As their names imply, a low-mass X-ray binary contains stars that are of relatively low mass, being of intermediate mass or lower, with a white dwarf as the compact companion. A high-mass X-ray binary, by contrast, often has one or two high-mass or very high-mass stars in the system, and the compact object is usually a neutron star or black hole. Though both systems emit copious amounts of X-ray radiation, their X-ray spectral signatures differ substantially because the physical conditions in those binary star systems are affected in different ways by the masses of the stars themselves.