The galaxy is to be found approximately 10° northwest ofAlpha Ursae Majoris (Dubhe) along with several other galaxies in theMessier 81 Group.[7][11] Its apparent magnitude due to its distance means it requires agood night sky and only rises very briefly and extremely low at its southernmost limit from Earth's surface, about the20th parallel south.
Messier 81 andMessier 82 are considered ideal for viewing usingbinoculars and smalltelescopes.[7][11] 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 with a very dark sky.[7][12] Telescopes withapertures of 8 inches (20 cm) or larger are needed to distinguish structures in the galaxy.[11]
Most of the emission at infrared wavelengths originates frominterstellar dust.[14][16] This interstellar dust is found primarily within the galaxy'sspiral arms, and it has been shown to be associated withstar formation regions.[14][16] The general explanation is that the hot, short-livedblue stars that are found within star formation regions are very effective at heating the dust and thus enhancing the infrared dust emission from these regions.
It is estimated M81 has 210 ± 30 globular clusters.[17] In late February 2022, astronomers reported that M81 may be the source ofFRB 20200120E, a repeatingfast radio burst.[8][9]
M81 (left) andM82 (right). M82 is one of two galaxies strongly influenced gravitationally by M81. The other,NGC 3077, is located off the top edge of this image.M81 with satellite galaxyHolmberg IX in the top center-right corner
Only onesupernova has been detected in Messier 81.[18] The supernova, namedSN 1993J, was discovered on 28 March 1993 by F. García in Spain.[19] At the time, it was the second brightest Type II supernova observed in the 20th century,[20] peaking at anapparent magnitude of 10.7. The spectral characteristics of the supernova changed over time. Initially, it looked more like a Type II supernova (a supernova formed by the explosion of a supergiant star) with stronghydrogenspectral line emission, but later the hydrogen lines faded and stronghelium spectral lines appeared, making the supernova look more like a Type Ib.[20][21]
Moreover, the variations in SN 1993J's luminosity over time were not like the variations observed in other Type II supernovae,[22][23] but did resemble the variations observed in Type Ib supernovae.[24] Hence, the supernova has been classified as aType IIb, a transitory class between Type II and Type Ib.[21] The scientific results from this supernova suggested that Type Ib and Ic supernovae were formed through the explosions of giant stars through processes similar to those taking place in Type II supernovae.[21][25] Despite the uncertainties in modeling the unusual supernova, it was also used to estimate a very approximate distance of 8.5 ± 1.3Mly (2.6 ± 0.4Mpc) to Messier 81.[20] As a local galaxy, theCentral Bureau for Astronomical Telegrams (CBAT) tracksnovae in M81 along withM31 andM33.[26]
In the center of M81 there exists a supermassive black hole (SMBH) with a mass of about7×107M☉. The SMBH is active, having an accretion disk and one-sided relativisticjet. The observation also demonstrate that there may exist a second SMBH that orbits the primary SMBH with a period of around 30 years. The mass of the secondary SMBH is estimated at 0.1 of the primary.[27]
^abcGordon, K. D.; Pérez-González, P. G.; Misselt, K. A.; Murphy, E. J.; Bendo, G. J.; Walter, F.; Thornley, M. D.; Kennicutt Jr., R. C.; Rieke, G. H.; Engelbracht, C. W.; Smith, J.-D. T.;Alonso-Herrero, A.; Appleton, P. N.; Calzetti, D.; Dale, D. A.; Draine, B. T.; Frayer, D. T.; Helou, G.; Hinz, J. L.; Hines, D. C.; Kelly, D. M.; Morrison, J. E.; Muzerolle, J.; Regan, M. W.; Stansberry, J. A.; Stolovy, S. R.; Storrie-Lombardi, L. J.; Su, K. Y. L.; Young, E. T. (2004). "Spatially Resolved Ultraviolet, Hα, Infrared, and Radio Star Formation in M81".Astrophysical Journal Supplement Series.154 (1):215–221.arXiv:astro-ph/0406064.Bibcode:2004ApJS..154..215G.doi:10.1086/422714.S2CID17283721.
^abPérez-González, P. G.; Kennicutt Jr., R. C.; Gordon, K. D.; Misselt, K. A.; Gil De Paz, A.; Engelbracht, C. W.; Rieke, G. H.; Bendo, G. J.; Bianchi, L.; Boissier, S.; Calzetti, D.; Dale, D. A.; Draine, B. T.; Jarrett, T. H.; Hollenbach, D.; Prescott, M. K. M. (2006). "Ultraviolet through Far-Infrared Spatially Resolved Analysis of the Recent Star Formation in M81 (NGC 3031)".Astrophysical Journal.648 (2):987–1006.arXiv:astro-ph/0605605.Bibcode:2006ApJ...648..987P.doi:10.1086/506196.S2CID13901458.
^Ripero, J.; Garcia, F.; Rodriguez, D.; Pujol, P.; Filippenko, A. V.; Treffers, R. R.; Paik, Y.; Davis, M.; Schlegel, D.; Hartwick, F. D. A.; Balam, D. D.; Zurek, D.; Robb, R. M.; Garnavich, P.; Hong, B. A. (1993). "Supernova 1993J in NGC 3031".IAU Circular.5731: 1.Bibcode:1993IAUC.5731....1R.
^abcSchmidt, B.P.; Kirshner, R.P.; Eastman, R.G.; Grashuis, R.; Dell'Antonio, I.; Caldwell, N.; Foltz, C.; Huchra, John P.; Milone, Alejandra A. E. (1993). "The unusual supernova SN1993J in the galaxy M81".Nature.364 (6438):600–602.Bibcode:1993Natur.364..600S.doi:10.1038/364600a0.S2CID4304547.
^Wheeler, J. C.; Barker, E.; Benjamin, R.; Boisseau, J.; Clocchiatti, A.; De Vaucouleurs, G.; Gaffney, N.; Harkness, R. P.; Khokhlov, A. M.; Lester, D. F.; Smith, B. J.; Smith, V. V.; Tomkin, J. (1993). "Early Observations of SN 1993J in M81 at McDonald Observatory".Astrophysical Journal.417:L71–L74.Bibcode:1993ApJ...417L..71W.doi:10.1086/187097.
