Death of massive stars
Stars more than about eight times the Sun's mass end their lives in spectacular supernova explosions, scattering the elements needed for planets and life.
Fusing heavier elements
Massive stars become hot enough to fuse carbon, then neon, oxygen, and silicon, building an onion-like structure of shells. Each stage is shorter: silicon fusion may last only about a day. Fusion stops at iron, because fusing iron absorbs energy rather than releasing it.
Core collapse
When the iron core grows to about 1.4 solar masses or more, it can no longer support itself. In a fraction of a second, it collapses: electrons and protons combine into neutrons, releasing a flood of neutrinos. The collapse halts when the core reaches nuclear density, and infalling material rebounds.
Type II supernovae
The rebound shock, powered by neutrinos, blasts the star's outer layers into space in a core-collapse (Type II) supernova, briefly shining as brightly as billions of Suns. Supernova 1987A, in a nearby galaxy, was the closest observed in centuries, and detectors on Earth caught about two dozen of its neutrinos.
Making the elements
Elements up to iron are made by fusion in stars. Many heavier elements form in supernovae and in collisions of neutron stars through rapid neutron capture. In 2017, a neutron star merger detected by gravitational waves was observed producing heavy elements such as gold and platinum. The iron in your blood and the calcium in your bones were made in stars: 'we are made of star stuff'.
Neutron stars
If the collapsed core has about 1.4-3 solar masses, it becomes a neutron star: a ball of neutrons about 20 km across, as dense as an atomic nucleus. A teaspoon would weigh about a billion tonnes. Neutron stars have intense magnetic fields and can spin hundreds of times per second.
Pulsars
In 1967, Jocelyn Bell Burnell, a graduate student, discovered regular radio pulses from space. They came from pulsars, rapidly spinning neutron stars whose beams of radiation sweep past Earth like a lighthouse. Pulsars are such precise clocks that they have been used to test general relativity and detect very-low-frequency gravitational waves.
Black holes
If the remnant core exceeds about 3 solar masses, no known force can stop its collapse, and it becomes a black hole, a region where gravity is so strong that not even light can escape. The boundary is the event horizon; its radius for a non-spinning black hole is the Schwarzschild radius, about 3 km per solar mass.
Worked example
Compare remnants: a 1-solar-mass star leaves a white dwarf about the size of Earth; a 20-solar-mass star might leave a 1.5-solar-mass neutron star about 20 km across; a 40-solar-mass star might leave a 10-solar-mass black hole with an event horizon about 30 km across. Increasing mass leads to ever more compact remnants.
Key takeaways
Massive stars fuse elements up to iron in shells; fusion stops at iron. The iron core collapses in under a second into neutrons, rebounding in a core-collapse supernova, as in SN 1987A. Stars and supernovae make the elements; neutron star mergers make gold and platinum. Neutron stars are about 20 km across and extremely dense; pulsars were discovered by Jocelyn Bell Burnell in 1967. Remnants above about 3 solar masses become black holes.
Flashcard
Layers of fusion in massive stars
Check your understanding
Why does fusion in a massive star's core stop at iron?