A teaspoon of this star weighs a billion tons

Somewhere out in the galaxy sits the crushed core of a dead giant star, spinning up to hundreds of times per second. It's called a neutron star, and it packs more mass than our Sun into a ball only about 20 kilometers wide — roughly the size of a city. Scoop up a single teaspoon of its material and it would weigh around a billion tons, more than every human on Earth combined.

The secret is that ordinary matter is mostly empty space. An atom is a tiny nucleus surrounded by a vast cloud of electrons, like a fly in a cathedral. When a massive star runs out of fuel and collapses, gravity becomes so ferocious it crushes that emptiness away — forcing electrons and protons to merge into neutrons packed shoulder to shoulder. What's left is essentially one gigantic atomic nucleus.

The gravity at the surface is so intense that a marshmallow dropped onto a neutron star would hit with the energy of an atomic bomb. And yet these objects aren't rare monsters — they're the ordinary corpses of big stars, scattered throughout the cosmos as a reminder of just how much nothing the solid world around you actually contains.