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  1. 1 The Andromeda Galaxy, also called M31, is the nearest major galaxy to our own Milky Way. NASA, M31
  2. 2 It is a spiral galaxy in the constellation Andromeda. NASA, M31
  3. 3 Britannica lists its other names: the Andromeda Nebula, M31 and NGC 224. Britannica
  4. 4 It shares various characteristics with the Milky Way. Britannica
  5. 5 Astronomers use it as a stand-in for spiral galaxies in general: we would know much less about our own spiral if we did not have Andromeda to study. NASA, M31
  6. 6 NASA says studying the Milky Way from inside is like trying to understand the layout of New York City while standing in the middle of Central Park. NASA, M31
  7. 7 We see Andromeda almost edge on, tilted by 77 degrees. NASA, M31
  8. 8 NASA says Andromeda is as much as twice as massive as the Milky Way. NASA, 2025
  9. 9 The Royal Observatory says it has more than twice as many stars as the Milky Way. Royal Observatory
  10. 10 The Milky Way, Andromeda and more than 50 other galaxies make up the Local Group. NASA, galaxies
  11. 11 NASA gives its distance as 2.5 million light-years. NASA, M31
  12. 12 Britannica says about 2,480,000 light-years from Earth. Britannica
  13. 13 The Royal Observatory’s ‘closest galaxy’ page says about 2 million light-years. Royal Observatory, closest
  14. 14 So the light we see left Andromeda 2.5 million years ago: we see it as it was then. EarthSky
  15. 15 Royal Observatory: about 152,000 light-years wide. Royal Observatory
  16. 16 Britannica: approximately 200,000 light-years across. Britannica
  17. 17 NASA: some 220,000 light-years across. NASA, how big
  18. 18 It is so close that it looks six times as wide as the full Moon in the sky. NASA, M31
  19. 19 NASA’s 2025 satellite story says it appears to the naked eye roughly the size of the full Moon. NASA, 2025
  20. 20 EarthSky says it stretches about 3 degrees by 1 degree, twice as wide as the full Moon. EarthSky
  21. 21 ESA/Hubble says the whole galaxy contains over one thousand billion stars: more than a trillion. ESA/Hubble, 2015
  22. 22 Andromeda is getting about 110 km (70 miles) closer to us every second. Royal Observatory
  23. 23 NASA put its approach at about 250,000 miles per hour, fast enough to get from here to the Moon in an hour. NASA, 2012
  24. 24 That speed was measured with the Doppler effect: Andromeda’s motion towards us squashes the waves of its starlight. NASA, 2012
  25. 25 It was mentioned as early as 965 CE in the Book of the Fixed Stars by the Islamic astronomer al-Ṣūfī. Britannica
  26. 26 It was rediscovered in 1612, soon after the telescope was invented, by the German astronomer Simon Marius. Britannica
  27. 27 Marius said it looked like the light of a candle seen through a horn. Britannica
  28. 28 For centuries astronomers thought it was part of the Milky Way, a ‘spiral nebula’, hence the old name Andromeda Nebula. Britannica
  29. 29 In the early 1900s most astronomers thought the Milky Way was a single ‘island universe’ with nothing observable beyond it. NASA, V1
  30. 30 Andromeda was catalogued as just one of many faint, fuzzy ‘spiral nebulae’. NASA, V1
  31. 31 As far back as 1912, astronomers realised Andromeda, then thought to be only a nebula, was heading our way. NASA, 2025
  32. 32 On the night of 5 October 1923 Edwin Hubble began an observing run that lasted into the early hours of 6 October. NASA, V1
  33. 33 In poor conditions he made a 45-minute exposure and marked three suspected novae, exploding stars, with an ‘N’. NASA, V1
  34. 34 Comparing that plate with older ones, he found one ‘nova’ dimmed and brightened far faster than a nova should. NASA, V1
  35. 35 He crossed out the ‘N’ and wrote ‘VAR!’, for variable, with an exclamation mark. NASA, V1
  36. 36 The star, Hubble variable number one or V1, is a Cepheid variable in Andromeda’s outer regions. NASA, V1
  37. 37 A Cepheid brightens and fades in a regular pattern, like a lighthouse, and was already known to be a reliable distance marker. NASA, V1
  38. 38 V1 showed that Andromeda lies beyond our galaxy, and settled the debate about the spiral nebulae. NASA, V1
  39. 39 Harlow Shapley, who believed the nebulae were part of the Milky Way, reportedly said of Hubble’s letter: “Here is the letter that destroyed my universe.” NASA, V1
  40. 40 By the end of 1924 Hubble had found 36 variable stars in Andromeda, 12 of them Cepheids. NASA, V1
