Neutron stars are the dense cinders left behind when massive stars explode in supernovae.
A neutron star has a tremendous gravitational force, containing the same mass as our sun squeezed into a sphere about 10 miles in diameter. The neutron stars in Jonker's studies are all in so-called binary star systems, where a neutron star and a normal star orbit each other. The neutron star's gravity rips gas away from its neighbour. This gas then spirals down onto the neutron star, gathering into an 'accretion disk' from where it eventually trickles onto the neutron star. As the gases move towards the neutron star, they become extremely hot and give off X-rays.