Well beyond our solar neighborhood are systems of orbiting neutron stars. These binary systems are born when the cores of two old stars collapse in supernova explosions. Neutron stars have the mass of our Sun but are the size of a city, so dense that boundaries between atoms disappear. In this simulation, the translucent spheres are the neutron stars. The flat circles are a cross-section through the equators.
Colors map matter density, which increases as colors move from yellow to blue to red.
Einstein's theory of general relativity predicts that the orbit shrinks from ripples of space-time called gravitational waves. The stars finally merge in a few milliseconds, sending out a burst of gravitational waves. New supercomputer models taking relativity fully into account will be able to predict the gravitational wave signal. With that, scientists can detect the possible formation of the more massive black holes and perhaps better estimate the expansion rate of the universe.