Well a question ive been asking myself for quite a while.
As we know hydrogen is the most abundant element in the universe.
It is responsible for nuclear fusion in stars, which makes it also the most common energy source.
The question is - since it is being used up constantly in massive amounts throughout the universe, and is not in any way regenerated, where the hell does it come from? And what happens when it runs out?
How do new stars form if they are said to form from the remains of old stars, which coincidentally have used up their hydrogen fuel?
Does physics even have an explanation for this?

It was pretty much created in the first 3 minutes after big bang. BB theory answers to your questions quite well, at least way better than i can explain this..

http://en.wikipedia.org/wiki/Big_Bang
http://csep10.phys.utk.edu/astr162/lect/cosmology/hotbb.html

Hydrogen was the first element to be formed after the big bang (as it is the simplest atom), and aside from small amounts of helium and lithium, was about the only element produced. A certain amount has been fused into heavier elements, but hydrogen is still by far the most abundant element in the universe.

As to what happens when it runs out, heavier elements than hydrogen can fuse, though less energy is released. Stars begin fusing heavier elements anywhere when their hydrogen runs low. Eventually though, all isotopes that release energy when they undergo fusion will have been fused, at which point stars are no longer possible. That is however a very long way off.

As for precisely where Hydrogen initially came from, a neutron is unstable and will naturally decay into a proton in about 15 minutes (maybe it was 7... something really short at any rate, the scale is what matters). Thus the only stable state as the universe cooled was a single proton, or a proton + Neutron pair. And so everything paired up to become Hydrogen 2, or decayed and stayed alone to be Hydrogen 1.

As the universe cooled even more, heat energy was finally low enough that the charge of a Proton could trap a wandering electron, and so neutral H2 and H1 began to form as well.


EDIT: And I realized I didn't explicitly state it: But all radiation of neutrons or Protons will result in the creation of more hydrogen. So the supply DOES get refreshed slowly. But there is so much available still that this slow resupply is mostly irrelevant.

A neutron has a half-life of 10.25 minutes; it decays into a proton and an electron.

:)

A neutron has a half-life of 10.25 minutes; it decays into a proton and an electron.

:)

youre right
I totally forgot about the neutron decay.

I was aware of how hydrogen and other elements first came into existence, i just totally forgot about this simple refreshing mechanism.

To clear up the decay numbers: ~10 minutes is the half-life and ~15 minutes is the mean lifetime. Both numbers are correct, they just mean something slightly different.

For the original questions:
Big Bang theory says, that at the point that helium was formed (the so-called "nucleosythesis") neutrons were outnumberd by protons about 1 to 7. As He-4, the most important product atom at this stage, needs one neutron for each proton, only about 25% of the baryonic matter could fuse into other atoms, so that the remaining 75% was left as single protons i.e. hydrogen.

Now protons can fuse into hydrogen in stars and produce energy that way. But if you for example take the energy produced by our galaxy and extrapolate it over its lifetime you will get to the conclusion, that in the entire lifetime of our galaxy only roughly 1% of the hydrogen was consumed by fusion.

So it will take a very long time until hydrogen runs out. When that happens, however, the universe will be pretty much dead (nothing much will happen anymore).

edit: As free neutrons are pretty rare, the neutron decay is not a significant refreshing mechanism

uppi, do you mean that long before the hydrogen is "used up" that the spacial expansion of the universe will cause to to be so spread out that there won't be sufficient concentrations of it to create new stars?

uppi, do you mean that long before the hydrogen is "used up" that the spacial expansion of the universe will cause to to be so spread out that there won't be sufficient concentrations of it to create new stars?

Yes. If the concentration of hydrogen gets too low, there won't be any fusion anymore, so there will still be lots of hydrogen when every star has gone dark. But as we don't really know what drives spacial expansion, it is difficult to predict what exactly the universe is going to do.