  41. 41 From them he worked out a distance of 900,000 light-years; better measurements later moved it to about 2 million. NASA, V1
  42. 42 Britannica: only in the 1920s did Hubble show conclusively that Andromeda is a separate galaxy. Britannica
  43. 43 An astronomer at the Space Telescope Science Institute called V1 “the most important star in the history of cosmology”. NASA, V1
  44. 44 In 2010 and 2011 amateur astronomers tracked V1 and the Hubble Space Telescope photographed it again, as a tribute. NASA, V1
  45. 45 V1 takes a little over 31 days to brighten and fade once. NASA, V1
  46. 46 Cepheids are still the first rung on the ‘cosmic distance ladder’ astronomers use for far more distant galaxies. NASA, V1
  47. 47 NASA says Edwin Hubble measured Andromeda’s vast distance by finding a variable star that served as a ‘milepost marker’. NASA, 2012
  48. 48 In January 2015 Hubble released a 1.5-billion-pixel mosaic of part of Andromeda, showing over 100 million stars. ESA/Hubble, 2015
  49. 49 Showing that 2015 image at full size would take more than 600 HD television screens. ESA/Hubble, 2015
  50. 50 NASA likened resolving single stars at that distance to photographing a beach and seeing individual grains of sand. NASA, M31
  51. 51 In January 2025 Hubble released an even bigger panorama: about 2.5 billion pixels and 200 million stars. NASA, M31
  52. 52 It took over 10 years and more than 600 overlapping Hubble snapshots. NASA, M31
  53. 53 It needed over 1,000 Hubble orbits. NASA, M31
  54. 54 Even 200 million stars is only a fraction of Andromeda’s population. NASA, M31
  55. 55 Cooler, yellowish stars crowd the centre; a blue ring of young stars and star-forming regions marks a spiral arm. NASA, M31
  56. 56 The dark silhouettes in the pictures are complex lanes of dust. NASA, M31
  57. 57 NGC 206 is the most conspicuous star cloud in Andromeda. NASA, M31
  58. 58 Some bright ‘stars’ in Hubble’s picture are photo-bombers in our own Milky Way, much closer to us. NASA, M31
  59. 59 There is a supermassive black hole at Andromeda’s heart. NASA, black hole
  60. 60 In 2005 Hubble measurements put its mass at 140 million Suns, three times more than once thought. NASA, black hole
  61. 61 A disc of hot, young blue stars whips round the black hole, rather like planets round the Sun. NASA, black hole
  62. 62 What looked like a double centre is a ring of old red stars about five light-years from the black hole. NASA, black hole
  63. 63 Andromeda has a past of collisions, swallowing smaller galaxies. Britannica
  64. 64 Deep surveys of its outskirts show huge streams and clouds of stars, the remains of smaller galaxies it has ‘eaten’. Britannica
  65. 65 In 2018 University of Michigan scientists concluded that Andromeda shredded a massive galaxy, now called M32p, about two billion years ago. U. Michigan, 2018
  66. 66 M32p would have been the third-largest galaxy in the Local Group, after the Milky Way and Andromeda. U. Michigan, 2018
  67. 67 It was at least 20 times larger than any galaxy that ever merged with the Milky Way. U. Michigan, 2018
  68. 68 They suggest the dense little galaxy M32 is its surviving centre, like the pit of a plum. U. Michigan, 2018
  69. 69 One of the scientists, Eric Bell, said: “M32 is a weirdo.” U. Michigan, 2018
  70. 70 NASA says the Milky Way has been relatively placid, but Andromeda has had a more dynamic history, probably including a major merger a few billion years ago. NASA, 2025
  71. 71 Hubble has studied the whole known population of 36 small satellite galaxies orbiting Andromeda. NASA, 2025
  72. 72 The survey built a 3D map of the dwarfs and worked out how they formed stars over nearly 14 billion years. NASA, 2025
  73. 73 Half of them seem confined to one plane, all orbiting in the same direction. NASA, 2025
  74. 74 One researcher, Daniel Weisz, called that “weird”: a total surprise nobody fully understands yet. NASA, 2025
  75. 75 The brightest companion is M32, a compact galaxy that may be the leftover core of a larger galaxy that hit Andromeda a few billion years ago. NASA, 2025
  76. 76 Many of Andromeda’s dwarfs kept forming stars slowly for far longer than expected, something computer simulations don’t show. NASA, 2025
  77. 77 Britannica’s pictures show M110, an elliptical galaxy that is another satellite of Andromeda. Britannica
