Where the facts come from
Every factual claim on this site links to its source. This is the full list.
How we checked
Each fact was written in our own words alongside its source. Our build script stores the text of every source and, for each fact, checks that the key words and figures really appear in it. 115 of 115 checks passed when this site was last built (13 November 2026). Where we couldn’t find a named source saying something, we left it out. Where good sources disagree, we say so and give both.
Where sources differ
- Distance. 2.5 million light-years (NASA), about 2,480,000 (Britannica) or about 2 million (the Royal Observatory’s closest-galaxy page).
- Width. About 152,000 light-years (Royal Observatory), approximately 200,000 (Britannica) or some 220,000 (NASA).
- Size in the sky. Six times as wide as the full Moon (NASA’s Messier 31 page), roughly the size of the full Moon to the naked eye (NASA, 2025), or about 3 by 1 degrees, twice the Moon’s width (EarthSky).
- Bigger than the Milky Way. As much as twice as massive (NASA) or more than twice as many stars (Royal Observatory).
- Brightness. Magnitude 3.1 (NASA) or 3.5 (EarthSky).
- How lopsided the satellites are. Seen in 0.3% of similar simulated systems (Nature Astronomy summary) or 0.1% after removing look-alikes (Sky & Telescope).
- Meeting the Milky Way. A head-on collision in about four billion years (NASA, 2012), or only a 50-50 chance in ten billion years, with about 2% for a head-on hit in 4 to 5 billion years (NASA, 2025).
All sources (20)
- NASA Science: Hubble’s Messier Catalog, Messier 31 (the Andromeda Galaxy) (17 facts)
- NASA Science: Hubble views the star that changed the universe (2011) (16 facts)
- Encyclopaedia Britannica: Andromeda Galaxy (13 facts)
- EarthSky: The constellation Andromeda the Chained Lady (12 facts)
- NASA Science: NASA’s Hubble shows Milky Way is destined for head-on collision (31 May 2012) (10 facts)
- NASA Science: NASA’s Hubble provides bird’s-eye view of Andromeda galaxy’s ecosystem (27 February 2025) (9 facts)
- NASA Science: Apocalypse when? Hubble casts doubt on certainty of galactic collision (2 June 2025) (6 facts)
- University of Michigan News: The Milky Way’s long-lost sibling finally found (23 July 2018) (5 facts)
- NASA Science: Hubble finds mysterious disk of blue stars around black hole (2005) (4 facts)
- Royal Museums Greenwich: A guide to the Milky Way, our home galaxy (Jake Foster, Royal Observatory) (4 facts)
- Royal Museums Greenwich: What is our closest galaxy? (4 facts)
- ESA/Hubble heic1502: Hubble’s high-definition panoramic view of the Andromeda Galaxy (5 January 2015) (3 facts)
- Royal Museums Greenwich: Night sky highlights, February 2026 (3 facts)
- Sky & Telescope: Why are most of Andromeda’s dwarf galaxies on our side? (3 facts)
- ESA/Hubble heic2508: Hubble casts doubt on certainty of galactic collision (2 June 2025) (1 fact)
- NASA Science: Galaxies (1 fact)
- NASA Science: Our Milky Way Galaxy: How big is space? (1 fact)
- Nature Astronomy: Andromeda’s lopsided galaxy system challenges standard cosmology (11 April 2025) (1 fact)
- Royal Museums Greenwich: Astronomy without a telescope, what can I see in the night sky? (1 fact)
- Visit Isle of Man: Stargazing sites (1 fact)