Figure 12-4 RIVUXG A Supernova in a Distant Galaxy (a) Discovered on October 13, 1773, by Charles Messier, the Whirlpool Galaxy, also known as Messier Number 51 (M51), is more than 20 million ly away. (b) A progenitor star that later exploded into supernova SN 2005cs can barely be seen by the Hubble Space Telescope image just before the explosion. (c) The supernova is easily seen in the second image made January 11, 2005, just 12 days after it was first discovered.