A teaspoon of neutron star material would weigh billions of tons.

When a massive star runs out of fuel and its core collapses, the crush can be violent enough to force protons and electrons to merge into neutrons, packing an entire star’s worth of mass into a sphere only about 20 kilometers across. The result is a neutron star, one of the densest objects known to exist.

The density is difficult to grasp: a single teaspoon of neutron star material would weigh on the order of a billion tons, roughly comparable to a mountain compressed into a sugar cube. That extreme density also gives neutron stars intense gravity and, in many cases, powerful magnetic fields and rapid rotation — some spin hundreds of times per second, sweeping beams of radiation past Earth like a lighthouse and appearing to us as pulsars.