Chandrasekhar limit

There is a hard ceiling on how heavy a white dwarf can be — about 1.4 times the mass of the Sun. Pile on more than that and the star can no longer hold itself up; it must collapse or explode. This ceiling is the Chandrasekhar limit, and it is one of the most consequential numbers in astrophysics, because it decides which stars die quietly and which die in a flash bright enough to be seen across the universe.

The reason for the limit is a beautiful clash between two pieces of physics. A white dwarf is supported by electron degeneracy pressure — electrons refusing to be squeezed together. But add mass and gravity squeezes harder, forcing the electrons to move faster and faster until they approach the speed of light. Once they are this fast, Einstein's relativity says they cannot push back any harder no matter how much you compress them. Above about 1.4 solar masses, gravity simply wins. A 19-year-old student, Subrahmanyan Chandrasekhar, worked this out in 1930 on a ship sailing from India to England.

The limit's real power shows in what happens at the edge. A white dwarf siphoning gas from a companion creeps toward 1.4 solar masses, and as it does, runaway nuclear burning ignites and the entire star detonates as a Type Ia supernova. Because they all blow up at nearly the same mass, these explosions reach nearly the same peak brightness — which is exactly what makes them standard candles for measuring cosmic distances and, ultimately, the accelerating expansion of the universe. Note the limit is not exactly 1.4; the precise value depends on composition and rotation.