Supernova: How a Massive Star Collapses and Explodes

Fusion up to iron, then nothing

A star roughly 8 or more solar masses spends its life fusing progressively heavier elements in its core once each lighter fuel is exhausted: hydrogen to helium, then helium to carbon and oxygen, then on through neon, oxygen and silicon burning, each stage faster and hotter than the last. This ladder stops at iron. Fusing iron does not release energy — iron-56 sits at the peak of nuclear binding energy per nucleon, so fusing it costs energy rather than producing it. The core builds up an inert iron ball with no further fuel to burn.

Collapse in under a second

Without fusion to generate outward pressure, the iron core is held up only by electron degeneracy pressure — a quantum-mechanical effect from the Pauli exclusion principle, independent of temperature. That support has a maximum: the Chandrasekhar limit, about 1.4 solar masses. As the core grows past this limit, electrons are forced to combine with protons (electron capture, producing neutrons and neutrinos), which removes the very pressure holding the core up. The core collapses catastrophically in roughly a quarter of a second, from about the size of the Earth to a ball a few tens of kilometres across.

The bounce and the neutrino engine

Collapse halts abruptly once the core reaches nuclear density, where neutron degeneracy pressure and the strong nuclear force suddenly stiffen — the inner core rebounds like a compressed spring, sending a shock wave outward into the still-infalling outer layers. On its own, this bounce shock usually stalls, losing energy to breaking apart infalling iron nuclei. What revives it, in the leading model, is an enormous burst of neutrinos radiated from the hot proto-neutron star; a small fraction of that neutrino flux is reabsorbed just behind the stalled shock, reheating it enough to push outward again and blow off the star's outer layers in the visible explosion.

What is left behind

The fate of the collapsed core depends on how much mass ends up trapped. Below roughly 2 to 3 solar masses, neutron degeneracy pressure permanently holds up a neutron star. Above that, no known force can stop further collapse and the remnant becomes a black hole. Meanwhile the ejected outer layers, moving at thousands of kilometres per second, plough into surrounding interstellar gas over centuries and form an expanding supernova remnant, seeding the surrounding medium with freshly synthesised heavy elements.

The light curve: two very different clocks

The explosion's brightness over time is not powered by one process throughout. The initial flash comes from shock breakout — the rebound shock reaching the star's surface — followed by a peak dominated by the expanding, radioactively heated envelope becoming transparent. The long, gently declining tail that follows, lasting months, is powered by the radioactive decay chain synthesised in the explosion:

Ni-56  --(half-life ≈ 6.1 days)-->   Co-56  --(half-life ≈ 77 days)-->   Fe-56 (stable)

The cobalt-56 decay, with its 77-day half-life, sets the slow, near-exponential fade seen in the light curve months after peak brightness — a direct, observable signature of freshly made radioactive nickel and cobalt, not of the shock or the collapse itself.

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