Thanks to the above effect, the fusion of iron nuclei to create a heavier nuclei requires a net input of energy. I hope you see that this must cause a crisis for a star. We are going to discuss the crisis. The 'iron problem' may be set in the context of the earlier analogy. In general, a nucleus consists of a mix of protons and neutrons.
Stable nuclei have roughly the same number of protons and neutrons.
Nuclei with identical Z but different numbers of neutrons are said to be isotopes of the same element. So the nucleus is a mix of protons and neutrons. If it is stable the forces of repulsion and attraction must balance. What are these forces. Gravity exists between the particles, but is too weak to be a factor.