2003: On 25 July 2003, astronomers using ESA’s X-ray observatory, XMM-Newton, announced that they had discovered a pair of X-ray tails, stretching 3 million million kilometres across the sky. They emanate from the mysterious neutron star known as Geminga. The discovery gave astronomers new insight into the extraordinary conditions around the neutron star.
A neutron star measures only 20-30 kilometres across and is the dense remnant of an exploded star. Geminga is one of the closest to Earth, at a distance of about 500 light-years. Most neutron stars emit radio emissions, appearing to pulsate like a lighthouse, but Geminga is 'radio-quiet'. It does, however, emit huge quantities of pulsating gamma rays making it one of the brightest gamma-ray sources in the sky. Geminga is the only example of a successfully identified gamma-ray source from which astronomers have gained significant knowledge.