After the discovery of M100 by Méchain, Charles Messier made observations of the galaxy depicting it as a nebula without a star. He pointed out that it was difficult to recognize the nebula because of its faintness. William Herschel was able to identify a bright cluster of stars within the nebula during observations he did before John Herschel expanded the findings in 1833. With the advent of better telescopes, John Herschel was able to see a round, brighter galaxy; however, he also mentioned that it was barely visible through clouds. William Henry Smyth extended the studies of M100, detailing it as a pearly white nebula and pointing out diffuse spots.
Star formation
Messier 100 is considered a starburst galaxy with the strongest star formation activity concentrated in its center, within a ring - actually two tightly wound spiral arms attached to a small nuclear bar with a radius of 1 kilo-parsec – where star formation has been taking place since at least 500 million years ago in separate bursts. As usual on spiral galaxies of the Virgo Cluster, in the rest of the disk both star formation and neutral hydrogen, of which M100 is deficient compared to isolated spiral galaxies of similar Hubble type, are truncated within the galaxy's disk, which is caused by interactions with the intracluster medium of Virgo.
Supernovae
Seven supernovae have been identified in M100. In March 1901 the first supernova of M100 was found, SN 1901B, a type I supernova found with a magnitude of 15.6 at 110"W and 4"N from its nucleus. SN 1914A was then discovered in February to March 1914; its type was undeterminable but was found with a magnitude of 15.7 at 24"E and 111"S from its nucleus. Observations of M100 from February 21, 1960 to June 17, 1960 led to the discovery of SN 1959E, another type I supernova, with the faintest magnitude, 17.5, among the five found, at 58"E and 21"S from its nucleus. On April 15, 1979, the first type II supernova found in the M100 galaxy was discovered; however the star SN 1979C faded quickly; later observations from x-ray to radio wavelengths revealed its remnant. The fifth supernova was discovered February 7, 2006; the star SN 2006X had a magnitude of 15.3 when discovered two weeks before fading to magnitude +17. Supernova SN 2019ehk, discovered on April 29, 2019, reached a peak magnitude of approximately 15.8. The seventh supernova, SN 2020oi, was discovered on January 7, 2020.
