  Thermonuclear, or Type Ia supernovae (SNe Ia), originate from the explosion
of carbon--oxygen white dwarfs, and serve as standardizable cosmological
candles. However, despite their importance, the nature of the progenitor
systems that give rise to SNe Ia has not been hitherto elucidated.
Observational evidence favors the double-degenerate channel in which merging
white dwarf binaries lead to SNe Ia. Furthermore, significant discrepancies
exist between observations and theory, and to date, there has been no
self-consistent merger model that yields a SNe Ia. Here we show that a spiral
mode instability in the accretion disk formed during a binary white dwarf
merger leads to a detonation on a dynamical timescale. This mechanism sheds
light on how white dwarf mergers may frequently yield SNe Ia.
