Ultracompact binary stars

Stars most often don’t shine alone: Many of the specks of light visible at night actually are binaries comprising two stars orbiting each other. The orbits in general are well described by Newton’s law of gravity, just like the planetary orbits in our solar system; the exceptional cases can be explained by Einstein’s theory of general relativity. Thus, the closer the stars are to each other, the shorter their orbital period as described by Kepler’s third law. At the end of the 1960s, binary stars with periods of less than one hour were discovered, and astrophysicists recognized that the two stars are so close together that an ordinary star like the Sun could not fit in their orbits. ^{1  ,  2} The close proximity of the objects suggests that both components are stellar remnants—white dwarfs, helium stars, neutron stars, or even black holes—formed after stars exhaust their nuclear fuel. Box...