On March 18, 1781, Charles Messier discovered the unusually large number of 8 nebulous objects, all in the region of today's border of the constellations Virgo and Coma Berenices, i.e. Virgo Cluster galaxies (plus one additional object, globular cluster M92 in Hercules). The last of these eight objects was cataloged by him as M91, but his position was erroneous.

The barred spiral galaxy M91 is an appealing member of the Virgo Cluster of Galaxies. It is of type SBb and its bar is very conspicuous, lying at position angle 150°. As its recession velocity is only about 400 km/sec, it has a considerable peculiar velocity toward us through the Virgo cluster, about 700 km/sec, as the cluster's recession velocity is about 1100 km/sec. The membership of Messier 91 in the Virgo Cluster of Galaxies was confirmed by a recent measurement of its distance as 52 +/- 6 million light years by detecting Cepheid variables. These measurements were done by the Hubble Space Telescope.

For the moderately equipped amateur, M91 is one of the most difficult Messier objects. Suggestions of this bar may be seen at medium power even in smaller telescopes, if the viewing conditions are good enough to see the galaxy at all. Photos show the bar more clearly, and show the spiral arms emanating from the ends of the bar.
Under poor conditions, only the bright elongated bar region of this galaxy shows up, while under good conditions the spiral arms show up and exhibit an almost round to slightly elongated shape.