Final answer:
A white dwarf explodes as a Type Ia supernova when it accretes matter from a companion star and exceeds the Chandrasekhar limit, triggering runaway nuclear fusion that releases a huge amount of energy.
Step-by-step explanation:
A white dwarf collapses and explodes in a Type Ia supernova due to the accretion of matter from a companion star. This accretion can either be gradual and then accelerate, or result from the merger of two white dwarfs in a binary system. When the white dwarf's mass approaches the Chandrasekhar limit, exceeding 1.4 M☉ (solar masses), it can no longer support itself against gravitational collapse. The resulting increase in density and temperature leads to a runaway fusion process, primarily of carbon, that releases a tremendous amount of energy and destroys the white dwarf, leaving no remnant.