Messier 81 Galaxy, located in the direction of the Sagittarius constellation and commonly referred to as Bode's Galaxy, is a spiral galaxy. It was first discovered in 1774 by Johann Elert Bode and was later cataloged by Charles Messier. M81 is one of the brightest spiral galaxies visible to the naked eye in the night sky and is one of the largest members of the local galaxy group known as the M81 Group.
Messier 81 Galaxy (NASA)
Location and Distance
Messier 81 is located approximately 11.8 million light-years (3.6 Megaparsecs) from Earth. Its coordinates, according to the J2000 epoch, are Right Ascension (RA): 09h 55m 33.2s and Declination (Dec): +69° 03′ 55″. This position indicates that the galaxy lies in the northern sky, within the Ursa Major constellation near the Sagittarius constellation.
Location of Messier 81 (NASA)
Structural Features
Messier 81 is classified as a classical large spiral galaxy (SA(s)ab). This classification indicates that the galaxy has well-defined spiral arms and a relatively large central bulge. The galaxy's diameter is estimated to be approximately 90,000 light-years, which is about 90% of the diameter of the Milky Way.
The galaxy's center hosts a dense stellar population and a prominent bulge (central swelling). M81 also contains an active galactic nucleus (AGN). This nucleus exhibits low-level Seyfert-type activity, meaning the galaxy's center consists of a supermassive black hole surrounded by hot gas and dust.
Mass and Dynamic Properties
The total mass of M81 is estimated to be approximately 7 x 10¹¹ solar masses. This mass estimate considers both the stellar mass and the mass of the dark matter halo surrounding the galaxy. Rotation curve measurements support the significant presence of dark matter, as the rotational velocities remain constant even in the outer regions of the galaxy.
The average rotational velocity of the stars in the galaxy is approximately 240 km/s, a value critical for modeling the galaxy's mass distribution and dynamics.
Stellar Population and Star Formation
Messier 81 hosts stellar populations of various ages. In the central regions, old star clusters dominate, while young and intermediate-age stars are more prevalent in the spiral arms. H II regions—areas of ionized hydrogen—within the spiral arms mark zones of new star formation.
The star formation rate in M81 ranges between 0.3 and 1.0 solar masses per year, indicating a moderately active phase of star formation. The galaxy is gravitationally interacting with its nearby neighbor, M82 (the Cigar Galaxy), and this interaction contributes to increased star formation in certain regions of M81's spiral arms.
Radio and Other Electromagnetic Observations
Messier 81 has been observed across a wide range of wavelengths. Radio observations have revealed a high concentration of neutron stars and supernova remnants in the galaxy's central region. In addition, hydrogen atom (HI) density maps have been produced for the spiral arms.
X-ray observations support the presence of hot plasma surrounding the supermassive black hole at the center of the galaxy. These data are used to better understand the active core structure of M81.
M81 Group and Surroundings
M81 is the primary member of the M81 Group, which includes several large galaxies such as M82 and NGC 3077. This group is one of the nearest major galaxy groups after the Local Group. Strong gravitational interactions among the galaxies in the M81 Group have been observed. In particular, the interactions between M81 and M82 have caused structural and morphological changes in both galaxies.
Messier 81 is a well-studied spiral galaxy in the scientific literature due to its structure and dynamics. Its proximity allows for detailed multi-wavelength observations, providing valuable data on galactic nucleus activity, star formation processes, and dark matter distribution. Studying M81 contributes significantly to our understanding of galaxy evolution and cosmological models.
