  Thermonuclear supernovae result when interaction with a companion reignites
nuclear fusion in a carbon-oxygen white dwarf, causing a thermonuclear runaway,
a catastrophic gain in pressure, and the disintegration of the whole white
dwarf. It is usually thought that fusion is reignited in near-pycnonuclear
conditions when the white dwarf approaches the Chandrasekhar mass. I briefly
describe two long-standing problems faced by this scenario, and our suggestion
that these supernovae instead result from mergers of carbon-oxygen white
dwarfs, including those that produce sub-Chandrasekhar mass remnants. I then
turn to possible observational tests, in particular those that test the absence
or presence of electron captures during the burning.
