M91, spiral galaxy in Coma Berenices

Image by Jim Dixon
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.

Thus, for a long time, M91 was a missing Messier object, as Messier had determined its position from M89 while he thought it was from M58, as the Texas amateur William C. Williams of Fort Worth has figured out in 1969 (Williams 1969). Thus, the identity of M91 with NGC 4548, which had been cataloged H II.120 by William Herschel on April 8, 1784, was finally uncovered. Previous opinions have been that M91 had either been a comet which the great comet hunter Messier mistook for a nebula, and Owen Gingerich had suspected that it had been a duplicate observation of M58. William Herschel had not found M91 at Messier's erroneous position and suspected that it might have been NGC 4571 (his H III.602), a beautiful but faint 11.3 mag barred spiral (NGC 4571 came into discussion in summer 1994 when a group of astronomers at the Canada France Hawaii Telescope (CFHT) used observations of 3 Cepheids in this galaxy for a determination of the Hubble constant).

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 65/245 degrees (as measured from the North direction to the East). 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.