A star of more than about 8 solar masses does not stop at helium. After it becomes a red giant (a red supergiant), its heavy core can squeeze itself hot enough for one fusion stage after another: carbon, neon, oxygen, then silicon, building up a core of layers like an onion. Each stage is shorter than the last; silicon burns in about a day. Then the core reaches iron, and the star's life ends in seconds.
Why it works
Fusion releases energy only when the products are more tightly bound than the fuel. Binding energy per nucleon climbs from hydrogen up to a peak at iron and nickel, about $8.8$ MeV, then falls for heavier nuclei. So fusing anything lighter than iron releases energy, but fusing iron would absorb it. With no energy source, the iron core, about as massive as the Sun and as big as Earth, loses its support and collapses in less than a second, to a ball of neutrons about 20 km across. The infalling layers rebound off it, and a huge burst of neutrinos helps drive them outward: a core-collapse supernova, for weeks as bright as billions of Suns.
Key idea
Massive stars fuse elements up to iron. Iron cannot release energy by fusion, so the core collapses and the star explodes, scattering the elements it made, and new ones made in the blast, into space.
Worked examples
Worked example: Energy from a fusion step
In a massive star's core, two carbon-12 nuclei fuse into magnesium-24. Binding energies per nucleon: C-12, $7.680$ MeV; Mg-24, $8.261$ MeV. How much energy is released?
- Before: $24 \times 7.680 = 184.32$ MeV of binding. After: $24 \times 8.261 = 198.26$ MeV.
- Released: $198.26 - 184.32 = 13.9$ MeV, because the product is more tightly bound.
Answer: About $13.9$ MeV.
Worked example: Dating the Crab
The Crab Nebula, left by a supernova, has a radius of about $5.5$ ly and is expanding at about $1500$ km/s. Roughly how long ago did the star explode?
- Radius: $5.5 \times 9.46 \times 10^{15} = 5.2 \times 10^{16}$ m.
- Time: $\frac{5.2 \times 10^{16}}{1.5 \times 10^{6}} = 3.5 \times 10^{10}$ s $\approx 1100$ years.
- Chinese astronomers recorded a brilliant new star in that spot in the year 1054.
Answer: About 1000 years ago.
Common mistake
It is easy to assume fusion can go on forever, but a star cannot make energy by fusing iron: iron nuclei are already the most tightly bound, so joining them would take energy in. That is exactly why the core collapses. Check the curve: binding energy per nucleon rises up to iron and falls after it.
Supernovae are how the elements in your body got here. Carbon, oxygen, calcium and iron were made in stars and blown into space by supernovae and stellar winds; many elements heavier than iron, like gold, come from the rapid neutron captures in supernovae and in collisions of neutron stars. A different kind, the type Ia supernova, is a white dwarf in a binary that grows past about 1.4 solar masses and blows itself apart.