As stars slowly using up hydrogen and ejecting heavier elements into space, the future stars will form from nebulae that are more metal and helium rich.

The question is: how does the increased ratio of helium and metals affect star formation and stellar evolution?

Does more helium and metals in the core at the beginning means the star begins it's life in an advanced state so it dies faster?


(Adams & Laughlin 1997) discuss the effect of increasing metallicity in the future. A higher metallicity increases the stellar burn rate since the density increases but the higher opacity reduces it a bit. The total effect is nonlinear; in their model the maximal lifespan happens for $Z\approx 0.04$ and beyond that it declines. Whether there are newer and better models of this I do not know.

They also point out that the maximum stable stellar mass decreases as metallicity increases, while the mass of the minimum mass mainsequence star declines, both as a result of opacity effects.

Overall, the metallicity growth is likely to continue across the stelliferous era. While extragalactic gas may fall in, it merely adds to the hydrogen pool and continued star formation. What could stop the metallicity growth is quenching due to a galactic merger or active galactic nucleus that blows away enough gas to stop further star formation.

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    $\begingroup$ From the paper: "For example, when the metallicity reaches several times the solar value, objects with mass M∗ = 0.04 M⊙ may quite possibly halt their cooling and contraction and land on the main sequence when thick ice clouds form in their atmospheres. Such “frozen stars” would have an effective temperature of T∗ ≈ 273 K, far cooler than the current minimum mass main sequence stars. The luminosity of these frugal objects would be more than a thousand times smaller than the dimmest stars of today, with commensurate increases in longevity." A main sequence star that doesn't burn you? wow $\endgroup$ – Calmarius Sep 11 '19 at 9:48
  • $\begingroup$ @Calmarius - Very wow if true. But I have not seen any references to them ever since this paper, so maybe we should get our hopes up. $\endgroup$ – Anders Sandberg Sep 11 '19 at 19:02
  • $\begingroup$ Surely the minimum mass declines with increasing metallicity because of the change in the number of electrons per mass unit? Such "stars" would be fully convective, so opacity shouldn't play a role. But maybe I should read the paper. $\endgroup$ – Rob Jeffries Sep 19 '19 at 23:11
  • $\begingroup$ I would really like to see if anybody can find more recent literature on this. $\endgroup$ – Anders Sandberg Sep 19 '19 at 23:55

There is no sign that novae and supernovae are dumping enough heavy elements into the interstellar medium to affect formation of stars in the future by causing a shortage of hydrogen. It has recently been discovered that the Milky Way is at the centre of a vast cloud of hydrogen extending at least 300,000 light years in all directions, enveloping the Magellanic Clouds and containing enough mass to solve the dark matter problem without the aid of WIMPS. In addition, Australian astronomers have discovered much smaller but still massive clumps of hydrogen racing around the galaxy at speeds of up to 56 miles per sec, with no regard to the direction of rotation of the galaxy itself. What might happen 100 billion years down the road is too far ahead to speculate, but in the foreseeable future there is no fear of a shortage of hydrogen, so stars will continue to form in much the same way as they do now, and have closely similar lifetimes.

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    $\begingroup$ I think the downs are because your statements are... uncommon, and you did not post reference to them. I suggest to extend with links or so. I did not vote to your post, but soon also the del votes will likely come. $\endgroup$ – peterh - Reinstate Monica Sep 10 '19 at 22:08
  • $\begingroup$ Visit ZME Science on the internet $\endgroup$ – Michael Walsby Sep 10 '19 at 22:29
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    $\begingroup$ It is not a reference. You should insert them as links in the answer. I don't believe that I should explain it to you with 1000 rep. $\endgroup$ – peterh - Reinstate Monica Sep 10 '19 at 22:53
  • $\begingroup$ Can you please consider this comment: astronomy.stackexchange.com/questions/33236/… $\endgroup$ – samcarter_is_at_topanswers.xyz Sep 11 '19 at 11:45
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    $\begingroup$ In addition to posting speculative, ummm ... stuff, that speculative stuff misses the point of the point of the question. Metallicity (to an astronomer, every element other than hydrogen and helium are "metals") strongly affects the behaviors of stars. $\endgroup$ – David Hammen Sep 11 '19 at 12:40

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