Artist's impression of a Be X-ray binary. At the top is a B-type star with emission lines of hydrogen in its spectrum. Be stars spin so quickly that they are often surrounded by a circumstellar disc of matter, material which has been flung off via centrifugal force like water wrung from clothes in a spin dryer. The emission lines are thought to emanate from this disc. The Be star is in orbit around a neutron star (bottom). In Be X-ray binaries the orbits are considerably elliptical. When the orbit brings the stars close together (periastron), as this image shows, gas from the Be star's disc flows towards the neutron star and surrounds it in a 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.