Hydrogen, believe the hype?

[hes not well again]

I have a question about hydrogen. As I understand it the story goes that there is huge cloud of very cold hydrogen slightly unevenly distributed. As time goes on the slightly denser patches start to pull together under their small gravitational attraction until the pressure and density is enough to start off a fusion reaction. We now have a star that converts hydrogen into the next heavier elements, helium etc etc. It works it's way up the periodic table until it reaches iron when, if it is big enough it makes a supernova in which are made the heavy elements, lead, gold, uranium et al. At which point the presenter who may or may not be an ex pop star that has won awards for extreme cleverness and his work on dead hard sums, says," and then the process starts all over again." Well my question is where does the hydrogen come from for the next generation of stars given that the last sun converted the hydrogen in the local area into other elements long ago? I have read that our sun is thought to be a third generation star, maybe it got more hydrogen on ebay

[hes not well again]

I forgot to say all the above happens in space

[JamesF]

There's an awful lot of hydrogen about. More than anything else. The hydrogen forming subsequent stars doesn't have to come from one that's already collapsed or gone supernova. It might just be floating around waiting for something to cause it to clump together.

James

[Moonshane]

Indeed. Just look in Orion.

[michael.h.f.wilkinson]

Only a small fraction of the star's hydrogen is used up (around 10 pct at most, I think). The primordial gas clouds are generally 75% hydrogen, 24% helium, and 1% other stuff (roughly from the top of my head). Most of the helium was formed during the big bang, only a fraction in stars.

[Planetesimal]

In main sequence stars, only the core is actually under enough pressure to fuse Hydrogen. You can calculate that there's actually 100 billion years worth of Hydrogen in our Sun - but only 10% of that will be burned which means it only has 10 billion years before it runs out of usable Hydrogen, because the nearer you get to the stellar surface the lower the pressure and so fusion can't take place. The other 90% can be recycled when the star blows up (although this only happens when the mass of the star is above a certain mass limit).

In terms of the hydrogen that's out there, most hydrogen is in its ionised state, i.e. free protons (p⁺). Protons can be made by the radioactive decay of free neutrons (n -> p⁺ + e^- + ν_e), being the formation of a proton, an electron and an electron neutrino (this is useful because we can detect these on earth and so measure processes going on in the sun). The opposite of this is called inverse beta decay, where protons and electrons combine producing neutrons and neutrinos - this can happen in supernovas because of the intense pressure caused by the gravitational collapse of the core. The heat and pressure in the collapsing star can fuse the neutrons to produce elements of increasing atomic mass (just add more neutrons). However you've still got free neutrons that can decay back into protons.

But you don't need these processes to replenish the universe's hydrogen stocks. As John says, most of the matter (about 74%) in the universe is hydrogen formed in the big bang (baryonic matter that is!).

[symesie04]

Also the generations you refer to are actually known as Populations and appear to be in reverse order. Our Sun and other current stars are referred to as Population I stars. The earliest stars to form in the Universe are referred to as Population III stars. Its all down to the Metalicity of the star, it’s thought that the earliest stars would of been formed purely of Hydrogen and Helium and the later Population I stars would contain heavier metals in their composition. However there is currently a bit of a puzzle as the Population III stars do seem to contain heavy metals as well, I believe that the theory has been changed to include the possibility that a slightly older Population evolved first which were very pure. These massive stars were short lived and provide the impure components of the Population III stars. These very early Pop III stars are currently referred to as "early Population III" stars (brilliant huh?) but could easily be thought of as Population IIII stars.

[Cath]

Plus, their are galaxy's where no new stars are being born due to their not being enough concentrated gas clouds left, but if another galaxy comes along and collides things then change and so starts a generation of new stars in those galaxy's.

[hes not well again]

Thanks so much all for taking the trouble to provide such full explanations. It is a measure of the excellent clarity of Planetesimal's answer that for nearly twenty minutes I was convinced that I had a handle on the radioactive decay of hydrogen. To my shame I had forgotten that only at the core of a sun does fusion take place, apparently the same effect means that there is a limit to the size of hydrogen bomb that can be made because if you add more than a certain amount it is simply blown away by the explosion. It was the absence of gas clouds in our solar system that led me to wonder where the next sun might come from as when a sun ignites it seems to push any remaining gas and debris away from it to form planets and such. As Cath points out there are regions of space where there is no more star formation due to lack of materials. I dare say it will be an interesting time for astronomers when Andromeda arrives at the milky way and stirs thing up a bit. [assuming they still exist then]

[bomberbaz]

Fascinating stuff this, I simply never tire of the science behind the formation of our universe

[hes not well again]

Also these speculations seem to imply that irrespective of the coming heat death of the universe due to its increasing infinite expansion the universe is "born" with a finite amount of energy [hydrogen] available to convert and thus a finite lifespan.