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 version but was added much later by Owen Gingerich in 1953. This was based on analysis of notes written by Messier and Méchain that referenced M108 suggesting that the object was intended for inclusion in a later version. William Herschel independently rediscovered M108 on April 17, 1789.

Locating M108 is easy since it's positioned only 1.5 degrees southeast of bright Merak (β UMa - mag. +2.3) the southwest corner star of the bowl of the famous Plough or Big Dipper asterism of Ursa Major. Located 50 arc minutes southeast of M108 is the planetary nebula "Owl Nebula" (M97) and both items fit easily in the same wide field telescope 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, the galaxy is circumpolar and therefore never sets.

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

Finder Chart for M108 (also shown M40, M51, M97, M101, M106 and M109)

Finder Chart for M108 (also shown M40, M51, M97, M101, M106 and M109) - pdf format

Finder Chart for M81 (also shown M40, M97, M82, M108 and M109)

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

M108 shines at apparent magnitude +10.2 and since it's aligned almost face-on the galaxy exhibits a high surface brightness. Therefore, it can be spotted with a small 80mm (3.1-inch) telescope; appearing as a faint strongly elongated streak of light with a slightly brighter central region. A larger 200mm (8-inch) scope reveals a well-defined thin needle structure that displays a mottled dusty complexion with subtle variations in brightness. M108 is a galaxy that can withstand high magnifications and is somewhat similar in appearance to the brighter galaxy 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 the galaxy 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, peaking at magnitude +13.9.

M108 Data Table

Messier108
NGC3556
Object TypeBarred Spiral galaxy
ClassificationSB(s)cd
ConstellationUrsa Major
Distance (kly)45000
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 - April 2017

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