For its whole life, a star is a standoff: gravity pulls inward, and the energy from fusion pushes back out. When fusion ends for good, that outward push disappears and the core collapses. What happens next depends on one thing — how much mass is in the collapsing core, which traces back to how massive the star was when it was born.
The collapse stops only if something can push back hard enough. Squeeze matter far enough and its particles resist being packed any tighter, even with no heat at all. Physicists call this degeneracy pressure. It comes in two strengths — one from electrons, a much stronger one from neutrons — and each has a breaking point.
| Starting mass | Remnant | What stops the collapse |
|---|---|---|
| Below ~8 M☉ | White dwarf | Electron degeneracy pressure |
| ~8 to ~20–25 M☉ | Neutron star | Neutron degeneracy pressure |
| Above ~20–25 M☉ | Black hole | Nothing — collapse continues |
Pick a remnant. Each one is drawn to scale next to something familiar.
When a Sun-like star sheds its outer layers as a planetary nebula, the exposed core is a white dwarf: mostly carbon and oxygen, with roughly the Sun's mass packed into a ball about the size of Earth. A single teaspoon of it would weigh around 5 tons — about as much as an elephant. There is no fusion inside. A white dwarf shines only because it's still extremely hot, and it slowly cools over billions of years.
In 1930, a 19-year-old Subrahmanyan Chandrasekhar worked out that electron degeneracy pressure has a limit. No white dwarf can hold more than about 1.4 Suns of mass; beyond that, the electrons lose the fight and the core collapses further. Strangely, adding mass makes a white dwarf smaller, because stronger gravity squeezes it tighter.
When a massive star's iron core collapses, it blows straight past the white dwarf limit. In a fraction of a second, electrons are crushed into protons, turning them into neutrons. The core becomes a neutron star: one and a half to two Suns of mass in a sphere only about 20 km across. A teaspoon of it would weigh billions of tons. The collapse bounces off this ultra-dense core and helps blast the rest of the star apart as a supernova.
Two things get extreme as the core shrinks. Its spin speeds up enormously — the way a figure skater spins faster by pulling in their arms — and its magnetic field is squeezed to around a trillion times Earth's. That field channels beams of radiation out of the magnetic poles. If a beam sweeps across Earth, we see a regular flash on every rotation: a pulsar.
A pulsar's beams sweep around as it spins. We only see a flash when a beam crosses Earth — like a lighthouse seen from a ship.
Jocelyn Bell Burnell discovered the first pulsar in 1967 as a graduate student. Its pulses were so regular that her team half-jokingly labeled it "LGM-1," for "Little Green Men." The Crab Pulsar, at the heart of the Crab Nebula, is the remnant of a supernova that Chinese astronomers recorded in the year 1054.
Neutron degeneracy pressure has a limit too — roughly 2 to 3 Suns. A collapsing core heavier than that can't be stopped by anything we know of. It keeps shrinking until its gravity is so strong that not even light can escape. That's a black hole.
A black hole's boundary is its event horizon: the distance inside which escape is impossible. It isn't a solid surface — just a point of no return. Its size depends only on the mass inside:
Slide the mass. The event horizon's size grows in direct proportion — double the mass, double the radius.
Rule of thumb: horizon radius ≈ 3 km for every Sun's worth of mass.
If black holes give off no light, how do we find them? Three ways. Gas pulled off a companion star heats to millions of degrees and glows in X-rays before crossing the horizon — that's how Cygnus X-1 was identified in the early 1970s. Stars can be seen orbiting something massive and invisible, as at the center of our galaxy. And when two black holes merge, they send ripples through space itself: gravitational waves, first detected by LIGO in 2015.
You can now answer the unit's second essential question from start to finish. A star's starting mass sets how fast it lives (Activity 4.2.1), which path it takes after the main sequence, and — here — which remnant it leaves behind:
| Born with | Lives on the main sequence | Dies as | Leaves |
|---|---|---|---|
| ~0.1–0.5 M☉ | Hundreds of billions to trillions of years | (Not yet — the universe is too young) | White dwarf |
| ~0.5–8 M☉ | Billions to ~100 million years | Red giant → planetary nebula | White dwarf |
| ~8–20 M☉ | Tens of millions of years | Red supergiant → supernova | Neutron star |
| Above ~20–25 M☉ | A few million years | Supergiant → supernova | Black hole |