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Our universe is supposedly 13+ billion years old and our Sun is a third generation star. It seems to me that we are now in a relatively young stage of the universe. How many generations of stars will there be before the end of the universe? When is 'now', relative to the all possible timepoints in the universe, that life could appear?

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    $\begingroup$ Excellent question, although the answer to the title probably is "in the very beginning", since it seems that we live in a universe that will exist forever. But your last sentence "…that life could appear", makes the answer more convolved. You may consider editing your title to "Where are we in a aproximate timeline of the habitable Universe" (the answer still isn't trivial). $\endgroup$ – pela Nov 2 '15 at 20:47
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    $\begingroup$ What is a "generation" and where does the idea that the Sun is a third generation star come from? The constituent components of the Sun and solar system were made in millions of previous stars. $\endgroup$ – Rob Jeffries Nov 2 '15 at 22:24
  • $\begingroup$ @RobJeffries There might be some confusion with Population I, II and III stars, given that the Sun is a Population I star and these stars are, in general, younger. $\endgroup$ – HDE 226868 Nov 3 '15 at 1:55
  • $\begingroup$ There is no distinct time limit on habitability, but eventually (perhaps already) the universe will enter a phase of very slow reduction of habitable zones in approximately an exponential decay. To really answer this question, one must consider every flavor of habitable zone, its abundance, how long they may last, and the rate at which they are still to be created. What is the most common zone now (don't even know what that might be) will almost certainly not be the most common much later, as some exotic zone(s) may be the longest lasting. $\endgroup$ – Eubie Drew Nov 3 '15 at 2:06
  • $\begingroup$ @RobJeffries term third generation star is widely used in popular literature to indicate that such a star (like our sun) is constructed of material that has been processed and reprocessed through previous stellar lifecycles. $\endgroup$ – Eubie Drew Nov 3 '15 at 2:17
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Here's a very roughly calculated partial answer.

A first generation star and solar-system would obviously not be a good candidate for life or even planets cause you can't do much with mostly hydrogen and helium. You can't even build an ocean with mostly those 2 elements. Stars do just fine, but planets don't. Gas Giants and Suns only.

After the big bang: 75%-76% Hydrogen by mass, 24-25% Helium (with a teeny-tiny trace amount of Lithium). I'm going to assume 76% / 24% for rough calculations.

Using our solar-system and the Milky-way as a model, elements by mass:

Milky-Way

73.9% Hydrogen

24.0% Helium

1.04% Oxygen

0.46% Carbon

0.13% Neon

0.11% Iron

0.10% Nitrogen

0.07% Silicon

0.06% Magnesium

0.04% Sulfur

Smaller amounts of Argon, Calcium, Nickel, Sodium, Phosphorus, Chlorine, etc.

and composition of the Sun by mass (pretty similar except for more Helium)

71.0% Hydrogen

27.1% Helium

0.97% Oxygen

0.40% Carbon

0.06% Neon

0.14% Iron

0.10% Nitrogen

0.10% Silicon

0.08% Magnesium

0.04% Sulfur

Using these numbers as a rough guideline, if 1.9% to 2.1% of the baryonic mass of the milky-way converts from Hydrogen to Carbon or heavier elements after (about) 2 generations, and we extrapolate a 1% increase in all Carbon or heavier elements to each stellar generation or every 6 billion years, then we can begin to make an estimate.

Helium percentage could stay somewhat constant at about 24% cause it's both consumed and created stars.

Your original question, how many generations, if we figure 1% increase in heavier elements (Carbon or higher) per stellar generation, 10 generations we're still looking at, very roughly, (24% helium), 66% hydrogen, 5% oxygen, 2%-3% carbon, 2%-3% other elements. It still looks like a workable scenario to me though the stars, being made of some denser elements might have denser cores and as a result, could burn hydrogen a bit faster. Past a certain point the ratios probably stop being ideal but I see no reason why quite a few generations couldn't still work create life-friendly solar-systems.

Our sun is expected to cast off about half it's matter into a planetary nebula before it settles down into a white dwarf and most of that cast off matter will be hydrogen and some helium. Some time later if our sun as a white dwarf accretes enough to meet the Chandrasekhar mass and goes Type-1 Nova, it will again cast off a significant percentage of it's matter, so, to put it simply, Stars recycle a pretty good percentage of their hydrogen and helium, which is kinda cool. If you see Rob Jeffries' brilliant answer here, stars of 7-9 solar masses will eject an even higher percentage of their elements before they go white dwarf, as much as 85% and most of what gets ejected is the lighter elements, hydrogen and helium from the outer layers. Even smaller red-dwarf stars expel a percentage of their elements due to more active solar flares and lower gravity throughout their lives.

Add to this, the possibility of continued merger of the galaxies and dwarf galaxies in our local group, and life capable solar-systems could continue to form for quite some time, perhaps several tens of billions of years.

This answer isn't meant to be definitive, but only a rough estimate. Corrections welcome.

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  • $\begingroup$ What is a "generation"? What is the basis for your claim that there is a 1% increase in carbon abundance per "generation"? Until we can get this straight, this question is unanswerable. Different masses of stars enrich the ISM with different elements in different amounts and those stars have lifetimes that vary by several orders of magnitude and most do not die at all on 10 Gyr timescales. Nowhere in your calculations is there anything about mixing lengths and timescales in the ISM and the mass spectrum with which stars are born. These are crucial. $\endgroup$ – Rob Jeffries Nov 4 '15 at 11:34
  • $\begingroup$ I realize that a "generation" is vague. I was going off the original question's claim that our Sun is a 3rd generation star. I realize, number of generations has a degree of meaninglessness. I just meant to give a kind of simple answer, but I can delete if you think it more misses the point than is helpful. Adding timescales would make things more accurate but also harder to calculate. $\endgroup$ – userLTK Nov 4 '15 at 11:52
  • $\begingroup$ Analogous question. In your lungs now, there are oxygen atoms that were present in the last breaths of how many generations of humans? This is an easier/more meaningful question since at least humans have a more modest range of lifetimes than stars and a more modest range of lung capacities than stellar masses. $\endgroup$ – Rob Jeffries Nov 4 '15 at 12:01

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