A neutron star is what's left after a massive star dies — a stellar remnant, meaning the compact leftover core after the outer layers of a star are blown away or the whole thing collapses. Take a star roughly 10 to 25 times the mass of our Sun, strip away everything except its core, and crush that core down to a sphere about 20 kilometers across — roughly the size of a city like San Francisco — and you have a neutron star. All the mass, none of the room.

To get a feel for how strange that is, compare densities. A sugar-cube-sized chunk of the Sun's core weighs over a hundred grams — many times what an actual sugar cube weighs, since the core is roughly 150 times denser than water. A sugar-cube-sized chunk of a neutron star would weigh about a billion tons — roughly the mass of Mount Everest, compressed into a volume you could hold in your hand. This is density, the amount of mass packed into a given volume, and neutron stars sit near the top of the scale for anything that isn't a black hole. A neutron star typically packs 1.4 to 2 times the Sun's mass into a ball 20 km across, whereas the Sun itself is about 1.4 million km across. Squeezing the Sun's mass down to city-size is the defining trick of a neutron star.

Why "neutron"? Ordinary matter is mostly empty space — electrons orbit far from a nucleus, and atoms are more vacuum than substance. Under the crushing gravity of a collapsing stellar core, that empty space disappears: electrons get forced into protons, converting them into neutrons and releasing ghostly particles called neutrinos. What remains is matter made almost entirely of neutrons, packed as tightly as they can go — essentially, the object becomes one gigantic atomic nucleus, 20 kilometers wide, held together by gravity instead of the nuclear force. Section 2 covers exactly how this transformation happens during the supernova.