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A Type Ia supernova is a Type I supernova (i.e., without hydrogen spectral lines) that has a particular silicon absorption line (Si II at 615 nm). They generally have a characteristic light curve including oxygen and calcium lines when the light is near its peak, and spectral lines of various other elements afterward, occurring in sequence. The 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, and they are considered a standard candle, a source for which EMR luminosity can be determined irrespective of distance. They can be brighter than supernovae in general, but represent a minority. They are observed in other galaxies of all types and provide a means of estimating distance to galaxies.
Type Ia supernovae are presumed to be white dwarf thermonuclear (i.e., fusion) explosions and are referred to as thermonuclear supernovae. For some reason they are much greater than novae, which are smaller such explosions that occur on the surfaces of white dwarfs. Theorized Type Ia supernova triggering mechanisms are generally classified either as a single degenerate model (often SD) or a double degenerate model (DD), for models involving, respectively, one or two white dwarfs. The former presumes accretion from a binary companion and the latter, an explosion due to the pair's merger, forming a white dwarf too large to be stable. The exact means by which the explosion grows to supernova-strength remains of research interest, i.e., the Type Ia supernova problem. If both the model trigger-mechanisms occur in nature, the amount of matter in a Type Ia supernovae may well vary, an upper limit being two maximum-sized white dwarfs, which 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 aka Phillips relation.
Given the very high interest in identifying Type Ia supernovae in large surveys, methods of identifying them using photometry (photometric Type Ia supernova) have been refined.
Generally, all other supernovae (Type II as well as other Type I) are thought to be core collapse supernovae of stars in various conditions, though with recent observations detecting many optical transients including some showing previously-unseen characteristics, there is now interest in other mechanisms that might produce supernova-strength transients.