M100 - MIRROR GALAXY

Messier 100 (NGC 4321), sometimes called the Mirror Galaxy, is a grand-design spiral galaxy located approximately 55 million light-years from Earth in the constellation Coma Berenices. It was discovered by Pierre Mechain in 1781 and is one of the brightest and largest spiral galaxies in the Virgo Cluster. M100 is classified as an SAB(s)bc spiral galaxy with a weak bar and beautifully symmetric spiral arms. The galaxy spans approximately 107,000 light-years in diameter, comparable to the Milky Way, and has a visual magnitude of about 9.3. M100's spiral arms are exceptionally well-defined and populated with bright blue star clusters, HII regions, and dark dust lanes, making it one of the most photogenic galaxies in the Messier catalog. The galaxy played a historically important role in cosmology when the Hubble Space Telescope identified Cepheid variable stars in M100 as part of the HST Key Project on the Extragalactic Distance Scale. These observations provided a critical calibration point for measuring the Hubble constant, the rate of expansion of the universe, and were among the landmark early results of the Hubble Space Telescope that helped establish H0 at approximately 70 km/s/Mpc. Five supernovae have been observed in M100: SN 1901B, SN 1914A, SN 1959E, SN 1979C, and SN 2006X. SN 1979C, a Type II supernova, is particularly notable because X-ray observations suggest its remnant may contain a young black hole. M100 possesses a nucleus with low-level Seyfert or LINER activity. In amateur telescopes, M100 appears as a large, round, diffuse glow with a bright center. An 8-inch telescope under excellent conditions can begin to show the spiral arm structure.

M100 spans approximately 107,000 light-years in diameter and lies about 55 million light-years from Earth as one of the largest Virgo Cluster spirals.