Neutron stars are the stellar corpses left behind when a massive star goes supernova. They’re unimaginably dense: A tablespoonful of neutron star placed on Earth's surface would weigh roughly as much as Mount Everest (whereas a tablespoonful of the sun would weigh as little as about 5 pounds).
And while the mass range of neutron stars has been relatively well constrained over the years, it’s been harder to pin down precisely how wide they are. Most astronomers, however, think that mass is packed into a sphere about as big as a city.
