white dwarf star, composed primarily of carbon and oxygen. Eventually, the secondary star also evolves off the main sequence to form a red giant. Matter from the giant is accreted by the white dwarf, causing the latter to increase in mass. The exact details of initiation and of the heavy elements produced in the catastrophic event remain unclear. Type Ia supernovae produce a characteristic light curve—the graph of luminosity as a function of time—after the event. This luminosity is generated by the radioactive decay of nickel-56 through cobalt-56 to iron-56. The peak luminosity of the light curve is extremely consistent across normal type Ia supernovae, having a maximum absolute magnitude of about −19.3. This is because typical type Ia supernovae arise from a consistent type of progenitor star by gradual mass acquisition, and explode when they acquire a consistent typical mass, giving rise to very similar supernova conditions and behaviour. This allows them to be used as a secondary standard candle to measure the distance to their host galaxies. A second model for the formation of type Ia supernovae involves the merger of two white dwarf stars, with the combined mass momentarily exceeding the Chandrasekhar limit. This is sometimes referred to as the double-degenerate model, as both stars are degenerate white dwarfs. Due to the possible combinations of mass and chemical composition of the pair there is much variation in this type of event, and, in many cases, there may be no supernova at all, in which case they will have a less luminous light curve than the more normal SN type Ia. Non-standard Type Ia Abnormally bright type Ia supernovae occur when the white dwarf already has a mass higher than the Chandrasekhar limit, possibly enhanced further by asymmetry, but the ejected material will have less than normal kinetic energy. This super-Chandrasekhar-mass scenario can occur, for example, when the extra mass is supported by differential rotation. There is no formal sub-classification for non-standard type Ia supernovae. It has been proposed that a group of sub-luminous supernovae that occur when helium accretes onto a white dwarf should be classified as type Iax. This type of supernova may not always completely destroy the white dwarf progenitor and could leave behind a zombie star. One specific type of supernova originates from exploding white dwarfs, like type Ia, but contains hydrogen lines in their spectra, possibly because the white dwarf is surrounded by