Astronomers have used accurate timing of pulsar signals to determine the mass of a tiny neutron star and its white dwarf companion, showing that the neutron star is the most massive ever identified. However this is more than an exercise in finding new record holders, neutron stars are held up from collapsing into a black hole by quantum mechanics and nuclear physics, and these extreme conditions are at the limits of our theoretical understanding. So finding the upper mass limit of neutron stars gives important information on the conditions inside nuclear matter.