NGC 4321

Messier 100 (NGC 4321) is one of the brightest and most photogenic spiral galaxies in the Virgo Cluster, located approximately 55 million light-years away in the constellation Coma Berenices. It was discovered by Pierre Méchain on March 15, 1781, and catalogued by Charles Messier shortly after. NGC 4321 is a face-on Sc spiral with tightly wound, luminous arms rich in young blue stars and active star-forming regions — one of the Virgo Cluster's finest examples of a grand-design spiral. At magnitude 9.3, it is one of the brightest Virgo Cluster members observable through small telescopes.

NGC 4321 gained particular fame in the 1990s when it was observed by the Hubble Space Telescope as part of the Hubble Key Project to measure the Hubble constant — the rate of expansion of the Universe. Astronomers identified Cepheid variable stars within M100 and used them as precise distance indicators to calibrate the cosmic distance ladder, placing the galaxy at a well-determined distance and helping to pin down the value of the Hubble constant to an accuracy then unprecedented. The galaxy's face-on orientation reveals its grand spiral arms and a prominent central bar in detail; two companion dwarf galaxies are also visible in this image.

In a small telescope M100 appears as a round, moderately bright glow with a compact nucleus; moderate apertures (150–200 mm) reveal a hint of the disk's extent and the brighter core. This image was taken with the WIYN 0.9-meter telescope at the Kitt Peak National Observatory on the night of December 20, 2002.