[EvaH (OP)]

David asks:

When stars explode, that's when they fuse all the heavier elements, but where do the neutrons come from? If a zinc and a tin nucleus bumped into each other during a supernova, why would the nuclei stick and form a mercury atom when their mutual repulsion could only be overcome if there were excess neutrons already present?

What do you think?

[Halc]

When stars explode, that's when they fuse all the heavier elements, but where do the neutrons come from?

Neutrons are formed as part of the nuclear combustion process of the consumption of free protons.

A proton-proton reaction (see https://en.wikipedia.org/wiki/Proton%E2%80%93proton_chain_reaction ) occurs with a high energy collision of a pair of protons, producing a Deuterium nucleus (a proton and neutron) plus an ejected neutrino and positron.  Smaller stars like our own are dominated by such proton-proton reactions, but larger stars accomplish the same thing with a catalytic CNO cycle.  The end result is the same: New neutrons and expulsion of a neutrino and positron, raising the neutron to proton ratio as the star ages.

As for the reactions during a supernova, the reactions are complicated, but similar creation of excess neutrons from high energy protons can produce the balance needed for the heavier elements.

[OliVDB]

I wanted to answer that question, but it would have been much heavier, much wordier, for the same meaning; thanks for explaining it much better than I could have done myself!

[evan_au]

where do the neutrons come from?

When two protons (Hydrogen nuclei) bump into each other in the high-temperature/high pressure interior of a star, they temporarily form a Helium-2 nucleus, which is incredibly unstable, and flies apart in an instant.
- The two protons repel each other strongly, and their are no neutrons to provide additional binding through the Strong Nuclear Force.

Deuterium is stable, and so it is energetically favorable for Helium-2 to turn into Deuterium - but it (mostly) doesn't.
- Otherwise all of the Sun's Hydrogen would have fused into Deuterium long ago

The reaction:  proton -> neutron + neutrino + positron
is governed by the Weak Nuclear Force. It occurs very rarely
- During the fleeting instant which is the lifetime of Helium-2, it is extremely unlikely that the Weak Nuclear Force would turn a proton into a neutron.
- An estimate in the Wikipedia article above suggests that in the high pressure/high-temperature conditions in the core of the Sun, the average lifetime of a proton is about 9 billion years before it would turn into a neutron (as a component of Deuterium). This reaction is so rare that it is hard to measure it in the laboratory.
- It is energetically unfavorable for an isolated proton to turn into a neutron.
See: https://en.wikipedia.org/wiki/Weak_interaction

Heavier nuclei repel each other much more strongly than protons, so it requires much higher temperatures and pressures to force these nuclei together. This requires stars that are more massive than the Sun, with higher internal temperatures and pressures.
- Stars will burn Hydrogen to Helium first, then enter a Red Giant phase, where they burn heavier and heavier elements
See: https://en.wikipedia.org/wiki/Red_giant

The electrostatic repulsion of zinc and a tin nuclei is so high that they are unlikely to approach each other
- In fact, Iron and Nickel are the most stable elements
- Elements heavier than this (like Zinc and Tin) will not be produced in normal fusion of a star
- Some amount of these elements may be produced in the fury of a supernova
- It is now thought that much of the heavy elements (like Mercury) are released into space in neutron star collisions.
See: https://en.wikipedia.org/wiki/Nuclear_binding_energy#Nuclear_binding_energy_curve