You need a hot star to forge heavier atoms

Forging a heavier atom than a hydrogen, you have to bring a couple of protons together. But protons hate being crammed into a small place. Neutrons save the situation as pacifiers. They keep protons in one place, in the nucleus of an atom. It happens in stars, where protons and neutrons have a chance to meet.

However, even a neutron is not convincing enough to bring protons close to each other. The process needs a furnace, very hot environment, because the hotter is the matter the faster is their motion. So while zipping around like crazy, protons don’t even notice that they got into touch with their hated siblings. The more complex an atom, the higher temperature is needed to forge them.