What powers the visual light curve in a Type Ia supernova?

Radioactive decay, primarily Nickel-56 decaying to Cobalt-56 and then Iron-56.

Unlike core-collapse supernovae where neutrino emission dominates the total energy budget, the mechanism powering a Type Ia supernova, which results from a white dwarf exceeding the Chandrasekhar limit, is fundamentally different. The immense energy output observed visually over months in a Type Ia event is driven by the radioactive decay of newly synthesized heavy isotopes created during the thermonuclear runaway. Specifically, the primary energy source responsible for the characteristic, predictable light curve is the decay sequence starting with Nickel-56 ($ ext{Ni}^{56}$). This unstable isotope decays into Cobalt-56 ($ ext{Co}^{56}$), which subsequently decays into stable Iron-56 ($ ext{Fe}^{56}$). This sequential radioactive decay releases the energy that makes Type Ia supernovae consistent enough to be used as standard candles for measuring cosmological distances.