Hydrogen??

[Steve_meg]

After reading a few things about the formation of stars and there life cycle, it got my thinking.

Now stars fuse hydrogen into helium, albeit not quickly, are there any sources the produce hydrogen.

If not surely over trillions and trillions of years stars will use up all available hydrogen to form new stars.

Just to add this was only a random thought without much research so I might be miles off the truth.

[JulianO]

There are a few processes that can split Helium back to hydrogen, but they are the exception. So mostly the universe is turning from H to He. However as it started out as 75% H, it has a way to go.

There is vastly more H just floating around than there is in stars, so there are lots of reserves. Big clouds of it float in our own galaxy, and there is more in the extragalactic environment. The difficulty is getting it to form new stars. The clouds in our own galaxy tend to get disturbed from time to time, by supernovae and the like, triggering star formation.

The inter galactic medium is a different matter, as there is outward pressure from light, shockwaves and heating that stop it collapsing into the galaxy. However there is a concept of a "cold flow" where relatively cold H swirls into galaxies like water going down a drain. These have been predicted by models, but have not been reliably detected. Its things like these that keep galaxies ticking over.

[Knight of Clear Skies]

Very little hydrogen is produced from heavier elements, because it is an endothermic (energy absorbing) rather than an exothermic (energy producing) process. Iron is the most stable element, so generally speaking fusing elements lighter than Iron or fissioning heavier elements releases energy.

If not surely over trillions and trillions of years stars will use up all available hydrogen to form new stars.

That's correct. The average rate of star formation was far higher in the past than it was now. There was a recent estimate that 95% of the stars that will ever form already have, but I don't know how reliable that is (I'll probably start a thread to discuss it sometime). While the amount of hydrogen in the universe will decline over time, the biggest constraint will probably be the fraction of that hydrogen that is available in sufficient concentration to take part in new star formation. The universe is expanding, so its eventual fate is to be cold and dark. We've a very long way to go before we reach that point though.

[acey]

are there any sources the produce hydrogen.

The big bang. Also some nuclear processes that produce protons (which are hydrogen nuclei). Hydrogen doesn't run out as such (the vast majority of hydrogen in the universe isn't in stars, it's spread out thinly in the space in between). But wait long enough and everything will end up in black holes, the black holes evaporate (Hawking radiation) and all you've got is lot of low-energy photons, neutrinos and other useless junk, then the whole lot's dead.

[JulianO]

Hydrogen is produced in supernovae explosions, by photodisintegration. Essentially high energy gamma rays breaking up bigger nuclei. That's why you get all those neutrons formed by inverse beta decay in the middle, leaving a neutron star, but at the same time it's making even heavier elements.