[katieHaylor (OP)]

Sadiq asks:

Since lighter elements are transforming into heavier elements in stars, is it true to predict that one day there will be no new born star?

What do you think?

[chiralSPO]

Based on our current understanding of physics and cosmology, yes, it does appear that there will come a time that no stars form any more, and ultimately the whole universe will be cold and dark. (see https://en.wikipedia.org/wiki/Heat_death_of_the_universe ).

But it is difficult to estimate when that might happen, other than: a REALLY long time from now. The roughly 1.4e10 years since the big bang represent a very tiny fraction of a percent of the expected life of the universe.

However, it is also important to stress that we really don't have a good idea what kind of physics will dominate on these sorts of time and length scales. Given how much our understanding of cosmology has changed over the last 200 years, I think it is fully reasonable to expect that this answer will change drastically over the next 200 years. (I still don't think it will be any time soon)

[chiralSPO]

And specifically regarding the supply of hydrogen and helium:

Supposedly the primordial elements were essentially only hydrogen and helium (92% of atoms were hydrogen, and 75% of mass was hydrogen). While stars have been consuming these light elements, current estimates of the composition of the universe still have >90% of atoms being hydrogen. If the time since the big bang represents a 2% decrease in hydrogen abundance, and we take this rate as constant (probably a bad assumption, see below), then we would expect the universe to drop below 25% hydrogen (atom count) in 455 billion years.

I don't know off hand whether the rate of hydrogen consumption would increase or decrease over time. As the concentration of C, N and O increases we can expect that hydrogen fusion will increase in rate because these atoms catalyze hydrogen fusion (see https://en.wikipedia.org/wiki/CNO_cycle ) But we also have to take into account that the matter in the cosmos is getting more dispersed due to cosmic expansion, which may eventually slow the rate at which stars form. However, as the composition of the cosmos become more diverse, star formation will also become favored, because the different particles can radiate light energy across a wider range of frequencies, while still maintaining thermal equilibrium with each other (radiative cooling of hydrogen clouds is very inefficient at temperatures below the lowest energy emission, but heavier elements have lower energy emissions; see http://www.star.ucl.ac.uk/~tgreve/lectures/lecture-5.pdf )

[evan_au]

There are large clouds of hydrogen in intergalactic space that fall like rain on galaxies, fuelling new star formation.

Dramatic events like galaxy collisions trigger a burst of new stars, but the chaotic process of merging their black holes will see many stars consumed by the new, larger, central black hole.

Red dwarf stars are thought to last for a trillion years, not just billions like our Sun.