Messier 81 (M81, NGC 3031) in Ursa Major is one of the most conspicuous galaxies in the sky, and one of the nearest beyond the Local Group. It is a spiral galaxy and is one of the easiest and most rewarding galaxies to observe for the amateur astronomer on the northern hemisphere because with its total visual brightness of about 6.8 magnitudes it can be found with small instruments.
M81 is the first of the four objects originally discovered by Johann Elert Bode , who found it, together with its neighbor M82 , on December 31, 1774. Bode described it as a "nebulous patch", about 0.75 deg away from M82, which "appears mostly round and has a dense nucleus in the middle," and included it as No. 17 in his list. Pierre Méchain independently rediscovered both galaxies as nebulous patches in August 1779 and reported them to Charles Messier , who added them to his catalog after his position measurement on February 9, 1781.
Using the Hubble Space Telescope, a team under Wendy Freedman of the Carnegie Institution of Washington determined the distance to be 11.0 million light years, in 1993 well before the HST was refurbished. Together with the new distance scale correction implied by the results of ESA's Hipparcos satellite, the true distance of M81 is probably closer to 12.0 million light years.
On Sunday, March 28, 1993, a type II supernova (1993J) occured in M81, which was discovered by the Spanish amateur astronomer Francisco Garcia Diaz from Lugo (Spain), and reached a brightness of about mag 10.5 in its maximum.
Investigations performed in 1994 have indicated that M81 has probably only little dark matter, as its rotation curve was found to fall off in the outer regions; this is in contrast to many galaxies, including our own Milky Way , for which the rotation curve increases outward. To explain the velocity of the stars in these regions, the galaxy must have a certain amount of mass. However, the total mass observed in luminous matter - stars and nebulae - is typically insufficient to explain this behaviour; thus it is assumed that there is a significant portion of mass in galaxies is non-luminous, dark matter (or at least low-luminosity matter). For M81, the percentage of dark matter is now estimated to be lower than average.