Thermal Runaway
A white dwarf star may accumulate sufficient material from a stellar companion (either through accretion or via a merger) to raise its core temperature enough to ignite carbon fusion, at which point it undergoes runaway nuclear fusion, completely disrupting it. The vast majority are thought to be produced by the gradual accretion of hydrogen and some helium. Because this type of supernova ignition always occurs in stars with almost identical mass and very similar chemical composition, type Ia supernovae have very uniform properties and are useful as standard candles over intergalactic distances. Some calibrations are required to compensate for the gradual change in properties or different frequencies of abnormal luminosity supernovae at high red shift, and for small variations in brightness identified by light curve shape or spectrum.
Read more about this topic: Super Nova, Current Models
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