They are built up over time inside stars, as they fuse lighter
elements into heavier ones through nuclear fusion at their cores.
In the case of the 250-millimeter shells, the detonator and fuse lighter
were enclosed in the shell with tip or nose up.
Then, when the burning switches to a detonation, the higher density of the exploding star makes it easier to fuse lighter
nuclei into heavier ones to produce nickel-56.