|In the sun, how come hydrogen atoms, the
simplest, only form helium? Why aren't other more
complex atoms formed in the fusion?
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
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
Right now, the Sun is still in the
hydrogen-to-helium phase, but it will eventually
move on to forming heavier elements.
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.
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
supernova, higher mass elements can be
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