A Type Ia supernova is a supernova with a characteristic light curve including oxygen and calcium spectral lines when the light is near its peak, with spectral lines of other elements afterward in sequence. Its light curve, specifically, the rate at which light declines after the peak, is correlated with its peak brightness and with knowledge of its brightness, its distance can be estimated. They are observed in other galaxies of all types and provide a means of estimating distance to galaxies. For this reason they are considered a standard candle, a source of electromagnetic radiation for which the luminosity can be determined, irrespective of distance.
The accepted model is a white dwarf accreting matter until it reaches the point where it collapses. Another model, of two white dwarfs merging, has recently been revived, positing that if a third body is present, the chaotic orbits cause a likely collision within a plausible timescale. If that is true, the amount of matter in a Type Ia supernovae could be larger, the limit being two maximum-sized white dwarfs, perhaps could be brighter and not function as well as a standard candle. This could affect otherwise-reliable cosmological conclusions based upon observations of standard candles.
The means of determining brightness from the light curve has been refined over time, a currently-used version being the Phillips relationship or Phillips relation.