Is it possible for a star to return to the main sequence after entered in the post-main sequence phase? I have no clue if there's a mechanism such as acquiring hydrogen from from the environment or from a substellar body might bring again the star into the main sequence. Was it ever explored as a possibility?
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3$\begingroup$ I don't think you can do that because the state was changed at its core. Adding hydrogen to outer layer by interaction does not change the core's condition. $\endgroup$– Kornpob BhirombhakdiCommented Dec 10, 2020 at 19:04
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$\begingroup$ What happens in older stars is a big complicated topic. Wikipedia has an extensive series of articles. See en.wikipedia.org/wiki/Red_giant and the other articles linked there, eg en.wikipedia.org/wiki/Red-giant_branch & en.wikipedia.org/wiki/Asymptotic_giant_branch There's a lot of information there, so hopefully it's not too overwhelming. ;) $\endgroup$– PM 2RingCommented Dec 10, 2020 at 20:38
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$\begingroup$ Perhaps during the subgiant stage (between red giant and main sequence) mass loss through some mechanism could cause the star to return to main sequence. $\endgroup$– WarpPrimeCommented Dec 10, 2020 at 22:09
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$\begingroup$ Thank you for all your comments @fasterthanlight That's exactly what I was wondering... do you have any idea on what kind of mechanism or whatever might be a candidate... or someone that studied and definitely disproofed the hypothesis? $\endgroup$– Dac0Commented Dec 11, 2020 at 6:08
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$\begingroup$ Red giants lose lots of mass. Our Sun will lose about 50% of its mass in the form of gas and dust. But the big issue is all the helium (which is of course heavier than hydrogen) that builds up in the core. $\endgroup$– PM 2RingCommented Dec 11, 2020 at 8:13
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The evolution of a star is irreversible process, so strictly speaking, it is impossible to return the star back to its earlier phase (it is impossible to collect all energy it has radiated to space and use it for conversion of helium back to hydrogen).
But it is possible for a star plotted on HR diagram to return to a point corresponding to main sequence phase. These stars are called blue stragglers. They are usually formed when stars merge.