  78. 78 The Royal Observatory says its two dwarf elliptical satellites can be seen with a small telescope. Royal Observatory, closest
  79. 79 Most of Andromeda’s dwarf galaxies lie on our side, between Andromeda and the Milky Way, and better distances show that lopsidedness is real. Sky & Telescope
  80. 80 A 2025 Nature Astronomy summary says that degree of lopsidedness turns up in only 0.3% of similar simulated systems. Nature Astronomy
  81. 81 Sky & Telescope reports that, after removing look-alike cases, only 0.1% of simulated galaxies were as lopsided as Andromeda. Sky & Telescope
  82. 82 The researchers say this needs either a very unusual history, or may hint that our standard picture of the cosmos is missing something. Sky & Telescope
  83. 83 The Triangulum galaxy, M33, is a smaller companion at a similar distance, and much harder to see. Royal Observatory, closest
  84. 84 In 2012 NASA said Hubble measurements predicted a head-on meeting with Andromeda in about four billion years. NASA, 2012
  85. 85 NASA said the Sun would probably be flung into a new part of the galaxy, but Earth and the solar system would not be destroyed. NASA, 2012
  86. 86 In that picture the two would merge into a single elliptical galaxy about two billion years after the first encounter. NASA, 2012
  87. 87 Stars are so far apart that they would not hit each other. NASA, 2012
  88. 88 NASA’s illustrations showed Andromeda filling the night sky in 3.75 billion years, and the sky ablaze with new stars at 3.85 billion. NASA, 2012
  89. 89 Triangulum would join in and might later merge with the pair; there was a small chance it would hit the Milky Way first. NASA, 2012
  90. 90 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
  91. 91 It left only about a 2% chance of a head-on collision in just 4 to 5 billion years. NASA, 2025
  92. 92 The team considered 22 variables and ran 100,000 computer simulations reaching 10 billion years ahead. NASA, 2025
  93. 93 Lead author Till Sawala said the two could still go past each other. NASA, 2025
  94. 94 The Large Magellanic Cloud can change the Milky Way’s path and make a collision less likely. ESA/Hubble, 2025
  95. 95 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
  96. 96 The merged galaxy has been unofficially nicknamed Milkomeda. Royal Observatory
  97. 97 Hubble’s deep views show galaxy collisions were more common in the past, when the universe was smaller. NASA, 2012
  98. 98 It is one of the few galaxies you can see with your eyes alone, as a milky blur. Britannica
  99. 99 On clear, dark nights it shows faintly as a hazy patch; binoculars or a small telescope reveal its bright core and glow. Royal Observatory, Feb 2026
  100. 100 Through binoculars it looks like a lens-shaped smudge. Royal Observatory, closest
  101. 101 Give your eyes 15 minutes to adapt to the dark, and don’t look at your phone. Royal Observatory, Feb 2026
  102. 102 If you use a star app, switch on its red night-vision mode. Royal Observatory, Feb 2026
  103. 103 The W-shaped constellation Cassiopeia points towards Andromeda. EarthSky
  104. 104 Use the right-hand V of the W as an arrow: the galaxy is a little over 15 degrees from its tip, and a bit to the right. EarthSky
  105. 105 15 degrees is about the span of your index finger to little finger, held at arm’s length. EarthSky
  106. 106 Andromeda the constellation sits next to the Great Square of Pegasus; one of the square’s stars belongs to it. EarthSky
  107. 107 November and December are excellent months to look: after dark, the constellation is already high in the east. EarthSky
  108. 108 EarthSky adds that most people will need binoculars or a telescope to find it. EarthSky
  109. 109 Its brightness is magnitude 3.1 on NASA’s page. NASA, M31
  110. 110 EarthSky gives magnitude 3.5. EarthSky
  111. 111 Visit Isle of Man lists the Great Andromeda Galaxy among the sights you can see from the island’s dark sky sites with your eyes alone. Visit Isle of Man
  112. 112 Full Moonlight counts as light pollution for the fainter sights. Royal Observatory, naked eye
  113. 113 The constellation is named after Andromeda, daughter of Queen Cassiopeia and King Cepheus in Greek myth. EarthSky
  114. 114 She is the Chained Lady: chained up as a sacrifice to the sea monster Cetus, she is saved by Perseus. EarthSky
  115. 115 All the characters in the story are now constellations in the night sky. EarthSky