X-ray binaries are a class of binary stars that are very luminous in X-rays. The X-rays are produced by matter falling from one component (usually a relatively normal star) to the other component, which is a neutron star or a black hole. The infalling matter releases gravitational potential energy, up to several tens of per cent of its rest mass as X-rays. (Hydrogen fusion releases about 0.7 per cent of rest mass)

X-ray binaries are further subdivided into several (sometimes overlapping) subclasses, that perhaps reflect the underlying physics better: