M99
St. Catherine's Wheel
Spiral
Properties
Position & Identifiers
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About M99
Description
M99 (also called St. Catherine's Wheel) is a face-on unbarred spiral galaxy in Coma Berenices, about 55 million light-years from Earth. It has strongly asymmetric, sweeping spiral arms — one arm is significantly more prominent and extended than the others, likely pulled out by a gravitational interaction with a neighboring galaxy. M99 spans about 80,000 light-years.
Observing Tips
Located about 1 degree southeast of 6 Comae Berenices and near M98. In a telescope it appears as a moderately bright, round, diffuse glow. The spiral arms are faint and require 10 inches or more under dark skies. The overall impression is of a soft, round haze with a bright center. Best observed from March through June.
History
Discovered by Pierre Mechain on March 15, 1781. Charles Messier cataloged it on April 13, 1781. It was one of the first galaxies recognized as having spiral structure by Lord Rosse in 1846.
Fun Facts
M99's lopsided spiral structure is a textbook example of tidal interaction. One arm extends much further than the others, pulled out by a past close encounter with the galaxy VIRGOHI21 — a dark matter concentration with almost no visible stars. M99 has also hosted three observed supernovae (1967, 1972, 1986).
Community Photos (1)
Credit: Credit Line and Copyright Adam Block/Mount Lemmon SkyCenter/University of Arizona. License: CC BY-SA 3.0. (Wikimedia Commons)
Skybred Feb 28, 2026