Most type Ia supernovae are attributed to the
thermonuclear explosion of white dwarfs.  A star becomes a
white dwarf before it has completely consumed its thermonuclear
fuel.  The amount of thermonuclear energy locked within a white
dwarf is of order 0.1% of the white dwarf's rest mass energy.
Theorists have three theories that explain what triggers the
release of this energy, with each theory relying on the
white dwarf being a member of a binary system.  The preferred
theory is that the white dwarf grows in mass by pulling gas from
its companion onto itself until it becomes gravitationally
unstable; when the white dwarf collapses, its internal pressure
and temperature rise until thermonuclear reactions cause it
to explode.