X-ray binary in night sky, artwork. Blue giant star (right) and a collapsed star (left) seen from the rocky surface of a planet. The collapsed star, which could be a neutron star, has a bow shockwave (curved line) due to the strong stellar wind of the blue giant companion star. The extreme energies involved mean that X-rays are being emitted from the shockwave and the material accreting around the collapsed star. The two stars form a binary system, and because of the X-rays and the high mass of the blue giant star, it is called a high-mass X-ray binary system. Neutron stars are super-dense forms of matter, containing the mass of a star in a volume a few kilometres across.