Most supergiants, having blasted away their outer layers in supernovas, will contract so violently that protons and electrons are squashed together to form neutrons. Now the entire massive blob is crushed into a neutron star, an object made of neutrons packed together as closely as the particles in an atomic nucleus. This is a very unusual and extremely dense form of matter, as most atoms consist mainly of empty space. A neutron star just twenty kilometers across would weigh twice as much as our sun, and a teaspoon of neutron-star stuff would weigh a billion tons.
