M91 - NGC 4548

Messier 91 (NGC 4548) is a barred spiral galaxy located approximately 63 million light-years from Earth in the constellation Coma Berenices. It has one of the most complicated identification histories in the Messier catalog. Charles Messier observed an object in 1781 that he cataloged as M91, but for nearly two centuries the identity of M91 was debated because Messier's recorded position did not correspond to any obvious nebulous object. It was long considered a missing Messier object, with some researchers suggesting Messier had accidentally re-observed M58 or even a comet. In 1969, amateur astronomer William C. Williams convincingly argued that M91 is NGC 4548, a barred spiral galaxy that matches Messier's description when a simple coordinate error is corrected. This identification is now widely accepted. M91 is classified as an SBb barred spiral galaxy with a visual magnitude of about 10.2 and a physical diameter of approximately 100,000 light-years. It has a prominent central bar from which two spiral arms emerge, and the galaxy is seen at a moderate inclination. M91 is one of the faintest objects in the Messier catalog and has relatively low surface brightness, making it a somewhat challenging target for visual observers. The galaxy shows signs of gas depletion in its outer regions, consistent with ram pressure effects from the Virgo Cluster environment. M91 possesses a mildly active nucleus classified as a LINER. In amateur telescopes, M91 appears as a faint, oval glow with a slightly brighter center. An 8-inch telescope under dark skies shows the basic shape, while larger instruments may reveal hints of the central bar.

M91 spans approximately 100,000 light-years in diameter with a visual magnitude of 10.2, located about 63 million light-years from Earth.