scenarios where the star is simply destroyed for the sake of it's destruction, but rather where it is used for a logical purpose by an uncaring 3rd party.
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If "the local star" isn't our Sun, I think I can help you. Actually, I think I can even if it is the Sun, but in the far future.
In "The End of the Sun", two such scenarios are described. These will occur naturally during the star's lifespan, without the need for any third-party intervention, advanced technology or sorcery. You say "thousands of years", so I hope the timescales of 10,000-100,000 years are within the range of what you're looking for.
- The first of these is the "helium flash", which occurs at the end of the red giant phase for a star that was less than $2M_{\odot}$ in mass during its main sequence phase. It's the point at which the star enters its helium-fusing phase. One such star is our Sun, which is of course $1M_{\odot}$. Over the course of tens of thousands of years, the red giant shrinks to less than 1/50 of its previous radius and luminosity:
By galactic standards, however, the red giant has been shot through the heart. The sudden expansion of the core results in cooling so severe that it is something like the onset of an Ice Age. The cooling immediately leads to much lower pressure in the hydrogen-burning shell that surrounds the core, and therefore to a calamitous drop in the energy output. On a timescale which is almost instantaneous compared to the usual timescale that stars run on (perhaps as little as 10,000 years), the red giant's diameter and luminosity plummet to less than 2% of their former values. For stars the mass of our Sun, the result of the helium flash is a collapse into an orangeish-yellow star with perhaps ten times the current solar diameter and 40 times the luminosity. It is quite a comedown.
- The second scenario occurs roughly one hundred million years later, as the Sun (which has become a red giant again) reaches the end of its helium-fusing phase. During this time, it's been burning both helium and hydrogen. The explanation is rather complicated, but:
In four or five huge bursts, spaced roughly 100,000 years apart, the outer layers of the Sun will separate from the core and be completely blown away. They will form an enormous, expanding shell around the solar system, and move outward to rejoin the interstellar gas. Roughly 45% of the Sun's mass will escape in this way. The remaining 55% of the Sun's mass is soon compressed into the white-hot, ultra-dense core.
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