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Every fact

116 facts, every one sourced

Every fact on this site in one list, each linked to its source. Our build stores the source text and checks that the key words and figures in each fact really appear in it; see the sources page.

  1. 1 The Milky Way is the galaxy we live in: a huge collection of stars, gas, dust and dark matter, all held together by gravity. Royal Observatory
  2. 2 It is a large barred spiral galaxy. NASA Imagine
  3. 3 Every star you can see in the night sky is part of our own Milky Way. NASA Imagine
  4. 4 From Earth it looks like a faint, milky band of light arching across the whole sky, which is how it got its name. NASA, galaxies
  5. 5 That band is the galaxy’s central disc, seen edge on from inside. NASA, galaxies
  6. 6 Seeing the stars in one band, rather than spread all round the sky, tells astronomers our galaxy is basically flat. NASA Imagine
  7. 7 NASA says our galaxy probably contains 100 to 400 billion stars. NASA, how big
  8. 8 ESA’s guide puts it at about a hundred billion stars. ESA, guide
  9. 9 Britannica says several hundred billion stars, one of which is the Sun. Britannica
  10. 10 Counting the stars is very hard because we are inside the galaxy. NASA Imagine
  11. 11 The Royal Observatory estimates 100 to 400 billion planets too. Royal Observatory
  12. 12 The Royal Observatory gives its age as roughly 13.6 billion years. Royal Observatory
  13. 13 ESA’s Gaia team says about 13 billion years. ESA, Gaia 2018
  14. 14 Based on the deepest images so far, ours is one of about 2 trillion galaxies in the observable universe. NASA, how big
  15. 15 A light-year is the distance light travels in a year: about 9.5 trillion kilometres. NASA, how big
  16. 16 No spacecraft has gone far enough to photograph the Milky Way from outside, so every picture of the whole galaxy is an artist’s impression or a data map. Royal Observatory
  17. 17 A thick layer of dust hides much of the galaxy from ordinary telescopes, so astronomers map it with radio and infrared telescopes. Britannica
  18. 18 Its precise shape, true size and mass are still not known. Britannica, summary
  19. 19 NASA: about 100,000 light-years across. NASA, how big
  20. 20 NASA’s galaxy page says its disc of stars spans more than 100,000 light-years. NASA, galaxies
  21. 21 Royal Observatory: approximately 87,000 light-years wide, measured to a set level of brightness (the “isophotal” diameter). Royal Observatory
  22. 22 Britannica’s summary: roughly 150,000 light-years in diameter. Britannica, summary
  23. 23 The disc is only about 1,000 light-years thick at the spiral arms, rising to around 10,000 light-years at the central bulge. Royal Observatory
  24. 24 ESA describes the star disc as having a radius of about 50,000 light-years but a height of only 1,000 light-years. ESA, guide
  25. 25 Until dark matter is pinned down, the total mass can’t be measured: it is at least several hundred billion, and possibly a trillion, times the Sun’s. Britannica, structure
  26. 26 A roughly spherical halo of dark matter about a million light-years wide provides 90% of the galaxy’s total mass. Royal Observatory
  27. 27 Around the disc is a stellar halo of loose stars and globular clusters, thought to be over 300,000 light-years across but holding only 1% of the stars’ mass. Royal Observatory
  28. 28 There are around 160 known globular clusters in the Milky Way: tight balls of hundreds of thousands to millions of stars. Royal Observatory
  29. 29 Astronomers agree it is a spiral with a central bar, with arms of stars and gas coming off each end of the bar. Royal Observatory
  30. 30 The bar is roughly 30,000 light-years across. Royal Observatory
  31. 31 The bar wasn’t proven until 2005, when a team used NASA’s Spitzer Space Telescope to survey stars at the centre in infrared light, which gets through the dust. Royal Observatory
  32. 32 The central bulge holds about ten billion mainly old, red stars and is shaped like a peanut. ESA, guide
  33. 33 The bar joins the bulge on two opposite sides, like a pencil skewering a peanut lengthways. Royal Observatory
  34. 34 In 2008 NASA’s Spitzer team found two major arms, Scutum-Centaurus and Perseus, coming off the ends of the bar; before that four major arms had been assumed. NASA, Milky Way map
  35. 35 Later Gaia star maps gave clear evidence for four arms, an ESA scientist says. ESA, Gaia
  36. 36 The Royal Observatory names four major arms after the constellations they pass through: Perseus, Scutum-Centaurus, Sagittarius-Carina and Norma/Outer. Royal Observatory
  37. 37 Most of the galaxy’s stars are born in the nebulae of the arms. Royal Observatory
  38. 38 Arms don’t keep the same stars. ESA’s Jos de Bruijne compares them to traffic jams: stars leave at the front while more pile up at the back. ESA, Gaia
  39. 39 Jan Oort mapped the first hints of the spiral structure with radio observations in the 1950s. ESA, Gaia
