How would I make a natural, long-lived red giant star?

Question

I have a system in which the main habitable planets are on the many moons of a super-neptune. The ice giant has not migrated inwards, but rather the habitable zone has expanded outwards due to the red giant phase. However I need this red giant phase last long enough that life can form, evolve into an intelligent form, and colonize neighboring stars, all starting from that super-neptune system. Red giants are unfortunately not particularly known for their longevity, and 1 billion years (as our sun would last) simply isn't long enough.

There is the potential for life having a head start here if it formed in a tidally-heated subsurface ocean like that of Europa, so the red giant doesn't need to last 4 billion years+, as life would only need time to colonize land and then land life develop intelligence and technology. However the longer it lasts, the better.

My first guess was that a lower mass star would also have a longer lasting red giant phase, However I found out it was impossible for low mass stars to even become red giants; so the problem remains that its too short-lived.

Magic and aliens are not allowed as this is the source system in which the first life developed. At least in this galaxy.

I have also considered a couple options for older universes, such as making the central star a red dwarf that evolves into its hotter phases (Helium dwarves and blue dwarves for example) and thats how its habitable zone expands, however that necessitates a very old universe to accomodate the time for a red dwarf to evolve that far, which is inconvenient considering I want really massive stars to still be present and dominating the color of galaxies.

or by having a high metallicity star. However I haven't been able to find much information on how stellar metallicity effects the lifetime of stars and their red giant phases, so I don't even know if this would lengthen the phase.

Edit: High metallicity stars last shorter according to the answer by HDE 226868, so that option is out of the question

Answer 1

This is a good question, and you actually might be able to achieve your goal naturally. The lower cutoff for helium fusion is $\sim0.5M_{\odot}$; given the main sequence lifetime relation $\tau_{\text{MS}}\propto M^{-2.5}$, we can estimate that such a star would stay on the main sequence for approximately 56 billion years. A good rule of thumb for red giant lifetimes is approximately 10% of the main sequence lifetime$^{\dagger}$, so a star just above this fusion cutoff would spend $\tau_{\text{RG}}\approx$ 5.6 billion years on the red giant branch, which should be adequate for your purposes.

Increasing the metallicity may shake this up a bit. Metals do aid in the cooling of molecular clouds as protostellar cores begin to collapse, which means that it's much easier to form lower-mass stars now than in the beginning of the universe. It also may lower the maximum mass of stars (see Adams & Laughlin 1997), but in the distant future this limit is expected to be no lower than $\sim30M_{\odot}$, which should only affect the most massive of stars - nothing like the ones we care about.

Effects on stellar populations en masse aside, will increasing the mean metallicity decrease the lifetime of a star of a given mass? In the extreme case, yes. Adams & Laughlin estimate a metallicity dependence of $$\tau_{\text{MS}}\propto Z(1-4Z)\left(1-\frac{64}{27}Z\right)^{7.5}$$ This leads to a peak lifetime at $Z\sim0.04$. The Sun's metallicity is $Z_{\odot}\approx0.02$; increasing this to $Z\sim0.10$ would decrease the main sequence lifetime (and, roughly, the red giant lifetime) by roughly 30%. In the expected maximum metallicity case of $Z\sim0.20$, we would see extremely short lifetimes. (Whether the extrapolation to that case is truly valid is another question.) The upshot is that even when mean stellar metallicities are five times higher than they are today - still very much in the far future - the decrease in age shouldn't be significant enough for you to cause problems. It's only at the very end of the stelliferous era that you'd run into problems.

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$^{\dagger}$ See Stellar Interiors by Hansen, Kawaler and Trimble.

Answer 2

You need to refuel your star somehow

The best way to do this is probably a Stellar merger. This needs to be a very lucky collision that gets in the sweet spot where they merge gently, such that it doesn't cause a supernova, or create a black hole, or melt your ice world, or create a gamma ray burst that sterilises the system. This is quite the trick shot.

0.99 billion years into the red giants lifetime, the helium flash just months away, a rogue small yellow dwarf enters the system, gets captured into a tight elliptical orbit, and after a few laps (that luckily stay far away from your planet) it's eventually brushing past the sun at low speed, forming a contact binary, eventually merging. The yellow dwarf's hydrogen and helium resupply the red drawf, buying it another ~billion years.

This will be quite the heat-wave on your planet, and probably with quite a death toll, but with some careful tuning of the numbers there should be regions that survive.