Nuclear-fuelled supernovae

Not all supernovae result from the collapse of a red giant star: type Ia (“one A”) supernovae occur when a white dwarf has accumulated sufficient matter to exceed the Chandrasekhar limit (any white dwarf with less than this limit – 1.4 times the mass of the Sun - will stay a white dwarf forever, while a star that exceeds this mass is destined to become a supernova).

The explosion is driven by the fusion of carbon into iron by a ‘nuclear burning front,’ which travels through the star. Though much material will be dispersed during this explosion, simulations suggest it is possible that a bound remnant will remain. In such thermonuclear supernovae, nuclear decay plays a role in the brightness of the event. As fusion progresses, nickel-56 is produced. This decays into cobalt-56 with the release of gamma rays, which excite surrounding nuclei of oxygen, silicon, sulphur and calcium. These nuclei then emit radiation, increasing the brightness of the supernova explosion. Peak brightness of this kind of supernova typically occurs two to three weeks after the onset of the supernova and decreases as levels of nickel-56 fall. The cobalt-56 gradually decays into iron-56 leading to a gradual decrease in brightness of the supernova.