Chandrasekhar Limit

Subrahmanyan Chandrasekhar worked it out in 1930 at the age of nineteen, on the boat from Madras to Cambridge. Electron degeneracy pressure, the quantum refusal of electrons to share states, can hold up a cooling stellar core, but only until the electrons become relativistic, at which point the pressure stiffens too slowly to keep pace with gravity and there is a hard ceiling near 1.44 M⊙ for typical carbon-oxygen composition. Eddington ridiculed the result publicly for years; Chandrasekhar received the Nobel Prize for it in 1983.

The limit is the reason Type Ia supernovae are useful. A white dwarf accreting from a companion detonates as it approaches the limit, and because the trigger mass is always about the same, the explosions have nearly uniform peak brightness: the standard candles that revealed cosmic acceleration in 1998. The value quoted is the classical result for a mean molecular weight per electron of 2; rotation, composition and general-relativistic corrections shift it by a few per cent.