  40. 40 The disc is warped, because a ripple is passing through it. Royal Observatory
  41. 41 The Sun is about 26,000 light-years from the centre, roughly halfway between the centre and the outskirts. ESA, guide
  42. 42 Royal Observatory: around 26,000 light-years, about halfway out. Royal Observatory
  43. 43 NASA’s Imagine the Universe: about 25,000 light-years, so we live in the galaxy’s suburbs. NASA Imagine
  44. 44 Britannica’s summary: about 27,000 light-years. Britannica, summary
  45. 45 NASA: Earth sits along one of the spiral arms, about halfway from the centre. NASA, galaxies
  46. 46 The Sun lies near a small, partial arm called the Orion Arm, or Orion Spur, between the Sagittarius and Perseus arms. NASA, Milky Way map
  47. 47 The Orion Arm is an estimated 20,000 light-years long. Royal Observatory
  48. 48 Most of the stars you can see with your eyes alone are in our own arm, and so are the Orion Nebula and the Pleiades. Royal Observatory
  49. 49 The solar system is tilted at about 60 degrees to the galaxy’s disc, which is why the Milky Way crosses our sky at a steep angle to the paths of the planets. Royal Observatory
  50. 50 The solar system travels round the galaxy at about 828,000 km/h (515,000 mph). Royal Observatory
  51. 51 Royal Observatory: one orbit takes about 230 million years, and is called a galactic year. Royal Observatory
  52. 52 ESA: about 230 million years. ESA, Gaia
  53. 53 NASA’s galaxy page: about 240 million years. NASA, galaxies
  54. 54 NASA’s Imagine the Universe: 250 million years. NASA Imagine
  55. 55 As we go round, we bob up and down through the disc, one full bob every 60 to 70 million years. Royal Observatory
  56. 56 The Sun lies about 500 light-years from the Radcliffe Wave, a 9,000-light-year ripple of gas clouds found in 2019, and crossed it about 13 million years ago. ESA, Gaia
  57. 57 The disc spins at about 220 km per second. Royal Observatory
  58. 58 At the heart of the bulge sits the supermassive black hole Sagittarius A*, with the mass of four million Suns. Royal Observatory
  59. 59 ESO puts the black hole about 27,000 light-years from Earth. ESO, 2022
  60. 60 NASA puts it 26,000 light-years away, and says there is no danger of Earth being pulled in. NASA, 10 questions
  61. 61 From that distance it looks about as big in our sky as a doughnut on the Moon. ESO, 2022
  62. 62 A few hundred light-years from the black hole, a ring of giant gas clouds, the Central Molecular Zone, holds the galaxy’s densest gas. Royal Observatory
  63. 63 The bar pulls gas from the arms and funnels it in towards the centre. Royal Observatory
  64. 64 In ordinary visible light the centre can’t be seen at all: dust between us and it blocks the view, but infrared light gets through. NASA JPL
  65. 65 A Spitzer infrared picture shows hundreds of thousands of stars crowded into the centre. NASA JPL
  66. 66 Stars have been seen orbiting the black hole as close as a few light-days. ESA, guide
  67. 67 The Milky Way sits in a neighbourhood of more than 50 other galaxies called the Local Group. NASA, galaxies
  68. 68 ESA’s Gaia team describes the Local Group as the Milky Way, the Andromeda galaxy and about 60 smaller galaxies. ESA, Gaia 2018
  69. 69 The Local Group lies just off the edge of the Virgo cluster and is part of the Laniakea supercluster. NASA, galaxies
  70. 70 Our nearest large neighbour, the Andromeda Galaxy, is 2.5 million light-years away. Royal Observatory
  71. 71 The Royal Observatory says Andromeda is about 152,000 light-years wide, with more than twice as many stars as the Milky Way. Royal Observatory
  72. 72 NASA says Andromeda is some 220,000 light-years wide. NASA, how big
  73. 73 Andromeda is getting about 110 km (70 miles) closer to us every second. Royal Observatory
  74. 74 About 60 satellite galaxies are known to orbit the Milky Way, and there may be well over 100. Most are dwarfs, like the Large and Small Magellanic Clouds. Royal Observatory
  75. 75 Britannica gives the Large and Small Magellanic Clouds’ distances as 163,000 and 202,000 light-years. Britannica
  76. 76 The nearest companion, the Sagittarius dwarf galaxy, is falling into the Milky Way, captured by its much stronger gravity; its core is about 90,000 light-years away. Britannica
  77. 77 Gaia found millions of stars moving in a snail-shell pattern in the disc, probably a ripple from a past brush with the Sagittarius dwarf. ESA, Gaia
  78. 78 The Milky Way keeps stripping stars from dwarf galaxies and clusters; Gaia has traced streams of them across the sky. ESA, Gaia
  79. 79 From the northern hemisphere, under very dark skies, you may see two other galaxies with your eyes alone: Andromeda and M33. Royal Observatory, naked eye
  80. 80 In 2012 NASA said Hubble measurements predicted a head-on meeting with Andromeda in about four billion years. NASA, 2012
  81. 81 In that picture the two would merge into a single elliptical galaxy about two billion years after the first encounter. NASA, 2012
