Messier 91 — Galaxy in Coma Berenices

M91 is a barred spiral galaxy located about 63 million light-years away in the constellation Coma Berenices, within the Virgo Cluster. It is classified as type SBb, displaying a prominent central bar with two well-defined spiral arms unwinding from each end. M91 is one of the faintest objects in the Messier catalog at magnitude 10.2, and it has a historically confused identity — Messier may have originally recorded the wrong coordinates, leading to decades of debate about which object M91 actually referred to. The galaxy spans about 100,000 light-years across, similar to the Milky Way. Its nucleus contains a supermassive black hole surrounded by a ring of active star formation.

Located about 1 degree east of M88 in Coma Berenices, near the northern boundary of the Virgo Cluster. M91 is one of the more challenging Messier objects due to its low surface brightness. In a 4-inch telescope, it appears as a very faint, diffuse oval glow that can be easily overlooked. An 8-inch telescope under dark skies shows the brighter central bar region with a faint surrounding disk. The bar structure requires excellent transparency and at least a 10-12 inch telescope to discern. Patience and averted vision are essential. Best observed from March through June.

Cataloged by Charles Messier on March 18, 1781, but his recorded position did not match any obvious object, leading to M91 being considered a 'missing Messier object' for nearly two centuries. In 1969, amateur astronomer William C. Williams identified NGC 4548 as the likely candidate for M91, based on a careful analysis of Messier's observing notes and the positions of other objects cataloged that night. This identification is now widely accepted.