Messier 31 – The Andromeda Galaxy
Key Scientific Data
Object type: Barred spiral galaxy – SA(s)b
Constellation: Andromeda
Other designations: M31, NGC 224, UGC 454, PGC 2557, Andromeda Galaxy
Distance from Earth: approximately 2.5 million light-years
Diameter: approximately 150,000–200,000 light-years
Apparent size: approximately 3.2° × 1.0°
Apparent magnitude: approximately 3.4
Inclination: approximately 77°
Right Ascension: 00h 42m 44s
Declination: +41° 16′
First recorded observation: Abd al-Rahman al-Sufi, 964 AD
Nucleus: Supermassive black hole of approximately 100–140 million solar masses
Estimated number of stars: approximately one trillion
Light-travel time: approximately 2.5 million years
Description
Messier 31 (M31), the Andromeda Galaxy, is the nearest large galaxy to the Milky Way and the largest member of the Local Group.
Located approximately 2.5 million light-years away, it contains an estimated trillion stars. Its luminous yellowish bulge is dominated by older stellar populations, while its extensive disc reveals bluish spiral arms, star-forming regions and prominent dark dust lanes.
Viewed at an inclination of approximately 77°, Andromeda displays its characteristic elongated structure, extending across more than three degrees of the night sky.
Our Great Galactic Neighbour
Andromeda and the Milky Way are the two dominant galaxies of the Local Group, sharing broadly comparable masses and spiral structures.
Although Andromeda is physically larger and contains more stars, both galaxies are surrounded by numerous smaller satellites, providing evidence of their long histories of gravitational interactions and galactic accretion.
M32, M110 and a Sea of Stars
Two prominent satellite galaxies accompany Andromeda in this image.
M32 is a compact elliptical galaxy projected close to M31's disc, while M110 is a larger, more diffuse elliptical system. Both are gravitationally bound to Andromeda.
The numerous sharp stars scattered across the photograph belong primarily to our own Milky Way, creating a striking foreground against the distant galaxies.
Andromeda and the Milky Way – A Future Encounter
Andromeda is approaching the Milky Way, and a major gravitational encounter may occur several billion years from now.
Although once considered almost inevitable, recent measurements suggest that a direct collision is not guaranteed.
Such an encounter would distort both galaxies, producing tidal streams and potentially leading to their eventual merger. Individual stellar collisions, however, would remain extremely unlikely because of the enormous distances separating stars.
Curiosities
Andromeda was described in 964 AD by the Persian astronomer Abd al-Rahman al-Sufi as a small cloud.
Its true nature remained uncertain until the 1920s, when Edwin Hubble identified Cepheid variable stars within M31, demonstrating that it lay far beyond the Milky Way. This discovery fundamentally transformed our understanding of the Universe.
Andromeda spans approximately six times the apparent diameter of the Full Moon and can be seen with the naked eye under dark skies, making it one of the most distant objects readily visible without optical instruments.
Looking Back in Time
The light captured from M31 began its journey approximately 2.5 million years ago, near the beginning of Earth's Pleistocene epoch.
At that time, the planet was entering a period of repeated glaciations, while early human ancestors were evolving in Africa. The genus Homo had recently emerged, and early stone tools were already being used by ancient hominins.
Homo sapiens did not yet exist and would not appear until approximately 300,000 years ago. Agriculture, cities and the first great civilisations were still millions of years in the future.
Throughout the evolution of our ancestors, the emergence of modern humans and the entire history of human civilisation, these photons continued their journey across intergalactic space.
This photograph reveals Andromeda as it appeared when humanity was still at the earliest stages of its evolutionary history.
Image Data
Total integration time: 13h 18m
Integration by filter:
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L-Pro: 3h (60 × 180")
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Red: 2h 42m (54 × 180")
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Green: 2h 12m (44 × 180")
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Blue: 5h 24m (108 × 180")
Equipment:
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Telescope: William Optics RedCat 51 III WIFD
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Camera: ToupTek ATR2600M
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Mount: 10Micron GM2000 HPS II
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Filters: Optolong L-Pro, Red, Green, Blue 2"
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Accessories: ToupTek AAF Electric Focuser, WandererAstro WandererCover V4-EC
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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: