Discovery of supernova 1993J. X-ray telescope images of the central region of the galaxy M81 in October 1992 (left) and April 1993 (right), showing the appearance of the supernova SN 1993J (arrowed, lower right). A supernova is the explosive death of a large star. This supernova was discovered on 28 March 1993 by Spanish amateur astronomer Francisco Garcia Diaz. SN 1993J is around 11 million light years distant, in the constellation Ursa Major. The discovery was followed by other observations, including these images obtained with the PSPC sensors on the ROSAT satellite X-ray telescope. The scale bar (left) shows an angular distance of one arcminute.