M109 (NGC 3992) is a barred spiral galaxy located in the constellation of Ursa Major. It's estimated to be located 83.5 Million light-years from Earth, making it the furthest object in Messier's catalogue. Despite its large distance it's relatively bright; with an apparent magnitude of +10.3 the galaxy is within the range of small to medium sized amateur telescopes.
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 while the others were recorded by Messier as objects 98 and 99 in a rough draft. However, Messier never assigned positions for these items in the main catalogue and hence they were never included in the final version. Many years later in 1953, American astronomer and historian Owen Gingerich added draft objects 98 and 99 to the "official" Messier catalogue and they became items M108 and M109. 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 and not Méchain. Despite this, it's generally accepted that M109 is identified as NGC 3992.
Finding M109 is easy, it's located only 0.75 degrees to the southeast of Phecda (γ UMa - mag. +2.4) one of the stars 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.
M109 is visible in large 20x80 binoculars but requires good seeing conditions and is somewhat washed out from the resulting glare due to its close proximity to second magnitude Phecda. Through a 100mm (4-inch) scope the galaxy appears as a faint hazy elongated streak of nebulosity, which is best observed by switching to higher magnifications and moving Phecda outside the field of view. A 150mm (6-inch) telescope reveals a small sharp nucleus surrounded by a mottled nebulosity. An even larger amateur scope shows hints of structure including the bar shaped nucleus. Of course, it's much easier to photograph or image the bar shape than to actually observe it.
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 in M109.
M109 Data Table
|Object Type||Barred Spiral galaxy|
|RA (J2000)||11h 57m 36s|
|DEC (J2000)||53d 22m 28s|
|Apparent Size (arcmins)||7.6 x 4.7|
|Radius (light years)||90,000|
|Number of Stars||1 Trillion|
|Notable Feature||The most distant object in the Messier Catalogue|