Stars are giant balls of gas that shine because they’re constantly burning hydrogen into helium in their cores—a process called nuclear fusion. That fusion pushes outward with incredible force, balancing the pull of gravity. It’s what keeps stars glowing and stable.

But no star can shine forever.

When a large star runs out of hydrogen, the fusion slows, and gravity wins. The star collapses in on itself and explodes in a supernova. What’s left behind is something extraordinary—a neutron star.

Although it's only about the size of a city, (such as Manhattan), a neutron star is made almost entirely of neutrons, the dense core particles found in every atom. With no room between them, they’re packed tighter than anything else in the universe.

A single teaspoon of neutron star material would weigh around 4 billion tons. That’s more than the weight of Mount Everest—squeezed into a spoon.