Neutron stars pack more than the Sun's mass into a city-sized sphere

When a star between 8 and 20 times the mass of our Sun exhausts its fuel, it collapses in a supernova explosion. The core survives as a neutron star — an object typically 20 kilometres in diameter but containing 1.4 times the Sun's mass. A teaspoon of neutron star material would weigh approximately one billion tonnes on Earth. The densest neutron stars — called pulsars — emit beams of radio waves as they spin, sometimes thousands of times per second. When astronomers at Cambridge discovered the first pulsar in 1967, they briefly labelled it 'LGM-1' for 'Little Green Men', suspecting it was artificial.

Source: Hewish et al., Nature (1968); Lattimer, Annual Review of Nuclear and Particle Science (2012)