What mass limit triggers runaway carbon fusion in a white dwarf?

The Chandrasekhar limit (approximately $1.44$ times the mass of the Sun).

A white dwarf, the remnant of a smaller star, is stabilized against gravitational collapse by electron degeneracy pressure, a quantum mechanical resistance derived from the Pauli exclusion principle preventing electrons from occupying the same state. When a white dwarf accretes matter from a companion star in a binary system, its mass increases until this degeneracy pressure can no longer withstand the crushing force of gravity. This tipping point is precisely defined by the Chandrasekhar limit, noted as approximately $1.44$ times the mass of the Sun. Once this limit is surpassed, the gravitational pressure overwhelms the supporting force, leading to an immediate onset of runaway carbon fusion within the core. Because the white dwarf lacks the thermal structure necessary to regulate fusion via expansion or cooling, this results in a catastrophic thermonuclear runaway, characterizing a Type Ia supernova.