| Messier 100 | |
|---|---|
Galaxy Messier 100 imaged by ESO, revealing complex spiral arm structure | |
| Observation data (J2000 epoch) | |
| Constellation | Coma Berenices |
| Right ascension | 12h 22m 54.8616s |
| Declination | +15° 49′ 17.886″ |
| Redshift | 0.005240 |
| Heliocentric radial velocity | 1,571±1 km/s |
| Distance | 55 Mly |
| Group or cluster | Virgo Cluster |
| Apparent magnitude (V) | 9.3 |
| Characteristics | |
| Type | SAB(s)bc |
| Number of stars | 400 billion (4×1011) |
| Size | ~166,100 ly (50.93 kpc) (estimated) |
| Apparent size (V) | 7.4′ × 6.3′ |
| Other designations | |
| HOLM 387A, IRAS 12204+1605, NGC 4321, UGC 7450, MCG +03-32-015, PGC 40153, CGCG 099-030 | |
Messier 100 (also known as NGC 4321 or the Mirror Galaxy) is a grand design intermediate spiral galaxy in the constellation of Coma Berenices. It is one of the brightest and largest galaxies in the Virgo Cluster and is approximately 55 million light-years from the Milky Way, about 166,000 light-years in diameter. It was discovered by Pierre Méchain in 1781 and 29 days later seen again and entered by Charles Messier in his catalogue "of nebulae and star clusters". It was one of the first spiral galaxies to be discovered, and was listed as one of fourteen spiral nebulae by Lord William Parsons of Rosse in 1850. NGC 4323 and NGC 4328 are satellite galaxies of M100; the former is connected with it by a bridge of luminous matter.

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 his observations. His son John 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.
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 of radius: one thousand parsecs – where star formation has been taking place for at least 500 million years in separate bursts.

Seven supernovae have been identified in M100:
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