What immutable consequence dictates the fate awaiting any star?

Its initial mass

The final destiny of a star, whether it fades quietly or explodes spectacularly, is fundamentally determined by the amount of matter it possessed when it first formed. This initial mass dictates the internal pressure and temperature regimes the star can achieve throughout its lifetime. For low-mass stars, the core never reaches the conditions necessary to fuse elements beyond carbon and oxygen, leading to a gentle death. Conversely, only stars with sufficient initial heft can sustain fusion reactions all the way up to iron. When this final thermonuclear stage is reached, the subsequent catastrophic core collapse is inevitable, directly tracing back to the original bulk of the stellar material. The mass sets the limits on sustained thermonuclear reactions and ultimately defines the stellar corpse left behind, be it a white dwarf, neutron star, or black hole.