I know that first generation stars' main fuel was Hydrogen. I know the Big Bang happened at some point in time. Now if the strong force exists, then why aren't different, higher mass, number elements produced? why was there single proton nucleus like hydrogen?
[Physics] Why did the Big Bang produce hydrogen
big-bangcosmologynuclear-physicsnucleosynthesis
Related Solutions
Tis, a good question. Two related questions arise from it. The first one is, will the hydrogen all be used up in finite time? The second related one is, will star formation completely stop in finite time? They sound related, but the first result doesn't necessarily imply the same result for the second, or vice versa. I.E. a low but nonzero gas density might possibly not allow further star formation, and maybe we could have no hydrogen, but have other types of gas (or even solid objects) still collect into stellar mass objects.
I don't know for sure the answers. The rate of star formation (and hydrogen consumption) could decline slowly enough as to never formally reach zero. Or not.
We do know that a lot of gas gets blown out of galaxies by massive stars, supernova, and black hole activity, and becomes intergalactic gas -usually staying within the galaxy cluster. On a long time scale this should eventually fall back into the cluster's galaxies. So I would think the star formation rate would have a very long tail.
Actually, what you've read about the production of nuclei is not quite correct. There are several different processes by which atomic nuclei are produced:
- Big Bang nucleosynthesis is the fusion of hydrogen nuclei to form heavier elements in the early stages of the universe, as it cooled from the big bang. There are rather specific thermal requirements for this process to occur, so there was only a short time window in which heavier elements could form, meaning that the only fusion to actually happen in significant amounts was the conversion of hydrogen (and deuterium) to helium, and an extremely tiny amount of lithium.
Stellar nucleosynthesis is the fusion of hydrogen and other nuclei in the cores of stars. This is something separate from big bang cosmology, since stars didn't form until millions of years into the universe's lifetime.
Now, contrary to what you might have read, not all elements are formed in stellar nucleosynthesis. There are specific "chains" of nuclear reactions that occur, and only the elements that are produced by those reactions will exist in a star in appreciable quantities. Most stars produce their energy using either the proton-proton chain (in lighter stars) or the CNO cycle (in heavier stars), both of which consume hydrogen and form helium. Once most of the hydrogen has been consumed, the star's temperature will increase and it will start to fuse helium into carbon. When the helium runs out, it will fuse carbon into oxygen, then oxygen into silicon, then silicon into iron. (Of course the actual process is more complicated - see the Wikipedia articles for details.) Several other elements are produced or involved along the way, including neon, magnesium, phosphorous, and others, but lithium is not among them. In fact, stars have a tendency to consume lithium, rather than producing it, so stars actually tend to have only small amounts of lithium.
Supernova nucleosynthesis is the fusion of atomic nuclei due to the high-pressure, high-energy conditions that arise when a large star explodes in a type II supernova. There are certain similarities between this and big bang nucleosynthesis, namely the high temperatures and pressures, but the main difference is that an exploding star will have "reserves" of heavy elements built up from a lifetime of nuclear fusion. So instead of just forming a lot of helium as occurred just after the big bang, a supernova will form a whole spectrum of heavy elements. In fact supernovae are the only natural source of elements heavier than iron, since it actually requires an input of energy to produce those elements as fusion products. I believe some amount of lithium would be formed in a supernova along with all the other elements, but since a large star would have used up its hydrogen and helium in the central region where most of the action takes place, lithium is probably not a particularly common reaction product.
Related Question
- Cosmology – When Did the First Carbon Nucleus in the Universe Come Into Existence?
- [Physics] Does all hydrogen originate from the Big Bang
- Cosmology – How Does Hydrogen to Helium Ratio Prove the Big Bang Theory?
- Early Universe – Why Was the Early Universe Too Hot for Atoms to Form Despite High Pressure?
Best Answer
The answer touches upon the concept of Big Bang Nucleosynthesis (BBN), which is excellently explained on a graduate level in Baumann's lecture notes.
The key idea is the following: In order to form metals (anything heavier than hydrogen and helium), you need deuterium nuclei. But deuterium nuclei are only formed significantly when the temperature of the primordial plasma falls far below the binding energy of deuterium (T ~ 2.2 MeV). Why? Well, the formation of deuterium has to compete with the enormous amount of high-energy photons in the universe at the time, which split up the deuterium nuclei. So the photon bath has to be cool enough, so that most of the photons don't have enough energy to split the nuclei apart again. The relevant number is the baryon-to-photon ratio $\eta\sim10^{-9}$, i.e. for each baryon we have $10^9$ photons.
Once deuterium is produced, it is almost instantly fused to helium nuclei. However, basically no elements heavier than helium are formed in BBN because they require high enough number densities of helium nuclei from which they would be fused. But by the time helium fusion has begun, the reaction rates for those are already too slow.
For a more detailed and quantitative discussion see the link to the lecture script, chapter 3 'Thermal history', the section about BBN.