<P> Type Ia supernovae have a characteristic light curve, their graph of luminosity as a function of time after the explosion . Near the time of maximal luminosity, the spectrum contains lines of intermediate - mass elements from oxygen to calcium; these are the main constituents of the outer layers of the star . Months after the explosion, when the outer layers have expanded to the point of transparency, the spectrum is dominated by light emitted by material near the core of the star, heavy elements synthesized during the explosion; most prominently isotopes close to the mass of iron (iron - peak elements). The radioactive decay of nickel - 56 through cobalt - 56 to iron - 56 produces high - energy photons, which dominate the energy output of the ejecta at intermediate to late times . </P> <P> The use of Type Ia supernovae to measure precise distances was pioneered by a collaboration of Chilean and US astronomers, the Calán / Tololo Supernova Survey . In a series of papers in the 1990s the survey showed that while Type Ia supernovae do not all reach the same peak luminosity, a single parameter measured from the light curve can be used to correct unreddened Type Ia supernovae to standard candle values . The original correction to standard candle value is known as the Phillips relationship and was shown by this group to be able to measure relative distances to 7% accuracy . The cause of this uniformity in peak brightness is related to the amount of nickel - 56 produced in white dwarfs presumably exploding near the Chandrasekhar limit . </P> <P> The similarity in the absolute luminosity profiles of nearly all known Type Ia supernovae has led to their use as a secondary standard candle in extragalactic astronomy . Improved calibrations of the Cepheid variable distance scale and direct geometric distance measurements to NGC 4258 from the dynamics of maser emission when combined with the Hubble diagram of the Type Ia supernova distances have led to an improved value of the Hubble constant . </P> <P> In 1998, observations of distant Type Ia supernovae indicated the unexpected result that the Universe seems to undergo an accelerating expansion . Three members from two teams were subsequently awarded Nobel Prizes for this discovery . </P>

What is the difference between a nova and a type 1a supernova