X-ray binary star system, artwork. A star (lower centre) is seen in mutual orbit with a collapsed companion star (upper right). This star could be a neutron star or even a black hole. Material is being pulled from the star by the intense gravity of the collapsed star, forming an accretion disc (white). Extreme rotation speeds within the disc generate temperatures high enough for the emission of X-rays. Jets are forming at the poles of the collapsed star, showing the high-speed emission of material. The system is also called a microquasar. Neutron stars and black hole are super-dense forms of matter, containing the mass of a star in a volume a few kilometres across or less.