Messier Index/M81

Messier 81 (also known as NGC 3031 or Bode's Galaxy) is a spiral galaxy about 12 million light-years away in the constellation Ursa Major. M81 is one of the most striking examples of a grand design spiral galaxy, with near perfect arms spiraling into the very center. Because of its proximity to Earth, its large size, and its active galactic nucleus (which harbors a supermassive black hole) Messier 81 is a popular galaxy to study in professional astronomy research. The galaxy's large size and relatively low apparent magnitude (lower magnitude implies higher brightness) also make it a popular target for amateur astronomy observations.

Discovery
Messier 81 was first discovered by Johann Elert Bode in 1774. Consequently, the galaxy is sometimes referred to as "Bode's Galaxy". In 1779, Pierre Méchain and Charles Messier reidentified Bode's object, which was subsequently listed in the Messier Catalogue.

Dust emission


Most of the emission at infrared wavelengths originates from interstellar dust. This interstellar dust is found primarily within the galaxy's spiral arms, and it has been shown to be associated with star formation regions. The general explanation is that the hot, short-lived blue stars that are found within star formation regions are very effective at heating the dust and hence enhancing the infrared dust emission from these regions.

Supernova
Only one supernova has been detected in Messier 81. The supernova, named SN 1993J, was discovered on 28 March 1993 by F. Garcia in Spain. At the time, it was the second brightest supernova observed in the twentieth century. The spectral characteristics of the supernova changed over time. Initially, it looked more like a type II supernova (a supernovae formed by the explosion of a giant star) with strong hydrogen spectral line emission, but later the hydrogen lines faded and strong helium spectral lines appeared, making the supernova look more like a type Ib. Moreover, the variations in SN 1993J's luminosity over time were not like the variations observed in other type II supernovae but did resemble the variations observed in type Ib supernovae. Hence, the supernova has been classified as a "type IIb", a transitory class between type II and type Ib. The scientific results from this supernova suggested that type Ib and Ic supernovae were actually formed through the explosions of giant stars through processes similar to what takes place in type II supernovae. The supernova was also used to estimate a distance of 8.5 ± 1.3 Mly (2.6 ± 0.4 Mpc) to Messier 81.

Nearby galaxies and galaxy group information


Messier 81 is the largest galaxy in the M81 Group, a group of 34 galaxies located in the constellation Ursa Major. The distance from the Earth to the group is approximately 11.7 Mly (3.6 Mpc), making this one of the closest groups to the Local Group, which contains the Milky Way.

M81 is gravitationally interacting with Messier 82 and NGC 3077. The interactions have stripped some hydrogen gas away from all three galaxies, leading to the formation of filamentary gas structures in the group. Moreover, the interactions have also caused some interstellar gas to fall into the centers of Messier 82 and NGC 3077, which has led to strong starburst activity (or the formation of many stars) within the centers of these two galaxies.

Amateur astronomy information
Messier 81 is located approximately 10° northwest of Alpha Ursae Majoris along with several other galaxies in the Messier 81 Group. Messier 81 and Messier 82 can both be viewed easily using binoculars and small telescopes. The two objects are generally not observable to the unaided eye, although highly experienced amateur astronomers may be able to see Messier 81 under exceptional observing conditions. Telescopes with apertures of 8 inches or larger are needed to distinguish structure in the galaxy.