  82. 82 Stars are so far apart that they would not hit each other, but the Sun would probably be thrown much farther from the centre. NASA, 2012
  83. 83 Andromeda’s small companion, the Triangulum galaxy (M33), would join in and might later merge with the pair. NASA, 2012
  84. 84 In 2025 a new study using Hubble and Gaia data found only about a 50-50 chance of the two galaxies colliding in the next 10 billion years. NASA, 2025
  85. 85 It left only about a 2% chance of a head-on collision in just 4 to 5 billion years. NASA, 2025
  86. 86 The team considered 22 variables and ran 100,000 computer simulations reaching 10 billion years ahead. NASA, 2025
  87. 87 The Large Magellanic Cloud can change the Milky Way’s path and make a collision less likely. ESA/Hubble, 2025
  88. 88 NASA points out that a warming Sun will make Earth uninhabitable in roughly a billion years, so Andromeda is the least of our cosmic worries. NASA, 2025
  89. 89 As far back as 1912, astronomers realised Andromeda, then thought to be only a nebula, was heading our way. NASA, 2025
  90. 90 The merged galaxy has been unofficially nicknamed Milkomeda. Royal Observatory
  91. 91 With your eyes alone you won’t see the colours in photos: the Milky Way looks like fuzzy, colourless patches of light in a rough strip. Royal Observatory, naked eye
  92. 92 Let your eyes adjust to the dark for at least 15 minutes, and keep away from bright lights such as your phone screen. Royal Observatory, naked eye
  93. 93 A red-light torch won’t spoil your night vision, and a planisphere or app shows where to look. Royal Observatory, naked eye
  94. 94 Full Moonlight counts as light pollution for the fainter sights. Royal Observatory, naked eye
  95. 95 From the northern hemisphere we mostly look towards the outer arms, away from the dusty centre; from the south the bright core climbs much higher in the sky. Royal Observatory
  96. 96 UK Dark Sky Discovery Sites come in two grades: at “Orion” sites you can see the seven main stars of Orion; at “Milky Way” sites, much darker and found only in rural areas, the Milky Way is visible to the naked eye. Dark Sky Discovery
  97. 97 On 19 October 2012 seven Isle of Man sites became Dark Sky Discovery Sites: Port Soderick Brooghs, Axnfell Plantation, Smeale Nature Reserve, Niarbyl, The Sound, Fort Island and Sulby Reservoir Car Park. Manx National Heritage
  98. 98 Manx National Heritage said that put the island on the map with the largest concentration of dark sky sites in Britain. Manx National Heritage
  99. 99 NASA astronaut Nicole Stott said she struggled to find the Isle of Man from the International Space Station because it was so dark. Manx National Heritage
  100. 100 The Isle of Man now has 26 official dark sky sites. Visit Isle of Man
  101. 101 From them you can see the Milky Way, and the Andromeda Galaxy, with your eyes alone. Visit Isle of Man
  102. 102 In Manx Gaelic the Milky Way is ‘Raad Moaar re Gorree’, the Great Way of Orry, after a tale of the Viking king Orry pointing to it when asked where he had come from. Visit Isle of Man, guide
  103. 103 The island’s dark-skies guide says the Milky Way is best seen there from autumn through to spring. Visit Isle of Man, guide
  104. 104 In winter it stretches from the left of Orion, through Gemini and Cassiopeia, to the north-western horizon; the guide picks Port Soderick, Axnfell, Fort Island and Tholt-y-Will for winter skies. Visit Isle of Man, guide
  105. 105 An ancient Greek story said the Milky Way was milk from the goddess Hera, spilt across the sky as she nursed the baby Heracles. Royal Observatory
  106. 106 Our word ‘galaxy’ comes from the Greek ‘galaxias’, meaning ‘milky’. Royal Observatory
  107. 107 In ancient Greece, Democritus suggested the band might be made of distant stars. Library of Congress
  108. 108 In his 1610 book Sidereus Nuncius, Galileo reported that through a telescope parts of the Milky Way resolved into many stars. Library of Congress
  109. 109 In 1750 the English astronomer Thomas Wright suggested the Milky Way was a flat layer of stars, with our solar system part of it. Library of Congress
  110. 110 In 1755 the philosopher Immanuel Kant called the faint nebulae “island universes”. Library of Congress
  111. 111 In the 1780s William Herschel surveyed the stars in different directions and found them much denser on one side of the sky. Library of Congress
  112. 112 In 1917 Harlow Shapley used globular clusters to show the galaxy is immense, about 100,000 light-years across, with the Sun nearer the edge than the centre. Britannica, structure
  113. 113 Only in the 1920s was it confirmed that many nebulous ‘stars’ were whole other galaxies far beyond our own. Library of Congress
  114. 114 The Magellanic Clouds were recognised early in the 20th century as companions of our galaxy. Britannica
  115. 115 In April 2018 ESA’s Gaia released a catalogue of nearly 1.7 billion stars, after 22 months of charting the sky. ESA, 2018
  116. 116 Even so, Gaia’s survey covered almost two billion stars, only about 1% of the galaxy’s total. ESA, guide