[ww520]

It's the electromagnetic force of their 26 electrons that pushes two iron atoms apart such that the gravitational force of the star cannot overcome.

Nuclear fusion happens when two atoms come close enough that their strong nuclear force binds them together. Strong nuclear force only has effect in very short distance at the nuclear. Smaller atoms with their smaller size allow gravity of the star to push them together for fusion to happen. Bigger atoms with their more numerous electrons make it more difficult.

Bigger stars with stronger gravity can force fusions all the way to iron. Although iron resists fusion, a worse fate awaits it. With no more fusion, the star collapses further and becomes much hotter. The iron nuclei are broken down into alpha particles. The electrons and protons combine to form neutrons, releasing a flood of neutrinos, which cause supernova. Supernova has enough strength to cause fusions of all the heavier elements.

[dotancohen]

Thank you, this is phrased so well!

[ww520]

Actually I missed one crucial detail. Not just the electrons of the atoms are repelling each other, the positively charged protons of the atoms are repelling each atom as well, even more so. As the protons are tightly packed in the nucleus, it's even more difficult to force the nucleus of two atoms together.