Neutron stars: stellar corpses stranger than fiction

Take one and a half Suns and squeeze them into a ball the size of a city. That is a neutron star: the collapsed heart of a massive star after its supernova. Its density defies intuition — a teaspoon weighs a billion tonnes — and surface gravity runs 100 billion times Earth's. After the black hole, it is the most extreme object the universe makes. But unlike a black hole, a neutron star can be seen — and what we see is spectacular.

How does such a thing form?

When a massive star's core collapses, protons and electrons are literally crushed together into neutrons. What remains is effectively one giant atomic nucleus, ten to fifteen kilometres in radius. The collapse works like a pirouette: as the core shrinks, its spin accelerates — from once a month to tens of times per second. The magnetic field gets compressed along with it, to trillions of times Earth's.

Pulsars: cosmic lighthouses

Beams of radiation shoot from a neutron star's magnetic poles. If those poles are tilted relative to the spin axis — as on Earth — the beams sweep around like a lighthouse. When one sweeps across Earth, we see a pulsar: a star ticking with a regularity that rivals atomic clocks.

Jocelyn Bell found the first in 1967; the signal was so rhythmic it was half-jokingly labelled "LGM-1" — Little Green Men. We now know thousands, including millisecond pulsars spinning up to 700 times per second: their equators move at a quarter of light speed.

Magnetars: magnetism as a weapon

Roughly one in ten neutron stars is born a magnetar, with a field a thousand times stronger still — the strongest magnetism known. Such a field literally deforms nearby atoms. When a magnetar's crust cracks (a "starquake"), the flash is measurable across half the galaxy: the 2004 outburst of SGR 1806-20 disturbed satellites and Earth's ionosphere — from 50,000 light-years away.

Kilonovae: the gold factory

When two neutron stars orbit each other, they lose energy to gravitational waves and spiral inexorably together. The collision — a kilonova — is among the most violent events in the universe: a gamma-ray burst, a gravitational wave and a cloud of freshly forged elements all at once. The famous 2017 detection GW170817 proved the long-held suspicion: this is where gold, platinum and uranium are made. The ring on your finger is, most likely, kilonova ash.

The border with the black hole

Neutron stars have a limit: somewhere between two and three solar masses, even neutron pressure gives way. What exactly happens near that limit — and how matter behaves deep inside a neutron star — remains one of physics' open questions. The answer may come with the next kilonova we watch live.

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