Stars evolve through distinct phases driven by their internal dynamics, ultimately reaching white dwarfs, neutron stars, or black holes depending on core mass. In 1930, at just 20 years old, Indian astrophysicist Subrahmanyan Chandrasekhar proved that a star's fate hinges on this mass.
Chandrasekhar calculated the critical mass limit beyond which a star cannot support itself against gravity. Known as the Chandrasekhar limit, it stands at 1.44 solar masses (2.9×1030 kg), determining whether a white dwarf explodes or a massive star collapses into a neutron star or black hole.
Main-sequence stars with masses between 0.05 and 10 solar masses (1) end as white dwarfs. Stability arises from the balance between gravitational contraction and radiation pressure from core fusion.
When hydrogen fuses to helium, fusion slows, reducing pressure. The star contracts, heating the core to 100 million Kelvin and igniting helium fusion into carbon and oxygen via the triple-alpha process.
This energy expands the star into a red giant. Helium fusion ends quickly; without sufficient mass for carbon fusion, the core collapses into a white dwarf, ejecting outer layers as a planetary nebula of hydrogen and helium.
The carbon-oxygen core contracts under gravity until extreme density triggers electron degeneracy pressure, governed by the Pauli exclusion principle, which prevents electrons from occupying the same quantum state.
This pressure stabilizes white dwarfs under 1 solar mass (2), leading to gradual cooling into black dwarfs absent external factors.
In binary systems, a white dwarf accretes material from its companion (3), gaining mass and compressing its core. At 1.44 solar masses—the Chandrasekhar limit—carbon fusion ignites runaway thermonuclear reactions, exploding as a Type Ia supernova (4). No compact remnant survives in this single-degeneracy scenario.
Theoretically, rapid accretion might allow collapse to a neutron star (5), though this remains debated.
Stars over 10 solar masses fuse carbon to neon-magnesium, then neon to iron-nickel. Electron degeneracy pressure initially resists gravity, but accumulating iron-nickel mass reaches the Chandrasekhar limit, causing collapse.
Electrons merge with protons into neutrons, emitting neutrinos and forming a 20-30 km neutron star.
Outer layers rebound, creating a shock wave amplified by neutrinos, exploding as a Type II supernova.
Neutron degeneracy pressure stabilizes the star up to 3 solar masses.
Neutron stars remain stable below 3 solar masses, per J. Robert Oppenheimer and George Volkoff. Beyond this, they collapse into black holes.
In binaries, accretion can push mass to this limit, forming an event horizon and stellar black hole.
Very massive cores may bypass the neutron star phase, collapsing directly.
