M109 (NGC 3992) is a barred spiral galaxy located in the constellation of Ursa Major. It's estimated to be 83.5 Million light-years from Earth, making it the furthest object in Messier's catalogue. Despite its large distance, it's relatively bright at magnitude +10.3 and therefore within the range of small to medium size scopes.

M109 has a complicated history. In March 1781, Pierre Méchain passed three "nebulae" he recently found to Charles Messier for confirmation. The first one was to become M97 whereas the others were recorded as objects 98 and 99 in a rough draft for the next catalogue version. However, Messier never assigned positions for these two items and they were never included in the final catalogue. In 1953, American astronomer and historian Owen Gingerich added draft objects 98 and 99 to the "official" catalogue and they became items M108 and M109 respectively. The story is further complicated by recent analysis that suggest Méchain may have not originally observed NGC 3992 but instead nearby galaxy NGC 3953. If so this implies that Messier in fact discovered NGC 3992. Despite this, it generally remains accepted that M109 is the same object as NGC 3992.

Finding M109 is easy as it's located only 0.75 degrees to the southeast of Phecda (γ UMa - mag. +2.4). This star is a member of the Plough asterism of Ursa Major. The galaxy is best seen from northern temperate latitudes during the months of March, April and May. From the Southern Hemisphere, it never rises very high above the northern horizon.

Finder Chart for M109 - pdf format (credit:- freestarcharts)

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

M109 is visible with large 20x80 binoculars but requires good seeing conditions. However, it's somewhat washed out from the glare of nearby Phecda. Through 100mm (4-inch) scopes, the galaxy appears as a faint hazy elongated streak of nebulosity that's best observed by switching to high magnifications and moving Phecda out of the field of view. A 150mm (6-inch) instrument reveals a small sharp nucleus surrounded by mottled nebulosity. Even larger amateur reflectors show hints of structure, including the bar shaped nucleus. Of course it's much easier to photograph or image the bar than to observe it visually.

In total, M109 measures about 7.6 by 4.7 arc minutes in apparent size. It's an extremely large galaxy with a physical diameter of 180,000 light-years and contains about a trillion stars. On March 17, 1956 a magnitude +12.8 type I supernova (1956A) was observed there.

M109 Data Table