# Messier 81
**Messier 81** (M81, NGC 3031), also called **Bode's Galaxy**, is a grand design spiral galaxy in the constellation [Ursa Major](https://www.edgepedia.org/ursa-major), about 11.6 million light-years from Earth and one of the brightest galaxies visible from the northern hemisphere.[1](https://science.nasa.gov/mission/hubble/science/explore-the-night-sky/hubble-messier-catalog/messier-81/) It was discovered by the German astronomer Johann Elert Bode in 1774 and added to [Charles Messier](https://www.edgepedia.org/charles-messier)'s catalog of nebulous objects in 1781.[1](https://science.nasa.gov/mission/hubble/science/explore-the-night-sky/hubble-messier-catalog/messier-81/) • [2](https://www.mpifr-bonn.mpg.de/563211/m81)
| Key fact | Detail |
|---|---|
| Type and catalog names | Grand design spiral galaxy; NGC 3031 in the New General Catalogue; Messier 81 in Messier's 1781 catalog[3](https://www.messier-objects.com/messier-81-bodes-galaxy/) • [2](https://www.mpifr-bonn.mpg.de/563211/m81) |
| Distance | 11.6 million light-years (about 3.6 Mpc); a 1994 Hubble Cepheid study gave 3.63 ± 0.34 Mpc[1](https://science.nasa.gov/mission/hubble/science/explore-the-night-sky/hubble-messier-catalog/messier-81/) • [4](https://ntrs.nasa.gov/citations/19950040592) |
| Brightness | Apparent magnitude 6.8 to 6.9, at the limit of naked-eye visibility; best observed in April[5](https://science.nasa.gov/missions/hubble/hubble-photographs-grand-design-spiral-galaxy-m81/) • [1](https://science.nasa.gov/mission/hubble/science/explore-the-night-sky/hubble-messier-catalog/messier-81/) |
| Size | Apparent size 26.9 by 14.1 arc minutes, corresponding to a diameter of about 90,000 light years; angular size similar to the Full Moon[3](https://www.messier-objects.com/messier-81-bodes-galaxy/) • [5](https://science.nasa.gov/missions/hubble/hubble-photographs-grand-design-spiral-galaxy-m81/) |
| Central black hole | 70 million solar masses, about 15 times the mass of the Milky Way's central black hole[5](https://science.nasa.gov/missions/hubble/hubble-photographs-grand-design-spiral-galaxy-m81/) |
| Notable events | Hosted supernova SN 1993J, the brightest supernova since SN 1987A; hosts the repeating fast radio burst FRB 20200120E in a globular cluster[6](https://www.nature.com/articles/364509a0) • [7](https://www.nature.com/articles/s41586-021-04354-w) |
| Group | Dominant member of the M81 Group, which includes at least a dozen gravitationally linked galaxies[8](https://www.jpl.nasa.gov/images/pia13454-wise-beholds-a-pair-of-dancing-galaxies/) |
## Discovery and observation
Bode found M81 and its companion M82 in December 1774; he sometimes referred to the pair as "Bode's Nebula", since astronomers at the time did not know galaxies existed.[2](https://www.mpifr-bonn.mpg.de/563211/m81) Messier added the pair to his catalog in 1781.[2](https://www.mpifr-bonn.mpg.de/563211/m81)
M81 lies about 10 degrees northwest of Dubhe ([Alpha Ursae Majoris](https://www.edgepedia.org/alpha-ursae-majoris)) in the circumpolar constellation Ursa Major, and an imaginary line from Phecda through Dubhe, extended by roughly the same distance, leads to it.[3](https://www.messier-objects.com/messier-81-bodes-galaxy/) At magnitude 6.8 it sits at the limit of naked-eye visibility. Binoculars show a faint patch of light in the same field of view as M82; a 3-inch telescope shows a hazy bright oval, and a 6-inch telescope reveals the bright core surrounded by a faint halo.[5](https://science.nasa.gov/missions/hubble/hubble-photographs-grand-design-spiral-galaxy-m81/) • [3](https://www.messier-objects.com/messier-81-bodes-galaxy/) The galaxy is one of the most remote large galaxies known to be moving toward us (blueshifted).[3](https://www.messier-objects.com/messier-81-bodes-galaxy/)
