Type Ia supernovae occur in a binary star system. Eventually, one of the main sequence stars burns through all of the hydrogen in its core, expanding into a red giant. This may bring part of the surface of the giant star closer to its companion, and mass can overflow from the red giant and be accreted onto the secondary star. When the outside of the red giant is stripped away, all that remains is a white dwarf star.
The second main sequence star will also eventually reach the end of its life and expand into a red giant. If the stars are close enough, matter will again overflow from the red dwarf onto the more gravitationally dense white dwarf star, forming an accretion disk of matter, and sometimes exhibiting relativistic jets. When the white dwarf reaches critical mass (about 1.4 times the mass of our Sun), a nuclear chain reaction occurs, causing the white dwarf to explode (and usually rip apart the system). The resulting light is 5 billion times brighter than the Sun. These chain reactions and resulting explosions are so consistent that they can be used as ‘standard candles’ to measure the distance to their host galaxies.
Credit: CAASTRO/Swinburne Astronomy Productions