Type Ia supernova

A Type Ia supernova is a thermonuclear stellar explosion associated with the disruption of a carbon-oxygen white dwarf. Rapid nuclear fusion releases enough energy to unbind the compact body and eject newly processed material. Several progenitor routes are studied, including the Double-degenerate Type Ia supernova scenario; the observational class does not identify one unique prior binary history.

The subsequent radioactively powered Type Ia supernova light curve is strongly influenced by the amount and distribution of radioactive material and by photon escape through expanding ejecta. Its peak luminosity is therefore not identical in every event. Empirical standardization using light curve shape and colour makes the class useful for distances, while composition, extinction and population differences must be accounted for in precision applications.

The nickel-56 to cobalt-56 to iron-56 decay chain supplies gamma rays and charged particles that heat supernova ejecta. Early expansion and photon diffusion delay the escaping optical light; after a peak on a scale of weeks, the luminosity falls as decay power declines and gamma rays escape more readily. With comparable diffusion times and deposition efficiencies, peak luminosity scales approximately with the nickel-56 mass. Using one fixed absolute luminosity without accounting for composition-dependent yields can therefore bias standard-candle distances.

Approximate a fixed mass of iron-group freeze-out ashes by nickel-56 and iron-54, with no further weak neutronization and negligible residual light particles. Their electron fractions are $1/2$ and $26/54$. Conservation of the initial electron fraction in stellar matter, $1/2-X_{22}/22$, then gives the displayed nickel mass fraction. Its argument is the initial neon-22 mass fraction, which need not equal the earlier nitrogen metallicity. The two-isotope mass-budget approximation is not an exact full nuclear equilibrium with all free-particle densities set to zero.