Definition
The Chandrasekhar limit is the maximum mass — approximately 1.4 solar masses — that a stellar remnant supported by electron degeneracy pressure (a white dwarf) can have without collapsing.
Above this limit, the quantum pressure of degenerate electrons cannot resist gravity, and the remnant collapses further into a neutron star or, if even more massive, a black hole. The limit was derived in 1930 by Subrahmanyan Chandrasekhar, then 19, on a steamship from India to Cambridge.
Why it matters
How it works
In a white dwarf, gravity is balanced not by thermal pressure but by electron degeneracy pressure — the Pauli exclusion principle forces electrons into higher momentum states as they are packed tighter. Chandrasekhar applied special relativity to the most energetic electrons: as the star becomes denser, those electrons approach the speed of light, and their momentum can no longer rise fast enough to keep up with gravity.
The limit emerges as a clean combination of constants: roughly M_Ch ≈ (ℏc/G)^(3/2) / m_p² ≈ 1.4 M_☉, where m_p is the proton mass. There is no adjustable knob — it is set by quantum mechanics, relativity, and the composition of a typical stellar remnant (carbon and oxygen).
The astrophysical consequences are dramatic. A solitary white dwarf below 1.4 M_☉ cools quietly for trillions of years. A white dwarf in a binary that accretes past the limit undergoes runaway carbon fusion and detonates as a Type Ia supernova — bright enough to serve as a "standard candle" across cosmological distances and the tool that revealed accelerating expansion in 1998. A neutron-star remnant has its own analogous limit (the Tolman–Oppenheimer–Volkoff limit, ~2–3 M_☉) above which a black hole is the only outcome.