It is imminent due to the exhausted Hydrogen atoms in the nuclear fusion of a sufficiently massive star and its strong gravity that collapses such a dead star that stopped its nuclear fusion. The lack of gravitational compression in the core will cause the process of nuclear fusion to stop, since there will no longer be any force capable of fusing atomic nuclei together.
Nuclear fusion is the process that powers active or main sequence stars and other high-magnitude stars, where large amounts of energy are released. Energy released in most nuclear reactions is much larger than in chemical reactions, because the binding energy that holds a nucleus together is greater than the energy that holds electrons to a nucleus.
The difference in mass between the reactants and products is manifested as either the release or the absorption of energy. So the difference in mass arises due to the difference in atomic binding energy between the nuclei before and after the reaction in a nuclear fusion.
After the supply of hydrogen is exhausted in the cores of stars heavy enough, their cores start fusing helium to carbon.
In the most massive stars, the process is continued until some of their energy is produced by fusing lighter elements to iron. Iron has one of the highest binding energies; reactions producing heavier elements are generally endothermic.
Therefore significant amounts of heavier elements are not formed during stable periods of massive star evolution, but are formed in supernova explosions. Most black holes form from the remnants of a large star that dies in a supernova explosion.
