Expansion of supernova SN 1993j, radio images. Composite of images showing the expansion over seven years (from 1993 to 2000) of the shell-like structure that formed following the supernova in the galaxy M81. SN 1993j was discovered on 28 March 1993. By observing at regular intervals with the Very Long Baseline Array (VLBA), the Very Large Array (VLA), and various tracking stations around the world, this sequence of images shows a shell-like radio structure expanding with circular symmetry. At first the expansion rate was over 16,000 kilometres per second, but it has decelerated since, and had slowed to less than 10,000 kilometres per second by February 2000. Here, blue is the faintest radio signal and red the strongest. M81 is 11 million light-years from Earth in the constellation of Ursa Major. This image combines X-band and 4-centimetre radio emissions.