M108 and M109 in Ursa Major are fine galaxies of dissimilar appearance that are curiously somewhat overlooked.
Early spring sees the return of the galaxies, monumental rotating structures of many millions of gravitationally bound stars amid interstellar dust and gas that span tens of thousands or more light years in diameter. They’re not termed ‘island universes’ for nothing! The constellation of Ursa Major, the Great Bear, is prime galaxy country, with five Messier-designated gems leading its pack of outstanding targets.
Messier 108 (NGC 3556) and 109 (NGC 3992) understandably take a back seat to the majesty of Messier 101, the Pinwheel Galaxy, and the peerless pairing of Messier 81 and 82, but this morphologically contrasting pair of spiral galaxies are worth more attention, being easy to locate and offering outstanding targets for imagers and visual observers armed with moderate-aperture telescopes. Furthermore, Ursa Major rides high, allowing for long observing periods at very favourable elevations.
Messier 109 is an exquisite and striking barred spiral galaxy as seen in deep amateur images. Presented face-on to our line of sight, it sports a short but strong central bar, which together with its multiple, graceful spiral arms give it a classic ‘θ’ (the Greek lower-case letter for ‘theta’) appearance.
Shining at magnitude +9.8 and spanning 7.6’ x 4.9’ at its fullest extent, M109 is just bright enough for its core to be seen through an 80mm (~three-inch) instrument under good conditions. M109 lies just 38’ south-east of magnitude +2.4 Phecda (gamma [γ] Ursae Majoris [UMa]), the star lying at the south-eastern corner of ‘The Plough’. This location is both a blessing and a curse: M109 is clearly easy to locate, but the overpowering glare from Phecda will wash-out low-power views. A 150mm (six–inch) telescope at high powers can show M109’s halo extending to 5’ x 3’.
Messier 108 lies seven degrees north-west of M109, just 1.5 degrees south-east of magnitude +2.3 Merak, the south-western ‘Plough’ star and one of the ‘Pointers’, along with Dubhe (alpha [α] UMa, magnitude +1.8). As a nice bonus, the Owl Nebula (Messier 97), a superb planetary nebula, lies under a degree to the south-east. Messier 108 is another spiral galaxy, but seen highly inclined from our vantage point and extending to 8.3’ x 2.5’ in deep images.
Although Messier 108 is listed as just fainter than M109 (at magnitude +10.0), it is slightly easier to see through a small telescope, owing to its marginally higher surface brightness. It could be described as a fainter, smaller and less dynamic version of Messier 82, with a telescope in the 250–300mm (10- to 12-inch) class showing uneven brightness across its form.
source: astronomynow.com