Messier 81 – Bode's Galaxy
Key Scientific Data
Object type: Grand-design spiral galaxy – SA(s)ab
Constellation: Ursa Major
Other designations: M81, NGC 3031, UGC 5318, PGC 28630, Bode's Galaxy
Distance from Earth: approximately 11.6 million light-years
Diameter: approximately 90,000 light-years
Apparent size: approximately 27 × 14 arcminutes
Apparent magnitude: approximately 6.9
Inclination: approximately 59°
Right Ascension: 09h 55m 33s
Declination: +69° 04′
Discovered by: Johann Elert Bode, 1774
Nucleus: active galactic nucleus hosting a supermassive black hole of approximately 70 million solar masses
Light-travel time: approximately 11.6 million years
Description
Messier 81, also known as Bode's Galaxy, is one of the brightest and most beautiful spiral galaxies visible from Earth. It lies in the constellation Ursa Major, approximately 11.6 million light-years away.
M81 is a classic example of a grand-design spiral galaxy, characterised by two remarkably well-defined spiral arms winding around a large and luminous central region.
Its yellowish central bulge is dominated by an older population of stars, while the spiral arms contain younger, hotter blue stars. Scattered along them are numerous regions of active star formation, where clouds of hydrogen gas are giving birth to new generations of stars. Dark lanes of interstellar dust trace the spiral structure and partially obscure the light coming from the stars behind them.
M81 is broadly comparable in size to our Milky Way, although its central bulge is considerably more prominent.
At the very centre of the galaxy lies a supermassive black hole with a mass of approximately 70 million Suns — around an order of magnitude more massive than Sagittarius A*, the black hole at the centre of our own Galaxy.
A Classic Grand-Design Spiral
M81 is considered one of the finest nearby examples of a grand-design spiral galaxy.
Unlike galaxies whose spiral structure appears fragmented or irregular, M81 displays two long, continuous and highly organised spiral arms. These can be followed from the outer regions of the galaxy almost all the way towards its nucleus.
The striking symmetry of M81 makes it an important object for studying how spiral structures form and evolve.
Its appearance, however, has not been shaped in isolation. M81 belongs to a nearby group of galaxies and has undergone powerful gravitational interactions with some of its neighbours, particularly M82 and NGC 3077.
These encounters have influenced the distribution of gas within the galaxies and their subsequent star formation, leaving signatures that astronomers can still observe today.
M81 and M82 – A Gravitational Encounter
One of the most fascinating aspects of M81 is its relationship with the nearby M82, the Cigar Galaxy.
Although the two galaxies appear peaceful when viewed through a telescope today, their history has been much more dramatic.
A close gravitational encounter hundreds of millions of years ago distorted the distribution of gas within the M81 Group. The immense gravitational forces involved pulled streams of hydrogen away from the galaxies and triggered intense episodes of star formation.
The consequences are particularly spectacular in M82, which is now undergoing an intense starburst. New stars are being formed at an extraordinary rate, and enormous quantities of gas are being expelled from its central regions.
M81 itself also bears traces of these interactions. Some of its unusual dust lanes and enhanced star-forming regions may be remnants of these ancient gravitational encounters.
In astronomical terms, M81, M82 and NGC 3077 form a remarkable natural laboratory for studying what happens when galaxies interact gravitationally without necessarily colliding directly.
Curiosities
M81 was discovered on 31 December 1774 by the German astronomer Johann Elert Bode, which explains its popular name, Bode's Galaxy.
A few years later, in 1779, Pierre Méchain independently observed M81 and M82. Charles Messier subsequently included both objects in his famous catalogue in 1781.
With an apparent magnitude of approximately 6.9, M81 is one of the brightest galaxies in the night sky. Under exceptionally dark conditions it lies close to the limit of naked-eye visibility, although binoculars or a small telescope make it much easier to observe.
Its apparent diameter is surprisingly large: the galaxy spans roughly half the apparent diameter of the Full Moonacross the sky. Its light is simply spread over such a large area that it appears much fainter than the Moon.
M81 has also played an important role in measuring the scale of the Universe. Observations of Cepheid variable starswithin the galaxy have allowed astronomers to determine its distance with great accuracy. Cepheids periodically change their brightness in a predictable way and therefore act as important cosmic “standard candles”.
M81 was also home to SN 1993J, one of the brightest and most extensively studied supernovae of modern times. Its observation provided astronomers with valuable information about the final stages in the lives of massive stars.
Looking Back in Time
The light captured in this photograph has travelled through space for approximately 11.6 million years.
This means that we are not seeing M81 as it exists today. We are seeing the galaxy as it appeared 11.6 million years ago.
When these photons began their journey towards us, the human species did not yet exist. Our distant ancestors were still millions of years away from the emergence of Homo sapiens.
During all that time, the photons continued travelling through intergalactic space at the speed of light.
Eventually, after a journey of roughly 110 billion billion kilometres, a tiny fraction of them reached Earth and ended their 11.6-million-year voyage on the sensor of the telescope used to create this image.
In this sense, an astronomical photograph is much more than an image of a distant object: it is a direct record of the Universe's past.
Image Data
Total integration time: 20h 24m 20s
Integration by filter:
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L-Pro: 4h 58m 40s
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Red: 5h 25m 20s
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Green: 5h 18m 20s
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Blue: 4h 42m
Equipment:
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Telescope: Artesky ARTEC 250 Pro
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Camera: ToupTek ATR2600M
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Mount: 10Micron GM2000 HPS II
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Filters: Optolong Blue 2", Optolong Green 2", Optolong L-Pro 2", Optolong Red 2"
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Accessories: Pegasus Astro Prodigy Microfocuser, ToupTek AFW-L 7×2", WandererAstro WandererBox Pro V3
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Software: Adobe Photoshop, Pleiades Astrophoto PixInsight, Stefan Berg Nighttime Imaging 'N' Astronomy (N.I.N.A. / NINA)
For further information and the full-resolution image, visit AstroBin: