Supernova remnant. Coloured Chandra X-ray image of the supernova remnant G21.5-0.9. This remnant is thought to be 40,000 years old, and the central region is about 5 light years wide. Supernovae occur when a star's nuclear reactions can no longer support it against gravity and it collapses in on itself. Collapsing material rebounds off a newly-formed neutron star at the supernova's heart to form the expanding shells of gas. X-rays are produced by high-energy electrons moving in the neutron star's magnetic field lines. G21.5-0.9 lies 16,000 light years from Earth in the constellation Scutum. This image was made by the Advanced CCD Imaging Spectrometer (ACIS) in 1999.