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. 116 of 116 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 to Sagittarius A*. About 26,000 light-years (NASA, 10 questions) or about 27,000 light-years (ESO, 2022).
- The mass of Cygnus X-1. 14.8 Suns (NASA Chandra, 2011), around 20 (ESA, 2024) or 21 (ESO, 2024).
- How heavy a star must be to leave a black hole. Around 20 Suns or more for the star (NASA, types), or more than three solar masses (Britannica).
- The size of the shadow. Roughly twice the black hole’s surface (NASA, anatomy) or about five and a half times the event horizon for M87 (Britannica).
- The most distant black hole. QSO J0313-1806, around 13 billion light-years (NASA’s main black hole page), or one about 13.1 billion light-years away (NASA, 2019).
- The name. Coined by John Wheeler in 1967 (Royal Society) or popularised by him (Britannica).
All sources (24)
- Royal Museums Greenwich: A guide to the Milky Way, our home galaxy (Jake Foster, Royal Observatory) (35 facts)
- ESA: Five fascinating Gaia revelations about the Milky Way (7 facts)
- NASA Science: Galaxies (7 facts)
- ESA: Guide to our Galaxy (6 facts)
- Library of Congress: The Milky Way, one of the many galaxies (Finding Our Place in the Cosmos) (6 facts)
- NASA Imagine the Universe: The Milky Way Galaxy (6 facts)
- Encyclopaedia Britannica: Milky Way Galaxy (5 facts)
- NASA Science: Apocalypse when? Hubble casts doubt on certainty of galactic collision (2 June 2025) (5 facts)
- NASA Science: Our Milky Way Galaxy: How big is space? (5 facts)
- Royal Museums Greenwich: Astronomy without a telescope, what can I see in the night sky? (5 facts)
- NASA Science: NASA’s Hubble shows Milky Way is destined for head-on collision (31 May 2012) (4 facts)
- Encyclopaedia Britannica: Milky Way Galaxy summary (3 facts)
- Manx National Heritage: Isle of Man awarded Dark Skies status (19 October 2012) (3 facts)
- Visit Isle of Man: Isle of Man Dark Skies guide (PDF) (3 facts)
- ESA: The billion star surveyor, Gaia Data Release 2 media kit (April 2018, PDF) (2 facts)
- ESO press release eso2208: Astronomers reveal first image of the black hole at the heart of our galaxy (12 May 2022) (2 facts)
- Encyclopaedia Britannica: Milky Way Galaxy, the structure and dynamics (2 facts)
- NASA JPL: A cauldron of stars at the galaxy’s center (Spitzer) (2 facts)
- NASA Science: The Milky Way Galaxy (Spitzer map of the spiral arms) (2 facts)
- Visit Isle of Man: Stargazing sites (2 facts)
- Dark Sky Discovery: Dark Sky Discovery Sites (1 fact)
- ESA/Hubble heic2508: Hubble casts doubt on certainty of galactic collision (2 June 2025) (1 fact)
- ESA: Gaia creates richest star map of our Galaxy, and beyond (25 April 2018) (1 fact)
- NASA Science: 10 questions you might have about black holes (1 fact)