If a white dwarf star slowly gains enough extra mass to pass a critical limit, what specific, powerful type of supernova explosion then destroys it completely?

The specific, powerful type of supernova explosion that destroys a white dwarf star after it slowly gains enough extra mass to pass a critical limit is a Type Ia supernova. A white dwarf is the very dense, compact remnant core of a star that has exhausted its nuclear fuel, with its gravity supported by electron degeneracy pressure, a quantum mechanical force preventing further collapse. This scenario typically occurs in a binary star system, where the white dwarf's strong gravitational pull strips, or "accretes," matter from a companion star. As the white dwarf slowly accumulates this additional mass, its core density and temperature steadily increase. There is a precise critical mass, known as the Chandrasekhar limit, which is approximately 1.4 times the mass of our Sun. This limit represents the maximum mass that electron degeneracy pressure can support against the immense inward force of gravity. When the white dwarf's mass approaches and then slightly exceeds this Chandrasekhar limit, the extreme conditions in its core—specifically, the high density and temperature—trigger the rapid ignition of carbon and oxygen fusion. Because the white dwarf's matter is degenerate, meaning its pressure is largely independent of temperature, this ignition does not lead to a gradual expansion and cooling. Instead, the fusion reaction becomes an uncontrolled, runaway thermonuclear explosion that propagates rapidly through the entire star. This incredibly energetic blast completely obliterates the white dwarf, fusing virtually all of its mass into heavier elements (like nickel-56, which subsequently decays), and leaves no stellar remnant behind. The energy released makes Type Ia supernovae among the brightest events in the universe.