Here is a different kind of x-ray binary. In the foreground (top) is a hot star called a Be star. It spins so quickly that it is surrounded by a circumstellar disc of matter, flung off via centrifugal force. The Be star is in orbit around a tiny and powerful neutron star (bottom). While the individual stars in many binary systems often have circular orbits, in Be x-ray binaries the orbits are considerably elliptical. When the orbit brings the stars close together, as this image shows, gas from the Be star's circumstellar disc flows towards the neutron star and surrounds it in a much smaller accretion disc. This gas is heated to very high temperatures, emitting x-rays. When the neutron star is far from the Be star, by contrast, it can no longer feed off its companion. Its accretion disc, and therefore the x-rays, then diminish.