M91

When Charles Messier originally added this object to his Catalogue in 1781, he recorded its coordinates inaccurately. For nearly two hundred years, it was unclear which object Messier's catalogue number '91' referred to, and it was not until the 1960s that Messier's error could be corrected, and the galaxy catalogued as 'M91' could be identified.

'M91' is now known to refer to a barred spiral galaxy within the Virgo Cluster that straddles the border between Coma Berenices and Virgo. M91 lies in the southern regions of Coma Berenices, but in the central parts of the cluster as a whole. The orientation of the galaxy is face-on to an observer on Earth, making its thick central bar and distinct spiral arms clearly visible.

Though the Virgo Cluster as a whole is receding from the Milky Way Galaxy, M91 represents something of an anomaly. This galaxy is passing through the cluster in the opposite direction, travelling towards the Milky Way at a relative rate approaching 500 km/s. This curiosity is most likely the result of past gravitational interactions between M91 and the numerous other galaxies that make up the Virgo Cluster.