The quasi-stellar object, the pulsar, the neutron star have all come onto the scene of physics within the space of a few years. Is the next entrant destined to be the black hole? If so, it is difficult to think of any development that could be of greater significance. A black hole, whether of “ordinary size” (approximately one solar mass, 1 M⊙) or much larger (around 10^6 M⊙ to 10^10 M⊙, as proposed in the nuclei of some galaxies), provides our “laboratory model” for the gravitational collapse, predicted by Einstein's theory, of the universe itself
