Messier 61 (NGC 4303) is a spiral galaxy in the constellation Virgo, and one of the larger members of the Virgo Cluster.
M 61 was discovered by Barnabus Oriani on May 5, 1779, while observing a comet. Charles Messier saw it the same night, but mistook it for the comet. By May 11th, Messier realized his mistake, and added it to his catalogue as the 61st entry. William Herschel, who normally avoided numbering Messier's objects, assigned his own number (H I.139) to M 61 when he observed and cataloged it in 1786.
Messier 61 has a visual magnitude of 9.7, and apparent dimensions of 6' x 5.5'. It is a giant spiral member of the Coma-Virgo Supercluster, on the southern fringes of the core of that supercluster. Its spiral arms bend at a rather sharp angle, making this galaxy unique among known spirals.
M 61 is one the largest galaxies in the Virgo cluster. At 65 million light years away, its absolute magnitude is -21.8, a luminosity of 44 billion suns, and its true diameter is in excess of 110,000 light years.
Six supernovae have been observed in Messier 61 (1926A, 1961I, 1964F, 1999gn, 2006ov, and 2008in). With its sixth supernova, M 61 has caught up to the record set by M 83, and now co-leads the statistics among Messier galaxies. As of March 2009, however, both galaxies are three supernovae behind the overall record holder, NGC 6946 in Cepheus.