Not all stars become black holes; the vast majority do not follow that path. A star’s ultimate destiny is governed by its initial mass, which dictates its lifespan, temperature, and the forces fighting against gravity. After a star exhausts its nuclear fuel, the stellar remnant—the dense core—can be one of three types, separated by precise mass thresholds.
Stellar Mass Determines the Outcome
A star spends most of its existence in hydrostatic equilibrium, balancing the inward pull of gravity with the outward pressure generated by nuclear fusion in its core. A star’s initial mass determines the intensity of its core temperature and pressure, setting the rate at which it consumes its fuel. More massive stars must generate greater internal pressure to resist stronger gravity, causing them to burn fuel quickly and have shorter lifespans. When core fusion ceases, the star loses its primary source of outward pressure, and gravity takes over, leading to a catastrophic collapse. The mass of the core determines which physical mechanism can halt the gravitational freefall.
The End of Low and Medium Mass Stars
The most common stellar fate belongs to low and medium-mass stars, those born with a mass up to about eight times that of the Sun. When these stars deplete core hydrogen, they swell into red giants and shed their outer layers, forming a planetary nebula. What remains is a dense, hot core called a White Dwarf, prevented from collapsing further by electron degeneracy pressure. This quantum mechanical resistance arises because no two electrons can occupy the same quantum state simultaneously. The stability of a White Dwarf depends on its mass remaining below the Chandrasekhar Limit, approximately 1.4 times the mass of the Sun. If the core remnant’s mass is less than this limit, electron degeneracy pressure is sufficient to counteract gravity. Since most stars result in a core remnant well below this threshold, they quietly fade away over billions of years as these dense, Earth-sized stellar embers slowly cool.
High Mass Collapse: Neutron Stars and Black Holes
Stars born with masses greater than eight times that of the Sun experience a violent end because their core is too heavy for electron degeneracy pressure to support. Once fusion stops in the iron core, gravity rapidly overwhelms internal resistance, causing the core to collapse in milliseconds. This rapid implosion triggers a massive Type II supernova, which blows off the star’s outer layers. The final remnant depends on the mass of the collapsing core.
If the core remnant’s mass is between the Chandrasekhar Limit (1.4 solar masses) and a higher threshold, the collapse is halted by neutron degeneracy pressure. This immense pressure forces electrons and protons to combine into neutrons, creating a Neutron Star. The theoretical maximum stable mass for a non-rotating Neutron Star is the Tolman-Oppenheimer-Volkoff (TOV) limit, estimated between 2.2 and 2.9 solar masses. Neutron stars are supported by the same quantum mechanical principle that stabilizes White Dwarfs, but applied to neutrons.
If the core remnant’s mass exceeds the TOV limit, neither electron nor neutron degeneracy pressure is powerful enough to stop the gravitational collapse. The core continues to compress without resistance, collapsing to a singularity—a point of infinite density. This forms a Black Hole, an object whose gravitational pull is so strong that nothing, not even light, can escape the event horizon. Only the most massive stars, leaving behind cores heavier than approximately 2.9 solar masses after a supernova, become Black Holes.