Scoop a single teaspoon of it and you would be holding roughly the weight of a mountain — a billion tonnes, in a spoon.

What's left when a star dies

A big star spends its life fusing light elements into heavier ones, pouring out energy that props it up against its own crushing gravity. When the core finally runs out of fuel, that outward push vanishes in seconds. The core collapses, the outer layers rebound in a supernova — briefly one of the brightest things in the universe — and what remains at the centre is squeezed past anything on Earth.

If the leftover core weighs between about one and two times the Sun, gravity rams its electrons and protons together into neutrons, packed to the density of an atomic nucleus. The result is a neutron star: the Sun's mass folded into a ball roughly twenty kilometres across, the size of a city. A sugar cube of it would outweigh all of humanity. It is, near enough, one colossal atomic nucleus the size of a mountain.

What holds it up

Nothing is fusing any more, so what stops it collapsing the rest of the way? The answer is the same rule that keeps your hand from passing through a table: neutrons, like electrons, refuse to share a quantum state. Crush them together and they are forced into ever higher-energy states, pushing back with a stubborn pressure that owes nothing to heat or fusion — pure quantum accounting, holding up an entire star.

Walter Baade and Fritz Zwicky proposed such objects in 1934, only two years after the neutron was discovered, and it sounded like fantasy. Then in 1967 Jocelyn Bell Burnell, combing through radio data, found a signal pulsing with impossible regularity every 1.3 seconds. The team half-jokingly tagged it LGM-1, for "Little Green Men," before working out what it really was: a neutron star, spinning fast and sweeping a beam of radiation past the Earth like a lighthouse. We call those pulsars.

A neutron star is what matter does when gravity almost wins — crushed to the very brink of a black hole, and stopped, at the last moment, by a rule about quantum bookkeeping.

The last stand before the dark

There is a ceiling to that defiance. Pile on more than roughly two Suns' worth of mass and even neutron pressure gives way. Nothing known can hold the line, and the core collapses the rest of the way into a black hole. A neutron star is the universe's last visible object — one notch short of a place where the future itself points inward.

They keep making news. In 2017, detectors caught the ripples of two neutron stars colliding some 130 million light-years away, and telescopes watched the afterglow confirm that such smash-ups forge much of the universe's gold and platinum. The ring on a finger may well have been minted in one.

And the honest edge sits at the very centre, where the pressure runs highest of all: we don't know what matter becomes there. The neutrons may dissolve into a soup of free quarks — a state we can't reproduce and can barely model. The densest thing we can point to is also, at its heart, a question we can't yet answer.