M81 is one of the brightest galaxies seen from Earth. It has a near-perfect spiral shape, with prominent, well-defined spiral arms winding from the central region outwards.
The galaxy is located approximately 11.8 million light years from Earth and has an apparent magnitude of 6.94. Its designation in the New General Catalogue is NGC 3031.
The galaxy’s size, brightness and proximity make it a popular object among both amateur and professional astronomers, as it is easy to observe even in smaller telescopes. M81’s active galactic nucleus contains a supermassive black hole with about 70 million solar masses, or 15 times the mass of the black hole in the Milky Way Galaxy, and has been an object of extensive study. The angular size of M81 roughly corresponds to that of the full Moon.
Messier 81 is tilted at an oblique angle when seen from Earth. Its spiral arms consist of young hot stars, bluish in colour, that were formed within the last few million years. The spiral arms also contain a stellar population formed about 600 million years ago.
The central bulge of M81 is home to significantly older stars, red in colour, and much larger than the central bulge of our own galaxy, the Milky Way.
Bode’s Galaxy was named after Johann Elert Bode, the German astronomer who discovered it on December 31, 1774, along with the nearby Cigar Galaxy (Messier 82). Pierre Méchain found both galaxies independently in August 1779 and reported the discovery to Charles Messier, who added them to the Messier catalogue as M81 and M82 on February 9, 1781.
Bode’s Galaxy is the largest member of the M81 Group, a group of 34 galaxies lying in Ursa Major constellation. It has a strong gravitational effect on Messier 82 (Cigar Galaxy) and NGC 3077, two other prominent members of the group.
The close encounter between the galaxies occurred about 300 million years ago. As a result, all three galaxies have had hydrogen gas stripped away. The gravitational interaction has also resulted in the formation of filamentary structures in the group. The filaments are made of the gas that is being stripped away from the three galaxies. The interaction has also caused the gas to fall into the central regions of Messier 82 and NGC 3077, resulting in increased star-forming activity.
Bode’s Galaxy’s infrared emissions mostly come from the interstellar dust found within its spiral arms. The dust is associated with starburst regions. The hot, young blue stars heat the dust, increasing the level of emissions in the infrared.
Messier 81 can be found about 10 degrees northwest of the bright star Dubhe, Alpha Ursae Majoris. Dubhe is the star that marks the northwest corner of the Big Dipper‘s bowl. Several other galaxies in the M81 Group are also located in the vicinity. The Cigar Galaxy is located 38 arc minutes north of Bode’s Galaxy.
Bode’s Galaxy can easily be seen in binoculars and amateur telescopes, but is not visible to the naked eye unless viewing conditions are exceptionally good. The structures within M81 can be seen in 8-inch or larger telescopes.
M81 and M82 are popular targets for both amateur and professional astronomers: M81 because it is an excellent example of a grand design spiral galaxy and M82 because it is a well-known and long studied starburst galaxy, with new stars being formed at exceptionally high rates.
A supernova was detected in Bode’s Galaxy on March 28, 1993. Designated SN 1993J, it was the second brightest supernova event seen in the 20th century at the time of discovery. It was classified as a Type IIb supernova. SN 1993J was the only supernova observed in Messier 81.
Bode’s Galaxy – Messier 81 (NGC 3031)
Constellation: Ursa Major
Coordinates: 09h 55m 33.2s (right ascension), +69°03’55” (declination)
Distance 11.8 ± 0.4 million light years (3.62 ± 0.12 Mpc)
Apparent magnitude: 6.94
Apparent size: 26.9 × 14.1 arc minutes
Designations: Bode’s Galaxy, Messier 81, M81, NGC 3031, UGC 5318, PGC 28630