Astronomy of the invisible

LEGEND has it that when word reached Copenhagen of the discovery of the neutron in 1932, Niels Bohr threw a great party at his home. Among his guests was a young Russian named Lev Landau, who is said to have quietly remarked that "stars could be made with this new particle". Astronomers like to think that this was said in earnest – and not under the influence of Danish hospitality – because we now know that neutron stars really do exist.