## Structure
M81 is tilted at an oblique angle to our line of sight, giving a bird's-eye view of its spiral structure, and it resembles the [Milky Way](https://www.edgepedia.org/milky-way) in overall form.[5](https://science.nasa.gov/missions/hubble/hubble-photographs-grand-design-spiral-galaxy-m81/) Its two well-resolved spiral arms wind all the way down into the nucleus and contain large quantities of interstellar dust associated with starburst regions.[5](https://science.nasa.gov/missions/hubble/hubble-photographs-grand-design-spiral-galaxy-m81/) • [3](https://www.messier-objects.com/messier-81-bodes-galaxy/) The arms are made of young, bluish, hot stars formed in the past few million years, along with a population formed in an episode of star formation that began about 600 million years ago; ultraviolet light from the young stars fluoresces the surrounding hydrogen gas.[5](https://science.nasa.gov/missions/hubble/hubble-photographs-grand-design-spiral-galaxy-m81/)
The central bulge holds much older, redder stars and is significantly larger than the Milky Way's bulge.[5](https://science.nasa.gov/missions/hubble/hubble-photographs-grand-design-spiral-galaxy-m81/) Hubble's high-resolution images show the inner bulge and nucleus as relatively structure free apart from fine dust lanes winding into the center, with easily visible globular clusters.[1](https://science.nasa.gov/mission/hubble/science/explore-the-night-sky/hubble-messier-catalog/messier-81/) A 1972 photometric survey found a total mass of 1.62 × 1011 solar masses.[9](https://adsabs.harvard.edu/pdf/1972ApJ...175..843C)
**Density waves.** M81 is a textbook case for spiral density wave theory, the idea, established by C. C. Lin and [Frank Shu](https://www.edgepedia.org/frank-shu) in the 1960s, that spiral arms are waves of compressed gas and stars rotating through the disk. H I velocity contours in M81 show a spiral velocity shock, strong evidence for a density wave in a grand design spiral.[10](https://ntrs.nasa.gov/api/citations/19870014861/downloads/19870014861.pdf) A study using Spitzer infrared data found the stellar spiral wave has a pitch angle of 23 ± 3 degrees, an amplitude rising from about 0.1 to 0.3 between radii of 300 and 600 arc seconds, and a corotation radius of 12 ± 3 kpc, with the inner Lindblad resonance at 4.3 kpc.[11](https://academic.oup.com/mnras/article/387/3/1007/1024504) The offset between the stellar arms and the gas shock indicates the pattern has persisted for at least a few hundred million years, and the authors argue the interaction with M82 could provide the driving force for the wave.[11](https://academic.oup.com/mnras/article/387/3/1007/1024504)
**Infrared view.** [Spitzer Space Telescope](https://www.edgepedia.org/spitzer-space-telescope) images show how the galaxy's appearance changes with wavelength. At 3.6 microns, which traces starlight, the image is virtually unaffected by dust and shows a smooth stellar mass distribution with subdued arms; at 8 microns, emission from dust heated by nearby luminous stars dominates, and the spiral arms become the galaxy's dominant feature.[12](https://www.nasa.gov/image-article/an-infrared-view-of-m81-galaxy/) The dust, composed of silicates, carbonaceous grains, and polycyclic aromatic hydrocarbons, traces the gas distribution and provides raw material for future star formation.[12](https://www.nasa.gov/image-article/an-infrared-view-of-m81-galaxy/)
## The M81 Group and galactic interactions
M81 anchors the M81 Group, which includes the starburst galaxy M82, the spiral galaxy [NGC 3077](https://www.edgepedia.org/ngc-3077), the spiral [NGC 2976](https://www.edgepedia.org/ngc-2976), and dwarf galaxies such as Holmberg I, Holmberg II, and Holmberg IX, along with the more distant [NGC 2403](https://www.edgepedia.org/ngc-2403); the group contains at least a dozen gravitationally linked galaxies.[8](https://www.jpl.nasa.gov/images/pia13454-wise-beholds-a-pair-of-dancing-galaxies/) • [2](https://www.mpifr-bonn.mpg.de/563211/m81) Neutral hydrogen maps show matter bridges connecting the galaxies, direct evidence of their interactions.[2](https://www.mpifr-bonn.mpg.de/563211/m81)
