Figure: Light curve of supernova

Type Ia supernovae have a characteristic light curve, their graph of luminosity as a function of time after the explosion. Near the time of maximum 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 iron-group 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.

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. The cause of this uniformity in the luminosity curve is still an open question. In 1998, observations of Type Ia supernovae indicated the unexpected result that the universe seems to undergo an accelerating expansion.

The light curves for type II supernovae is distinguished by the presence of hydrogen Balmer absorption lines in the spectra. These light curves have an average decay rate of 0.008 magnitudes per day; much lower than the decay rate for type I supernovae. Type II are sub-divided into two classes, depending on whether there is a plateau in their light curve (type II-P) or a linear decay rate (type II-L). The net decay rate is lower at 0.012 magnitudes per day for type II-L compared to 0.0075 magnitudes per day for type II-P. The difference in the shape of the light curves is believed to be caused, in the case of type II-L supernovae, by the expulsion of most of the hydrogen envelope of the progenitor star.

The plateau phase in type II-P supernovae is due to a change in the opacity of the exterior layer. The shock wave ionizes the hydrogen in the outer envelope, which greatly increases the opacity. This prevents photons from the inner parts of the explosion from escaping. Once the hydrogen cools sufficiently to recombine, the outer layer becomes transparent.

Credit: http://en.wikipedia.org/wiki/Supernova