Messier 109 — Galaxy in Ursa Major

M109 (NGC 3992) is a barred spiral galaxy in the constellation Ursa Major, located approximately 83.5 million light-years from Earth. It is one of the most distant objects in the Messier catalog and is the brightest member of a large galaxy group (the M109 Group or NGC 3992 Group) containing over 50 galaxies. M109 spans about 180,000 light-years in diameter, making it comparable in size to our Milky Way. It displays a prominent central bar structure with two main spiral arms emerging from the bar's ends, along with a bright, compact nucleus. The galaxy is classified as SBbc, indicating a barred spiral with moderately wound arms. Three satellite galaxies — UGC 6923, UGC 6940, and UGC 6969 — orbit in its vicinity. M109 is located very close to the star Phecda (Gamma Ursae Majoris) in the bowl of the Big Dipper, making it easy to find but challenging to observe due to the star's glare.

Located just 40 arcminutes southeast of Phecda (Gamma Ursae Majoris), the star at the bottom-left corner of the Big Dipper's bowl. At magnitude 9.8, M109 is faint and can be difficult to observe because of Phecda's brilliance nearby — position the bright star just outside your eyepiece field of view. A 4-inch telescope at 100x shows a faint, slightly oval haze. A 6-inch telescope begins to show the brighter central region with a faint surrounding disk. An 8-inch or larger telescope at 150-200x under dark skies may reveal hints of the bar structure crossing the nucleus, and with 12 inches or more, traces of the spiral arms become visible. Use averted vision and be patient. Best observed from February through June when the Big Dipper is high overhead.

Discovered by Pierre Mechain on March 12, 1781, and recorded in a letter to Bernoulli. Like M108 and others found by Mechain, it was not included in Messier's original published catalog. M109 was added to the catalog by Owen Gingerich in 1953. It was the last object added to the original list of Messier objects (before M110 was added in 1966). M109 has been a productive host for supernovae, with SN 1956A being one of the earliest supernovae used to test the expanding universe hypothesis. A more recent supernova, SN 2021aefx (Type Ia), was observed in M109 in November 2021 and reached magnitude 12.3.