M81, M82, and NGC 3077 swept past each other a few hundred million years ago. Simulations based on H I observations by [M. S. Yun](https://www.edgepedia.org/min-s-yun) place the three-way interaction at approximately 2 × 108 years ago, with M82's nearest approach to M81 about 2.2 × 108 years ago.[11](https://academic.oup.com/mnras/article/387/3/1007/1024504) The encounter triggered star formation in both M81 and M82, and in M82 it compressed gas near the core into a starburst whose radiation drives a galactic superwind of gas and dust.[8](https://www.jpl.nasa.gov/images/pia13454-wise-beholds-a-pair-of-dancing-galaxies/) The gravitational interaction with M82 has also enhanced M81's spiral density waves, making its arms more prominent than those of isolated spirals.[8](https://www.jpl.nasa.gov/images/pia13454-wise-beholds-a-pair-of-dancing-galaxies/) The two galaxies will likely orbit each other several more times before eventually merging.[8](https://www.jpl.nasa.gov/images/pia13454-wise-beholds-a-pair-of-dancing-galaxies/)
Recent work on the group's assembly history finds that M81's stellar halo formed early, about 9 billion years ago, and has experienced little star formation since.[13](https://arxiv.org/abs/2408.04627)
## Distance measurements
M81 was the first calibrating galaxy in the Hubble Space Telescope Key Project on the Extragalactic Distance Scale, whose aim was a [Hubble constant](https://www.edgepedia.org/hubble-constant) accurate to 10 percent.[4](https://ntrs.nasa.gov/citations/19950040592) A team under Wendy L. Freedman, an astronomer at the Carnegie Observatories, discovered 30 Cepheids in M81 with periods of 10 to 55 days and derived a true distance modulus of 27.80 ± 0.20 magnitudes, corresponding to 3.63 ± 0.34 Mpc.[4](https://ntrs.nasa.gov/citations/19950040592) Earlier, pre-refurbishment Hubble observations of 32 Cepheids had given about 11.0 million light-years.[14](https://messier.lamost.org/seds/seds.org/messier/en/m/m081.html) The proximity of M81, at only 3.6 Mpc, is one reason its supernova and fast radio burst could be studied in detail.[15](https://www.nature.com/articles/367621a0)
## Supernova 1993J
On 28 March 1993, a new supernova was discovered in M81. [SN 1993J](https://www.edgepedia.org/sn-1993j) was the brightest supernova since [SN 1987A](https://www.edgepedia.org/sn-1987a) and, like SN 1987A, a peculiar type II supernova.[6](https://www.nature.com/articles/364509a0) Soft X-rays from the event were detected six days after discovery, and the X-ray light curve was followed for the next 41 days; the emissions are explained by shock fronts produced as the ejecta expanded into the slow, dense wind of a red supergiant progenitor.[15](https://www.nature.com/articles/367621a0) Modeling showed the progenitor was a late-type supergiant that had lost almost all of its hydrogen-rich envelope before exploding, most likely through mass transfer to a companion star.[6](https://www.nature.com/articles/364509a0) Hubble images taken before the explosion identified the progenitor as a non-variable red supergiant, and later analysis found its energy distribution consistent with an early K-type supergiant with an initial mass of 13 to 22 solar masses.[16](https://arxiv.org/abs/astro-ph/0401090) • [17](https://arxiv.org/abs/astro-ph/0208382) SN 1993J has since become one of the best-observed supernovae in X-rays, with long-term time sampling better than any other supernova.[18](https://aas.org/sites/default/files/2020-01/AAS_224_Abstracts%20%281%29.pdf)
## FRB 20200120E
M81 hosts the nearest known extragalactic repeating fast radio burst source, [FRB 20200120E](https://www.edgepedia.org/frb-20200120e), discovered by the [Canadian Hydrogen Intensity Mapping Experiment](https://www.edgepedia.org/canadian-hydrogen-intensity-mapping-experiment) (CHIME) and localized with the European VLBI Network to a globular cluster associated with M81, about 2 parsecs from the cluster's optical center.[7](https://www.nature.com/articles/s41586-021-04354-w) The cluster is roughly 10 billion years old, so the source challenges FRB models that invoke young magnetars born in core-collapse supernovae; proposed alternatives include a highly magnetized neutron star formed through the accretion-induced collapse of a white dwarf or the merger of compact stars in a binary.[7](https://www.nature.com/articles/s41586-021-04354-w)
