56Ni

Nickel-56 (56Ni) is a radioactive isotope of nickel with 28 protons and 28 neutrons. It has a mass number of 56 and is unstable, decaying through a short sequence that powers energy release in stellar explosions. 56Ni decays by beta-plus decay (positron emission) to cobalt-56 (56Co) with a half-life of about 6.1 days. The resulting 56Co then decays, primarily by beta-plus decay and electron capture, to stable iron-56 (56Fe) with a half-life of about 77 days. The decay chain emits gamma rays that contribute to the observable luminosity of supernovae.

Production and role in astrophysics: 56Ni is produced in significant quantities during explosive silicon burning in supernovae, including thermonuclear (Type Ia) and core-collapse events. In these environments, temperatures and densities are high enough to synthesize iron-group nuclei, among which 56Ni is a major product. The energy released by the decay of 56Ni and its daughter 56Co heats the expanding ejecta, driving the early light curves of many supernovae. The amount of nickel synthesized correlates with peak brightness, making 56Ni a key metric in supernova energetics.

Observational significance: Gamma-ray lines associated with the decay of 56Co (and the preceding 56Ni decay) have been detected in supernova remnants, providing direct evidence of the isotope’s production. The decay chain also explains why supernova light curves brighten quickly after explosion and then fade over weeks to months.

Natural occurrence: 56Ni is not found in stable, natural abundances; it is produced during stellar explosions and decays to cobalt and then iron, influencing nucleosynthesis and chemical evolution in galaxies.