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In the sun, how come hydrogen atoms, the simplest, only form helium? Why aren't other more complex atoms formed in the fusion?
Question Date: 2001-05-11
Answer 1:

Combining protons and neutrons into a single nucleus requires a lot of energy because you have to overcome the tendency of the protons to repel each other, and the heavier the element, the more protons you have to push together, which requires even more energy. So you can see that helium is the easiest element to make in that sense. Successfully fusing hydrogen into helium ends up releasing more energy than it takes to push the protons and neutrons together, so you get a net gain in energy and the process can keep going.
Early in the life of the star, there's only enough energy around to form helium. As the star ages, it begins to run out of hydrogen, and the fusion slows. When this happens, there's nothing to keep the star from collapsing under its own gravity, so it begins to shrink. But as you might know, when you compress a gas, it gets hot. Eventually, the star gets hot enough so that there's enough energy around to allow heavier elements to form from the helium. This sort of cycle repeats until the star ends up forming iron
nuclei. (Why do you suppose the process stops at iron? Where do elements heavier than iron come from?)
Right now, the Sun is still in the hydrogen-to-helium phase, but it will eventually move on to forming heavier elements.

Answer 2:

It is easiest for hydrogen (one proton) to fuse into helium (2 protons, 2 neutrons) because it is the next element on the periodic table.Other elements have more protons and neutrons.
In a star, after all the available hydrogen is fused into helium, and if a star is massive enough, the helium can start fusing into higher mass elements. This process can continue all the way up to iron. For elements higher in mass than iron, you have to add energy to fuse them, so this doesn't happen during a star's normal life. If the
star goes supernova, higher mass elements can be created.

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