I have a rather naive question. In stars such as the Sun, what prevents the whole thing exploding at once? Why is the nuclear fusion happening slowly? I can only assume that something about the fusion is fighting the gravity and slowing the fusion down and when that process is done gravity starts the fusion process again.
[Physics] Why doesn’t the nuclear fusion in a star make it explode
astrophysicsfusionnuclear-physicspressurestars
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Energy of a fission nuclear bomb comes from the gravitational energy of the stars.
Protons and neutrons can coalesce into different kinds of bound states. We call these states atomic nuclei. The ones with the same number of protons are called isotopes, the ones with different number are nuclei of atoms of different kinds.
There are many possible different stable states (that is, stable nuclei), with different number of nucleons and different binding energies. However there are also some general tendencies for the specific binding energy per one nucleon (proton or neutron) in the nuclei. States of simple nuclei (like hidrogen or helium) have the lowest specific nucleon binding energy amongst all elements, but the higher is the atomic number, the higher the specific energy gets. However, for the very heavy nuclei the specific binding energy starts to drop again.
Here is a graph that sums it up:
http://en.wikipedia.org/wiki/File:Binding_energy_curve_-_common_isotopes.svg
It means that when nucleons are in the medium-atomic number nuclei, they have the highest possible binding energy. When they sit in very light elements (hidrogen) or very heavy ones (uranium), they have weaker binding. Thus, one can say that for the low "every-day" temperatures, the very heavy elements (like the very light ones) are quasistable in a sense.
Fission bomb effectively "lets" the very heavy atomic nuclei (plutonium, or uranium) to resettle to the atoms with lower number of nucleons, that is, with higher bound energies. The released binding energy difference makes the notorious effect. In terms of the graph cited above, it corresponds to nucleons moving from the right end closer to the peak.
Yet this is not the only way to let nucleons switch to the higher binding energy state than the initial one. We can "resettle" very light elements (like hydrogen) and let nucleons move to the peak from the left. That would be fusion.
Heavy nucleons emerge in the stars. Here the gravitational energy is high enough to let the nucleons "unite" into whatever nuclei they like. Stars usually are formed from the very light elements and the nucleons inside, again, tend to get to the states with lower energies, and form more "medium-number" nuclei. The energy difference powers stars and we see the light emission, high temperatures and all other fun effects.
However, sometimes the temperatures in the stars are so high, that nucleons form the very heavy nuclei from the medium-number nuclei. even though there is no immediate "energy" benefit.
These heavy elements then disseminate everywhere with the death of the star. This stored star energy can then be released in the fission bomb.
The potential energy explanation is correct. The mass explanation, as you have stated it, is not quite correct.
The other is the binding energy per nucleon explanation which essentially comes down to the fact that small nuclei have lower mass nucleons than slightly larger ones
Here, it is not the nucleons that have a lower mass, it is the nucleus. The child nucleus has a lower mass than the sum of the parent nuclei masses. But this mass deficit cannot be attributed to individual nucleons, it is the whole nucleus.
So, when small nuclei smash together and form the larger one, mass is lost, this mass being converted into the kinetic energy of the products (by E=mc^2) - so that mass-energy is conserved.
The whole idea of “converting” mass into energy is a poor explanation of the actual physics. If $E=mc^2$ and you convert some $m$ into $E$ then you would get an increase in $E$ and a decrease in $m$ so then the left hand and right hand sides of $E= mc^2$ would no longer be equal.
There is no “conversion” involved. Energy has mass and mass has energy. The full formula is $m^2 c^2=E^2/c^2-p^2$. But if $p=0$ you get the famous expression. So the correct way to understand that is not about “converting” energy or mass. Instead, energy at rest has mass and mass is energy at rest.
So, what actually happens is that a system of particles with energy and mass is converted into a system of different particles with the same system mass and energy. Then part of that system leaves and is no longer counted as part of the system. And the remaining pieces of the system have a lower mass snd energy (and maybe momentum).
For example, a deuterium and a proton fuse to make a helium 3 and a photon. The system of the helium 3 and the photon have the same energy and mass as the system of the deuterium and proton. But the photon soon leaves and is no longer counted. So the remaining helium nucleus has less mass than the whole system did. No mass was converted, the particles converted and some left the system.
Best Answer
The fusion that occurs in the core of the Sun occurs in nothing like the conditions you might be thinking of in a bomb, or a fusion reactor. In particular, it occurs at much lower temperatures and at a much lower rate. A cubic metre of material in the solar core is only releasing around 250 W of power by fusion.
The fusion rate is set by the temperature (and to a lesser extent, density) of the core. This in turn is set by the need for a pressure gradient to balance the weight of material pressing down on it from above. At 15 million kelvin (the core temperature, which is much lower than the temperatures in nuclear bombs or fusion reactors), the average proton has a lifetime of several billion years before being converted (with three others) into a helium nucleus. There are two reasons this is slow. First, you have to get protons, which repel each other electromagnetically, close enough together to feel the strong nuclear force. This is why high temperatures are needed. Second, because the diproton is unstable, one of the protons needs to change into a neutron via a weak force interaction, whilst it is in the unstable diproton state, to form a deuterium nucleus. This is just inherently unlikely and means the overall reaction chain to helium is very slow.
The reason there is no bomb-like explosion is because there is no problem in shifting 250 W per cubic metre away from the core, in the same way that a compost heap, which generates about the same power density, does not spontaneously explode. In the case of a star any additional heat goes into more radiation that diffuses away and in work done in expanding the star. As a result, the temperature of the core is stable. Ultimately, any additional energy emerges as sunlight at the solar photosphere.
If for some reason, the opacity to radiation in the core increased, then the temperature would rise and more energy would be generated by fusion. This is exactly what happens in the core as more hydrogen is turned into helium; the core temperature and luminosity do rise, but slowly, on timescales of billions of years.