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.

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Braxton Gallagher · Accepted 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.

Autumn Pham · Accepted Answer

If fusion were to proceed faster, the core would get hotter, it would expand and become less dense, and with less density, fusion would slow down.The main sequence in stars like the Sun does proceed much more slowly than other stages. This is because the p-p chain reaction starts with the fusion of two protons to form a diproton, or helium-2. The diproton is unstable, and usually immediately decays back into two protons, but Bethe realised that on rare occasions it decays by a weak reaction, releasing a neutrino and a positron to form a deuterium nucleus, hydrogen- 2. Because this second process is so rare, it limits the rate of stellar fusion so that stars spend the largest portion of their lives on the main sequence

In stars such as the Sun, what prevents the whole thing exploding at once? Why is the nuclear fusion

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