The Hard Weight Limit No Dead Star Can Cross

There is a hard weight limit, and no dead star is allowed to cross it.

It is 1.4 times the mass of our Sun, packed into an Earth-sized white dwarf. One step over, and the star cannot survive.

A white dwarf is held up by electron pressure, and that pressure has a limit. The physicist Subrahmanyan Chandrasekhar showed that above about 1.4 solar masses, electrons can no longer support the star. If a white dwarf steals enough gas from a companion and tips past that limit, it collapses and detonates as a Type Ia supernova. Here is the payoff: because every one of these blasts ignites at the same precise mass, they all shine with nearly the same brightness. That makes them standard candles. By measuring how bright they look, astronomers map distances across the universe.

Studying these exploding dead stars is how scientists discovered that the universe is not just expanding, but speeding up, the finding that revealed dark energy.

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