Neutron Stars Are So Dense a Teaspoon Would Weigh a Billion Tonnes

A neutron star is the collapsed core of a massive star that has gone supernova — compressed so densely that protons and electrons are forced together into neutrons. A teaspoon of neutron star material would weigh approximately one billion tonnes on Earth. They spin up to 700 times per second, and their magnetic fields are trillions of times stronger than Earth's.

Source: Black Holes and Time Warps — Kip Thorne