Naked Eye: No
Binoculars: No
Min Scope: 4 inch
Messier 99 (NGC 4254), also known as the Coma Pinwheel, 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 a prominent member of the Virgo Cluster. M99 is classified as an SA(s)c spiral galaxy and is seen nearly face-on, presenting a striking view of its spiral structure. The galaxy spans about 80,000 light-years in diameter and has a visual magnitude of approximately 9.9. M99 is one of the most asymmetric spiral galaxies known, with its spiral arms displaying a distinctly lopsided appearance. One arm is considerably more prominent and extended than the others, likely the result of a past gravitational interaction with other Virgo Cluster members, possibly the dark galaxy VIRGOHI21, a massive cloud of hydrogen gas discovered in radio wavelengths that appears to have no visible stellar counterpart. A bridge of hydrogen gas has been detected between M99 and VIRGOHI21, supporting an interaction scenario. M99 was the second galaxy in which spiral structure was recognized, after M51, by Lord Rosse in 1846. The galaxy has hosted three observed supernovae: SN 1967H, SN 1972Q, and SN 1986I. M99 is actively forming stars, with its spiral arms populated by bright HII regions and young blue star clusters. In amateur telescopes, M99 appears as a round, diffuse glow with a brighter nucleus. Its face-on orientation and moderate surface brightness make it somewhat challenging to observe, but 8-inch and larger telescopes under dark skies can begin to reveal the brightest spiral arm segments.
M99 spans approximately 80,000 light-years in diameter and lies about 55 million light-years from Earth in the Virgo Cluster.
The asymmetric arms are the interesting feature. Ha filter reveals HII regions in the arms.
It was the second galaxy where spiral structure was recognized and shows dramatic asymmetry possibly caused by interaction with the dark galaxy VIRGOHI21.