The overwhelming majority of the sun is made up of hydrogen, and specifically the isotope protium, which is just a proton with no neutrons.
When two protons collide, the overwhelming majority of the time they just bounce off eachother. The diproton (a particle with two protons and no neutrons) isn't just unstable, it's basically forbidden.
However there is an incredibly rare chance that when two protons come together instead of just splitting up, one of the protons will turn into a neutron, thus producing deuterium, the hydrogen isotope with one proton and one neutron. Deuterium is much more capable of fusing.
Now when I say incredibly rare, I mean like "unlikely to happen to any given hydrogen atom over the lifetime of a star" rare. Luckily stars are big, and dense, and thus contain a lot of hydrogen atoms hitting each other very frequently. Thus you have a slow but consistent burn of hydrogen over billions of years. Once it's converted to deuterium, it fuses within about 1 second. This produces helium-3 (2 protons, 1 neutron), which is also capable of fusion, but not with the abundant protium[1], so it takes some time to find something to fuse with. A helium-3 will on average survive about 200 years before it fuses, which is still pretty short as far as the sun is concerned.
This slow burn means that within any given chunk of the sun there aren't that many fusion events occurring. Even in the deepest core of the sun, power densities never exceed 275 Watts per cubic meter. By comparison, compost releases about 7000 Watts per cubic meter at its peak decay rate and even over longer time periods averages around 600 Watts per cubic meter. Human metabolism releases about 1600 Watts per cubic meter. But since the sun is so immensely large, it has a lot of cubic meters of fusing hydrogen. Overall, it's producing 3.8x10^26 Watts.
[1] Actually helium-3 can (at least in theory) fuse with a proton, it's just extremely rare. In fact it is so rare we've never actually observed it.
It should be noted that in stars slightly larger than the Sun, fusion energy production becomes dominated by the CNO cycle, which has a much stronger temperature dependence than the PP cycle.
When two protons collide, the overwhelming majority of the time they just bounce off eachother. The diproton (a particle with two protons and no neutrons) isn't just unstable, it's basically forbidden.
However there is an incredibly rare chance that when two protons come together instead of just splitting up, one of the protons will turn into a neutron, thus producing deuterium, the hydrogen isotope with one proton and one neutron. Deuterium is much more capable of fusing.
Now when I say incredibly rare, I mean like "unlikely to happen to any given hydrogen atom over the lifetime of a star" rare. Luckily stars are big, and dense, and thus contain a lot of hydrogen atoms hitting each other very frequently. Thus you have a slow but consistent burn of hydrogen over billions of years. Once it's converted to deuterium, it fuses within about 1 second. This produces helium-3 (2 protons, 1 neutron), which is also capable of fusion, but not with the abundant protium[1], so it takes some time to find something to fuse with. A helium-3 will on average survive about 200 years before it fuses, which is still pretty short as far as the sun is concerned.
This slow burn means that within any given chunk of the sun there aren't that many fusion events occurring. Even in the deepest core of the sun, power densities never exceed 275 Watts per cubic meter. By comparison, compost releases about 7000 Watts per cubic meter at its peak decay rate and even over longer time periods averages around 600 Watts per cubic meter. Human metabolism releases about 1600 Watts per cubic meter. But since the sun is so immensely large, it has a lot of cubic meters of fusing hydrogen. Overall, it's producing 3.8x10^26 Watts.
[1] Actually helium-3 can (at least in theory) fuse with a proton, it's just extremely rare. In fact it is so rare we've never actually observed it.