How dense is a neutron star?

Around 4 x 10^17 kilograms per cubic metre — comparable to an atomic nucleus. A teaspoon of the material would weigh roughly a billion tonnes on Earth.

How are neutron stars formed?

When a star of roughly 8 to 25 solar masses exhausts its fuel, its iron core collapses in less than a second. Protons and electrons merge into neutrons, the outer layers explode as a supernova, and the compressed core survives as a neutron star.

What happens when two neutron stars collide?

They merge in a kilonova, emitting gravitational waves and a burst of light, and forging heavy elements including gold and platinum. The event GW170817 in 2017 was the first observed in both gravitational waves and electromagnetic light.

What is a Neutron Star?

Q: What is a neutron star?

A neutron star is the collapsed core of a massive star, left behind after a supernova. It contains one to two solar masses in a sphere about 22 km wide, made almost entirely of neutrons and held up by degeneracy pressure.

A neutron star is what remains when the core of a massive star collapses but stops just short of becoming a black hole. It holds between about 1.1 and 2.3 solar masses in a sphere roughly 22 kilometres across — the densest matter that exists anywhere outside an event horizon.

The formation is violent and fast. When a massive star builds an iron core heavier than about 1.4 solar masses, fusion can no longer hold it up. The core collapses in well under a second, electrons and protons are crushed together into neutrons, and the infalling material rebounds off the newly rigid core, blowing the rest of the star apart as a supernova. What is left is held up by neutron degeneracy pressure and the repulsive core of the strong nuclear force.

The numbers are hard to hold in your head. A sugar-cube of neutron star material would weigh about a billion tonnes. Surface gravity is around 200 billion times Earth's; an object dropped from one metre hits the surface at roughly 7 million km/h. The surface is a crystalline iron crust under a plasma atmosphere maybe a centimetre thick, and beneath it the matter may become a superfluid of free neutrons — a state no laboratory can reproduce.

They are also nature's most precise instruments and one of its great forges. In 2017 the merger of two neutron stars was detected simultaneously in gravitational waves and light, confirming that such collisions produce a kilonova and forge much of the universe's gold, platinum and other heavy elements. Push a neutron star past roughly 2.3 solar masses and there is nothing left to stop the collapse: it becomes a black hole.