M108 is a nice edge on barred spiral galaxy located in Ursa Major that was discovered by Pierre Méchain on February 19, 1781. It's not one of the objects included by Messier in his final published catalogue but was added much later by Owen Gingerich in 1953. This was based on analysis of notes written by Messier and Méchain, suggesting that it was intended for inclusion in a later version. William Herschel independently rediscovered the galaxy on April 17, 1789.

Locating M108 is easy as it's positioned 1.5 degrees southeast of bright Merak (β UMa - mag. +2.3). This star marks the southwestern corner of the bowl of the Plough or Big Dipper asterism of Ursa Major. Located 50 arc minutes southeast of M108 is planetary nebula M97 (Owl Nebula) and both items fit easily in the same telescopic wide field of view.

M108 is best seen from Northern Hemisphere latitudes during the months of March, April and May. For observes located at latitudes greater than 35N, it's circumpolar and therefore never sets.

M108 Spiral galaxy by the Hubble Space Telescope (credit:- NASA, The Hubble Heritage Team (AURA/STScI))

Finder Chart for M108 (credit:- freestarcharts)

Finder Chart for M108 - pdf format (credit:- freestarcharts)

Finder Chart for M81 (also shown M40, M97, M82, M108 and M109) (credit:- freestarcharts)

Finder Chart for M81 (also shown M40, M97, M82, M108 and M109) - pdf format (credit:- freestarcharts)

The galaxy shines at apparent magnitude +10.2 and since it's almost edge-on from our perspective it has a high surface brightness. M108 can be spotted with small 80mm (3.1-inch) refractors, appearing as a faint strongly elongated streak of light with a slightly brighter core. Larger 200mm (8-inch) reflectors reveal a well-defined thin needle structure with a mottled dusty complexion and subtle variations in brightness. M108 is a galaxy that can take high magnifications well and is somewhat similar in appearance to M82.

In total, M108 spans 8.6 x 2.4 arc minutes of apparent sky and is located 45 million light-years from Earth. This corresponds to an actual diameter of 110,000 light-years. It's estimated to contain 400 billion stars and is believed to be an isolated member of the Ursa Major Cluster of galaxies.

A type II supernova (1969B) was observed in M108 on January 23, 1969. It peaked at magnitude +13.9.

M108 Data Table

Messier108
NGC3556
Object TypeBarred Spiral galaxy
ClassificationSB(s)cd
ConstellationUrsa Major
Distance (light-years)45 Million
Apparent Mag.+10.2
RA (J2000)11h 11m 31s
DEC (J2000)+55d 40m 24s
Apparent Size (arc mins)8.6 x 2.4
Radius (light-years)55,000
Number of Stars400 Billion

Sky Highlights - September 2017

Opposition
Neptune reaches opposition on September 5th

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for September

Northern Hemisphere
Evening
West:- Jupiter (mag. -1.7)
Southwest:- Saturn (mag. +0.5)
East:- Neptune (mag. +7.8)
Midnight
South:- Neptune
East:- Uranus (mag. +5.7)
Morning
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
Evening
West:- Jupiter
Northwest:- Saturn
East:- Neptune
Midnight
West:- Saturn
North:- Neptune
Northeast:- Uranus
Morning
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)

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