Where does the hydrogen for new stars come from?

Over in this thread Why doesn’t the sun just explode all at once? I was explaining the lifecycle of our sun.

It made me wonder where our sun (or future suns) get their hydrogen from in the first place. As I understand it our sun is probably a third generation star. That is some star in the distant past formed, exploded, formed again, exploded again and from those remnants our solar system formed.

However, if a star eats all (or nearly all) its hydrogen over the course of its life where did our sun get its hydrogen from? Where will future stars get their hydrgen from? Will all the galaxies in the universe snuff themselves out as stars no longer have any fuel to grab to form? I realize there is some hydrogen left in the universe that isn’t in stars (such as hydrogen right here on earth) but our sun comprises well over 99% of the mass of our solar system (all mass) so it doesn’t seem there is enough hydrogen lying about to make an entirely new sun.

What am I missing?

Just because the sun has most of the mass in our solar system doesn’t mean that all of space is like that. There is far more mass outside of star systems than in stars.

I recommend Atom, by Lawrence Krauss, as a good general public introduction to the formation of star systems, first, second, and third generation.

There is? With the exception of Dark Matter (which isn’t hydrogen) where is it? I thought on average there was something like one particle per cubic meter of ‘empty’ space. While I suppose if you add that up across all space it would be a lot I doubt there is enough in a local region to coalesce a star out of.

I have seen photos of stellar nurseries so I know there is still some free floating hydrogen out there. However, I don’t see how our sun could contribute to a future star. That is, it seems to me our sun will be the last generation and won’t contribute its stuff to the stellar gene pool. When our star dies it will leave a planetray nebula behind which I assume will be sorely lacking in hydrogen. Perhaps those atoms could contribute to the formation of a new planet someday but I still don’t see where the hydrogen will come from.

Stars do not eat most of the hydrogen. Only a small fraction of it is consumed. The fusion reaction only happens in the core where the density (pressure) is high enough, and since there is no convection there, fresh hydrogen cannot reach the core. Once the hydrogen in the core is gone the star dies, scattering most of the hydrogen-rich outer layers into space in one way or another.

That would explain it then.

In the other thread I linked in the OP it was suggested that the sun consumes about 10% of its hydrogen reserves.

Is it then reasonable to assume that we should get roughly 10-15 generations of stars out of the universe’s available hydrogen (hence putting a vague time limit on the ‘life’ of the universe even if the universe itself is still around)? Also, can one assume that there were many more stars in the past and that as time moves forward there will be fewer and fewer stars shining as free hydrogen becomes more scarce?

Basically this picture is correct, though the number of generations may be larger than 10-15.

Lower mass stars like the Sun create heavier elements in their cores, but this material is trapped there. Stars lose mass via the stellar wind, and when expelling the planetary nebula. But these are the outer layers, which have been enriched much less with heavy elements. So the expelled material has a distribution more like the original cloud that collapsed than like the distribution in an old star.

The 5 ages of the Universe by Adams and Laughlin says