M91, mag. +10.3, is a barred spiral galaxy located in the southern part of the constellation of Coma Berenices. It's a member of the Virgo cluster and was discovered by Charles Messier on March 18, 1781. This was a most 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 M58 instead of intended galaxy M89. It was only a degree mistake, however the result meant that the object was missing for almost 200 years! Some astronomy detective work by William Williams finally solved the mystery. In 1969, he pinpointed its location and concluded that M91 was almost certainly the same galaxy as NGC 4548.

Finding M91 can be challenging. The galaxy is positioned 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 located 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 a further two degrees directly north.

M91 and the Virgo cluster galaxies are best seen during the months of March, April and May.

M91 Barred Spiral galaxy (credit:- Thomas Haynes/Gail Haynes/Adam Block/NOAO/AURA/NSF)

Finder Chart for M91 (credit:- freestarcharts)

Finder Chart for M91 - pdf format (credit:- freestarcharts)

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) aperture, it appears as a faint ball of light that's brighter towards the centre. 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 size 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 TypeBarred Spiral galaxy
ConstellationComa Berenices
Distance (light-years)63 Million
Apparent Mag.+10.3
RA (J2000)12h 35m 27s
DEC (J2000)+14d 29m 48s
Apparent Size (arc mins)5.4 x 4.3
Radius (light-years)50,000
Number of Stars400 Billion
Notable FeatureMember of the Virgo Cluster of galaxies

Sky Highlights - September 2017

Neptune reaches opposition on September 5th

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for September

Northern Hemisphere
West:- Jupiter (mag. -1.7)
Southwest:- Saturn (mag. +0.5)
East:- Neptune (mag. +7.8)
South:- Neptune
East:- Uranus (mag. +5.7)
West:- Neptune
South:- Uranus
East:- Venus (mag. -3.9), Mars (mag. +1.8) (from second week), Mercury (mag. +0.5 to -1.3) (from second week)

Southern Hemisphere
West:- Jupiter
Northwest:- Saturn
East:- Neptune
West:- Saturn
North:- Neptune
Northeast:- Uranus
West:- Neptune
Northwest:- Uranus
Northeast:- Venus
East:- Mars (end of month)

Deep Sky

Small telescopes:-
Messier 13 - M13 - Great Hercules Globular Cluster
Messier 92 - M92 - Globular Cluster
Messier 11 - M11 - The Wild Duck Cluster (Open Cluster)
Messier 7 - M7 - The Ptolemy Cluster (Open Cluster)
Messier 6 - M6 - The Butterfly Cluster (Open Cluster)
Messier 4 - M4 - Globular Cluster
Messier 8 - M8 - Lagoon Nebula (Emission Nebula)
Messier 16 - M16 - Eagle Nebula (Emission Nebula with Open Cluster)
Messier 20 - M20 - Trifid Nebula (Emission and Reflection Nebula)

Shop at Amazon US


If you like the website and want to contribute to the running costs then please do so below. All contributions are most welcome.

PayPal - The safer, easier way to pay online.