Time-dependent evolution of the mass fraction of ⁵⁶Ni and ⁵⁶Co. The decay of these nuclei is the dominant source of energy for the light emitted by SN Ia. Because the half-life of ⁵⁶Ni to ⁵⁶Co is very short (6.1 days), this decay is only important in the first 50 days. ⁵⁶Co, on the other hand, decays to the stable nucleus ⁵⁶Fe within a much longer half-life (77.3 days) and is therefore the dominant source of energy for a period of 2 to 3 years.