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 Barred Spiral galaxy (Thomas Haynes/Gail Haynes/Adam Block/NOAO/AURA/NSF)

Finder Chart for M91 (also shown M49, M53, M58->M60, M64->M66, M84->M90 and M98->M100)

Finder Chart for M91 (also shown M49, M53, M58->M60, M64->M66, M84->M90 and M98->M100) - pdf format

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 TypeBarred Spiral galaxy
ConstellationComa Berenices
Distance (kly)63000
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 - March 2017

Comet 41P/Tuttle-Giacobini-Kresak now visible with binoculars as it heads towards perihelion

Mercury heading towards greatest elongation east

Minor Planet
Vesta now visible with binoculars and small telescopes.

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for March 2017

Northern Hemisphere
West:- Venus (mag. -4.8 to -4.1 - first half of month), Mars (mag. +1.3 to +1.5), Uranus (mag. +5.9), Mercury (mag. -1.5 to -0.4 - second half of month)
Southeast:- Jupiter (mag. -2.3 to -2.5)
Southwest:- Jupiter
Southeast:- Saturn (mag. +0.5)

Southern Hemisphere
West:- Venus (first half of month), Mars, Uranus
North:- Jupiter
East:- Saturn
West:- Jupiter
Northeast:- Saturn
East:- Neptune (mag. +8.0 - second half of month)

Deep Sky
Naked eye / binoculars:-
Melotte 111 - Mel 111 - The Coma Star Cluster (Open Cluster)
Messier 44 - M44 - The Praesepe (Open Cluster)

Messier 67 - M67 - Open Cluster
Messier 51 - M51 - The Whirlpool Galaxy (Spiral Galaxy)
Messier 97 - M97 - The Owl Nebula (Planetary Nebula)
Messier 101 - M101 - The Pinwheel Galaxy (Spiral Galaxy)
Messier 65 – M65 – Spiral Galaxy
Messier 66 - M66 - Intermediate Spiral Galaxy
Messier 95 - M95 - Barred Spiral Galaxy
Messier 96 - M96 - Intermediate Spiral Galaxy
NGC 4244 - Spiral Galaxy
NGC 4565 - Needle Galaxy - Spiral Galaxy

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