Monitoring with the 100-m Effelsberg telescope at 1.4 GHz from April 2021 to April 2022 detected 60 bursts, including a "burst storm" in which 53 bursts occurred within about 40 minutes; the bursts show no strict periodicity, and the source's dispersion measure remained stable over more than 10 months.[19](https://academic.oup.com/mnras/article/520/2/2281/7008530) Some bursts display pulse components only 10 to 100 nanoseconds wide, the shortest-duration radio emission observed from any FRB source.[20](https://doi.org/10.1038/s41550-024-02386-6) A 2024 to 2025 multiwavelength campaign placed the deepest persistent and prompt X-ray luminosity limits from any FRB source in the soft X-ray band, suggesting that some cosmological FRBs may be powered by X-ray-emitting magnetars.[20](https://doi.org/10.1038/s41550-024-02386-6)
## References
1. [Messier 81, NASA Science (Hubble Messier Catalog)](https://science.nasa.gov/mission/hubble/science/explore-the-night-sky/hubble-messier-catalog/messier-81/)
2. [Galaxiengruppe um M81 in verschiedenen Aufnahmen, Max-Planck-Institut für Radioastronomie](https://www.mpifr-bonn.mpg.de/563211/m81)
3. [Messier 81: Bode's Galaxy, Messier Objects](https://www.messier-objects.com/messier-81-bodes-galaxy/)
4. [Freedman, W. L. et al. (1994). The Hubble Space Telescope Extragalactic Distance Scale Key Project. I: The discovery of Cepheids and a new distance to M81, NASA NTRS](https://ntrs.nasa.gov/citations/19950040592)
5. [Hubble Photographs Grand Design Spiral Galaxy M81, NASA Science (2007)](https://science.nasa.gov/missions/hubble/hubble-photographs-grand-design-spiral-galaxy-m81/)
6. [The progenitor of supernova 1993J: a stripped supergiant in a binary system?, Nature (1993)](https://www.nature.com/articles/364509a0)
7. [Kirsten, F. et al. (2022). A repeating fast radio burst source in a globular cluster, Nature 602, 585–589](https://www.nature.com/articles/s41586-021-04354-w)
8. [WISE Beholds a Pair of Dancing Galaxies, NASA Jet Propulsion Laboratory (2011)](https://www.jpl.nasa.gov/images/pia13454-wise-beholds-a-pair-of-dancing-galaxies/)
9. [Photoelectric photometry of M81, Astrophysical Journal Supplement Series abstracts (1972)](https://adsabs.harvard.edu/pdf/1972ApJ...175..843C)
10. [Star formation and density waves in M81, NASA Conference Publication](https://ntrs.nasa.gov/api/citations/19870014861/downloads/19870014861.pdf)
11. [Tracing spiral density waves in M81, Monthly Notices of the Royal Astronomical Society 387, 1007 (2008)](https://academic.oup.com/mnras/article/387/3/1007/1024504)
12. [An Infrared View of the M81 Galaxy, NASA (2019)](https://www.nasa.gov/image-article/an-infrared-view-of-m81-galaxy/)
13. [A Timeline of the M81 Group: Properties of the Extended Structures of M82 and NGC 3077, arXiv:2408.04627](https://arxiv.org/abs/2408.04627)
14. [Messier Object 81, SEDS Messier database](https://messier.lamost.org/seds/seds.org/messier/en/m/m081.html)
15. [Detection of soft X-rays from supernova 1993J six days after outburst, Nature (1993)](https://www.nature.com/articles/367621a0)
16. [The massive binary companion star to the progenitor of supernova 1993J, arXiv:astro-ph/0401090](https://arxiv.org/abs/astro-ph/0401090)
17. [The Progenitor of Supernova 1993J Revisited, arXiv:astro-ph/0208382](https://arxiv.org/abs/astro-ph/0208382)
18. [SN 1993J: The X-ray Story of a Supernova Slowly Transitioning to a Remnant, AAS 224 abstracts](https://aas.org/sites/default/files/2020-01/AAS_224_Abstracts%20%281%29.pdf)
19. [A burst storm from the repeating FRB 20200120E in an M81 globular cluster, Monthly Notices of the Royal Astronomical Society 520, 2281 (2023)](https://academic.oup.com/mnras/article/520/2/2281/7008530)
20. [Multiwavelength constraints on the origin of a nearby repeating fast radio burst source in a globular cluster, Nature Astronomy 9, 111–127 (2025)](https://doi.org/10.1038/s41550-024-02386-6)
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*Topic: Encyclopedia › Physical world and mathematics › Astronomy › Stars and galaxies › Galaxies and large-scale structure › Named galaxies and the Local Group › Named galaxies (general and lists)*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*
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