Significance
Above about 1.4 solar masses, electron degeneracy pressure can no longer hold a dead star up against its own gravity, and collapse continues to a neutron star or a black hole. Worked out by Chandrasekhar at 19, aboard the ship carrying him to Cambridge, and disputed for a decade by Eddington, who thought the conclusion absurd. It sets the scale for Type Ia supernovae, which is why they can be used as distance markers at all — but the old line that they all detonate at exactly this mass has not survived: they are standardised after the fact, by correcting brightness against how fast the light curve fades, and a good part of them may come from white dwarfs that never reached the limit.