How does nuclear fusion in the Sun create new heavier elements?

Answer 1

Smaller nuclei fuse to become larger ones.

The sun is mostly hydrogen and helium. This is how stars start out. They make energy by squeezing the hydrogen nuclei together till they stick:

- two simple hydrogen (H1) nuclei (protons) get stuck into a Deuterium (H2) nucleus. One proton turns into a neutron by absorbing a nearby electron (the two atoms each had an electron to start with but see below).

- the Deuterium nucleus gets another proton stuck onto it and becomes either Tritium (H3) or Helium 3 (He3). Another proton will make it He4 which is regular helium. (or maybe H4 or Li4 but they decay into He4. He4 fills out the inner orbital so it's really stable.)

- and so on and so on. As a star gets older, it starts having more He and Li and Be and heavier elements from this process. If there's enough helium or Lithium, they'll start fusing with each other and make even bigger nuclei, faster.

At each step, energy comes off because it's in a lower energy state - like magnets that are stuck together instead of apart - but in the case of nuclei, it's the Protons that attract the Neutrons and vice versa, thru The Strong Force. But at the same time, all the protons are repelling each other! They're both + charged! Also, if they're cold atoms like you find on earth, they each have electron shells 100,000 times bigger than the nucleus, like a sand particle being suspended in the middle of a big balloon, so two neighboring atoms never get close enough to fuse.

To make it stick together, you have to get the protons and neutrons close enough for The Strong Force to kick in and overwhelm the proton-proton repulsion. Fortunately in the sun, it's so hot (the atoms and atom pieces are flying around so fast) that the electrons have been totally ripped off their nuclei and fly around free with the nuclei - that's called a Plasma. If two protons or other nuclei manage to stumble into each other hard enough, they'll fuse.

As time goes on, heavier and heavier nuclei form. Unfortunately these reactions require more compression and heat. Gravity is still working, so the nuclei keep fusing. the biggest the nuclei become is around where Iron is on the list of elements. At that point, there's no energy coming off at all- it's sortof an energy valley and this is the lowest point. The star becomes a white dwarf, or explodes as a supernova and the core becomes a neutron star or black hole, depending on how big it is.

Heavier nuclei are made either when the star explodes as a supernova, or by a process in red giant stars. The sun is too small for either of these two to happen. The heavy elements on earth are assumed to come from some other supernova like 5 billion years ago.