Definition of the Chandrasekhar Limit
The Chandrasekhar Limit is a fundamental constant in astrophysics, representing the maximum stable mass for a white dwarf star. Named after Nobel laureate Subrahmanyan Chandrasekhar, this limit is approximately 1.4 times the mass of our Sun (1.4 M☉). Beyond this critical mass, the electron degeneracy pressure that supports the white dwarf against gravitational collapse is insufficient, leading to further compression.
The Role of Electron Degeneracy Pressure
White dwarfs are the remnants of stars that have exhausted their nuclear fuel, primarily composed of carbon and oxygen. Their stability is maintained by electron degeneracy pressure, a quantum mechanical effect where electrons resist being squeezed into the same quantum state. This pressure acts outwards, counteracting the immense inward pull of gravity. As the mass of a white dwarf increases, so does the gravitational force, demanding greater degeneracy pressure to maintain equilibrium.
Implications for Stellar Fate: Type Ia Supernovae
When a white dwarf in a binary star system accretes matter from its companion star and exceeds the Chandrasekhar Limit, it can no longer support itself. This catastrophic collapse triggers a runaway thermonuclear fusion reaction, resulting in a Type Ia supernova. These supernovae are extremely luminous and have a consistent peak brightness, making them crucial 'standard candles' for measuring cosmic distances and understanding the expansion of the universe.
Beyond the Limit: Neutron Stars and Black Holes
If a star's core remnant is more massive than the Chandrasekhar Limit after a supernova, electron degeneracy pressure fails. Depending on the mass, the core continues to collapse, potentially forming an even denser object. If it falls within a certain range (typically 1.4 to 3 times the Sun's mass), it may become a neutron star, supported by neutron degeneracy pressure. For even larger masses, the collapse continues unchecked, leading to the formation of a black hole.