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.