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Red dwarfs have lifespans of trillions of years and are also the most abundant stars, making up 70% of the stars in the Milky Way. No red dwarf star has ever died and none will die for a very, very long time. Because it take so long for them to die, they're not releasing their hydrogen back into the interstellar medium to make new stars.

As more massive stars live and die over the course of years, those stars release their hydrogen into the interstellar medium, which can form new stars, 70% of which will be red dwarfs. Does this mean that, billions of years into the future, all of the matter will get locked within red dwarfs until they begin dying out trillions of years later?

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    $\begingroup$ Well, not all the gas, but you knew that right? Some ends up in black holes, neutron stars and white dwarfs for example. $\endgroup$
    – ProfRob
    Dec 15, 2017 at 16:13
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    $\begingroup$ To add, not all gas ends up as stars. For example, there's plenty of intergalactic dust out there that will never form into stars. $\endgroup$
    – Allure
    Dec 18, 2017 at 4:52
  • $\begingroup$ @user3727079 Your comment needs further justification as to why this gas will never form galaxies and stars. Such justification may plausibly arise from a consideration of the universal expansion and the cosmological constant, but it is not at all obvious. $\endgroup$
    – ProfRob
    Dec 19, 2017 at 12:26

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No. The reason is that gas recycling only recycles only about 40-50% of the gas in a sun-like star, leaving the rest as a white dwarf that slowly cools off. Heavier stars are even less effective in converting their gas into a condensed core. So the process will tend to produce a fair fraction of heavier white dwarfs now in the early stages of the stelliferous era, followed by a slow emergence of lighter white dwarfs from red dwarfs in the rest of the era.

Adams and Laughlin estimate that the final mass fractions will be brown dwarfs (never fused) 9.7%, white dwarfs 88%, and 2.4% neutron stars and black holes. In the post-stelliferous era some of the brown dwarfs will merge and make a few extra red dwarfs .

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Not necessarily. In order for a gas to collapse it needs to reach Jeans instability, which is determined by parameters on the radius of the nebulae and on its mass. It is a strongly local condition, and it doesn't necessarily determine which kinds of stars can come out of that unstable nebula, that could be determined after the already collapsing nebula fragments in different fractions of matter, each one of those could continue collapsing (if they are Jeans-unstable). If the fragments do collapse, the star that will be formed from that gas can be of basically any type of star, from red dwarf to blue luminous main sequence star, all depending on that fragment's mass. Obviously, this is true for basically every protostar in any given galaxy. Hence, will all new stars in a remote time from now be red dwarfs? not necessarily, it heavily depends on the forming nebulae

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  • $\begingroup$ This misses the point. Because their lives are much longer, the fraction of stellar mass existing in red dwarfs will gradually increase with respect to that residing in short-lived high-mass stars. $\endgroup$
    – ProfRob
    Dec 19, 2017 at 12:24
  • $\begingroup$ He's not asking about pre-existing stars, he's asking whether the remaining gas in the universe will collapse in Red Dwarfs. If he had asked about whether there will be more than Red Dwarfs rather than other kinds of main sequence stars in the future, you'd be right, but the question is different $\endgroup$
    – Birrabenzina
    Dec 19, 2017 at 16:19
  • $\begingroup$ You appear not to follow. The initial mass function can be absolutely constant with time, but the observed mass function of the stars that exist at any time will become more and more bottom heavy, since every low mass star that has been born is still alive, whereas high-mass stars die quickly. $\endgroup$
    – ProfRob
    Dec 19, 2017 at 18:11
  • $\begingroup$ If all gas is consumed in star formation then it all ends up in low-mass stars or the compact remnants of high mass stars - as per the accepted answer. $\endgroup$
    – ProfRob
    Dec 19, 2017 at 18:13
  • $\begingroup$ Ok now I get what you were saying now, yes that would be correct considering the present star distribution $\endgroup$
    – Birrabenzina
    Dec 19, 2017 at 20:05

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