NEUTRON STARS
The densest stars of all are more massive than the Sun but only ten miles across
Normal atoms are composed of electrons, protons and neutrons, but if they’re squashed together forcefully enough then the protons and electrons combine to form further neutrons, and eventually that’s all that’s left. The result is one of the most extreme forms of matter known to science, and it’s what neutron stars are made of. A neutron star is one of the possible outcomes when a star finally runs out of nuclear fuel and collapses under its own gravity. Small and medium-sized stars like the Sun end up as white dwarfs, while the most massive stars become black holes. In between the two, the end result is a neutron star, an object more massive than the Sun but compressed down into a volume the size of a city. Under normal circumstances it would be virtually impossible to observe a neutron star. But fortunately for astronomers, some of them give their presence away in a highly dramatic form as a pulsar. These are rapidly spinning neutron stars with strong magnetic fields which emit high-energy beams that can be detected as regular flashes of radiation when they periodically point towards Earth.