A Type Ia supernova is a Type I supernova (i.e., without hydrogen spectral lines) with silicon absorption lines and a characteristic light curve including oxygen and calcium lines when the light is near its peak, and 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 from a giant-star binary-star companion until it reaches the Chandrasekhar limit, the point where it collapses, (accretion induced collapse, abbreviated AIC). If it accretes hydrogen, there could be hydrogen lines. Another model, white dwarfs merging to achieve the limit, has recently been revived, positing that if a third body is present, the chaotic orbits cause a likely collision within a plausible timescale. The resulting collapse is presumed to trigger carbon fusion (a carbon detonation or carbon deflagration) leading to a thermal runaway.
If both triggers occur in nature, the amount of matter in a Type Ia supernovae could vary, 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 luminosity-decline rate relation), a currently-used version being the Phillips relationship or Phillips relation.
Given the very high interest in identifying Type Ia supernovae in large surveys, methods of identifying them with photometry (photometric Type Ia supernova) have been refined.