M91, mag. +10.3, is a barred spiral galaxy located in the southern part of constellation Coma Berenices. It's a member of the Virgo cluster of galaxies and was discovered by Charles Messier on March 18, 1781. This was a productive night for Messier; he discovered eight objects, all of them Virgo cluster galaxies and also rediscovered globular cluster M92 in Hercules. When recording the position of M91, Messier incorrectly referenced its location from galaxy M58 when he meant to use M89. It was only a one degree mistake, however the result meant that M91 was a missing object for almost 200 years!
It was not until 1969 when some astronomy detective work by William Williams solved the mystery of M91. In 1969, he pinpointed the location in the sky after applying Messier's measurements to a starting point of M89 and concluded that the missing object was almost certainly NGC 4548.
Finding M91 can be challenging. The galaxy is located about a degree north of the Coma Berenices-Virgo constellation boundary but there are no bright stars in the vicinity. The general area of sky can be found by imagining a line connecting Denebola (β Leo - mag. +2.1) with Vindemiatrix (ε Vir - mag. +2.8). About 60% of the way along this line is elliptical galaxy M89 (mag. +10.0) with M91 positioned two degrees directly north of it.
M91 and the Virgo cluster galaxies are best seen during the months of March, April and May.
M91 is one of the fainter Messier galaxies and is generally considered as one of the more difficult objects in the list. Through a medium size scope of the order of 150mm (6-inch) or 200mm (8-inch) it appears as a faint ball of light that's brighter towards the middle. In total the galaxy spans 5.4 x 4.3 arc minutes of apparent sky. On dark nights with good transparency the centre bar shape is evident with hints of the spiral structure visible. The spiral arms are better seen with larger amateur scopes.
M91 is about 63 million light-years distant and has a spatial diameter of 100,000 light-years. It's estimated to contain 400 billion stars.
M91 Data Table
|Object Type||Barred Spiral galaxy|
|RA (J2000)||12h 35m 27s|
|DEC (J2000)||14d 29m 48s|
|Apparent Size (arc mins)||5.4 x 4.3|
|Number of Stars||400 Billion|
|Notable Feature||Member of the Virgo Cluster of galaxies|