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  1. 1 A black hole is a place where gravity is so strong that nothing, not even light, can escape from it. NASA Space Place
  2. 2 Black holes aren’t really holes. They are huge amounts of matter packed into very small spaces. NASA
  3. 3 The edge of a black hole is the event horizon. It isn’t a solid surface like Earth’s: it is a boundary that contains all of the black hole’s matter. NASA
  4. 4 Inside the event horizon, the speed you would need to escape is greater than the speed of light, and nothing can go faster than light. NASA, anatomy
  5. 5 No light of any kind, not even X-rays, can get out from inside the event horizon. NASA, 10 questions
  6. 6 So black holes neither give out nor reflect light, and nothing that happens inside them can reach anyone outside. NASA, anatomy
  7. 7 Most of the light we see comes from the accretion disk: gas that settles into a hot, bright, fast-spinning disk and slowly works its way in until it falls through the event horizon. NASA, anatomy
  8. 8 Gas pulled towards a black hole is heated to millions of degrees, so it glows in X-rays. NASA, 10 questions
  9. 9 NASA says the dark zone called the event horizon shadow is roughly twice as big as the black hole’s actual surface. NASA, anatomy
  10. 10 Britannica says the shadow of the black hole in M87 is about five and a half times larger than its event horizon. Britannica
  11. 11 Thin rings of light at the edge of the shadow are distorted images of the disk, made by light that has gone round the black hole several times before escaping. NASA, anatomy
  12. 12 One side of the disk usually looks brighter: gas spinning towards us looks brighter and bluer, and gas spinning away looks dimmer and redder. NASA, anatomy
  13. 13 Above the disk is the corona, a thin, turbulent cloud at about a billion degrees that gives out high-energy X-rays. NASA, anatomy
  14. 14 A little of the material heading in can be redirected into a pair of jets that blast away in opposite directions at close to the speed of light. NASA, anatomy
  15. 15 The jets from supermassive black holes can be hundreds of thousands of light-years long. NASA, anatomy
  16. 16 Einstein’s general relativity predicts that the very centre holds a singularity, a point where matter is crushed to infinite density. NASA, anatomy
  17. 17 Nobody yet knows whether the singularity is a real physical thing or only a feature of the mathematics. It may mark the point where relativity stops working. NASA, anatomy
  18. 18 The radius of the event horizon is called the Schwarzschild radius, after the German astronomer Karl Schwarzschild. Britannica
  19. 19 The Schwarzschild radius is Rg = 2GM/c², where M is the mass, G is the gravitational constant and c is the speed of light. Britannica
  20. 20 The Schwarzschild radius goes up in proportion to the mass: twice the mass, twice the radius. Britannica
  21. 21 For a star like the Sun, the Schwarzschild radius is about 3 km (2 miles). Britannica
  22. 22 For a black hole with ten times the Sun’s mass, the radius would be 30 km (18.6 miles). Britannica
  23. 23 For something with the mass of a person it would be about 10⁻²³ cm, far smaller than the nucleus of an atom. Britannica
  24. 24 Large black holes can trap tens to millions of times the Sun’s mass in a point smaller than the tip of a pin. NASA Space Place
  25. 25 Fall too close and the black hole’s pull would squeeze you sideways and stretch you lengthways like a noodle. Scientists really do call it spaghettification. NASA, 10 questions
  26. 26 It has never happened to anyone: black holes are too far away to pull in any matter from our solar system. NASA, 10 questions
  27. 27 Scientists have watched black holes tear stars apart, which releases a tremendous amount of energy. NASA, 10 questions
  28. 28 All black holes spin. The fastest known, GRS 1915+105, turns more than 1,000 times a second. NASA
  29. 29 A hidden black hole can be weighed from the stars around it: their speeds show how massive the dark object is. NASA, 10 questions
  30. 30 Black holes bend light from things behind them. This gravitational lensing can reveal lonely black holes that would otherwise be invisible. NASA
  31. 31 Black holes are much studied but not fully understood. Nobody knows, for example, what matter looks like inside the event horizon. NASA
  32. 32 Gas falling quickly into a black hole is thought to give off more than 100 times as much energy as the same mass would release by nuclear fusion. Britannica
  33. 33 The details of a black hole’s structure are worked out from Albert Einstein’s general theory of relativity. Britannica
  34. 34 Astronomers usually sort black holes by mass into three groups: stellar-mass, supermassive and intermediate-mass. The boundaries are only approximate. NASA, types
  35. 35 When a star of more than eight times the Sun’s mass runs out of fuel, its core collapses, rebounds and explodes as a supernova. NASA, types
  36. 36 NASA says that if the star had around 20 times the Sun’s mass or more, its core collapses into a stellar-mass black hole. Near the threshold, it makes a city-sized neutron star instead. NASA, types
  37. 37 Britannica says only the most massive stars, those of more than three solar masses, become black holes. Smaller ones end as white dwarfs or neutron stars. Britannica
  38. 38 A stellar-mass black hole can probably form in seconds, after a massive star collapses. NASA, 10 questions
  39. 39 Newborn stellar black holes range from a few to hundreds of times the Sun’s mass. NASA, types
  40. 40 Nearly all the stellar-mass black holes found so far were found because they are paired with a star. NASA, types
  41. 41 Pairs like these have revealed around 50 suspected or confirmed stellar-mass black holes in the Milky Way, but there may be as many as 100 million in our galaxy. NASA, types
  42. 42 The stellar black holes found in the Milky Way before 2024 averaged about 10 times the Sun’s mass. ESO, 2024
  43. 43 Almost every large galaxy, the Milky Way included, has a supermassive black hole at its centre, with hundreds of thousands to billions of times the Sun’s mass. NASA, types
  44. 44 Some supermassive black holes formed in the first billion years after the birth of the universe. How they started is still a mystery. NASA, types
  45. 45 Supermassive black holes grow by swallowing smaller objects, and by merging with each other when galaxies collide. NASA, types
  46. 46 The black hole at the centre of the galaxy Holmberg 15A holds at least 40 billion times the Sun’s mass. NASA, types
  47. 47 Intermediate-mass black holes, from about a hundred to hundreds of thousands of Suns, should exist, but the candidates have proved hard to confirm. NASA, types
  48. 48 One candidate, 3XMM J215022.4−055108, has about 50,000 times the Sun’s mass. NASA, types
  49. 49 Primordial black holes, if they exist, would have formed in the first second of the universe, with masses from 100,000 times less than a paperclip to 100,000 times more than the Sun. NASA, types
  50. 50 Nobody has yet found definite proof that primordial black holes ever existed. NASA, types
  51. 51 Theory says black holes with less mass than a mountain would evaporate faster than bigger ones, so many small early ones may have vanished completely. NASA, types
  52. 52 The Sun will never become a black hole. It isn’t massive enough to explode, and will end its life as a white dwarf. NASA, 10 questions
  53. 53 The nearest known black hole, Gaia BH1, is about 1,500 light-years away. NASA
  54. 54 Gaia BH3, announced in April 2024, has about 33 times the Sun’s mass and lies about 2,000 light-years away in the constellation Aquila, making it the second-closest known black hole. ESO, 2024
  55. 55 It was the first black hole of stellar origin this big ever spotted in the Milky Way. ESA, 2024
  56. 56 Its companion is an ancient giant star that formed in the first two billion years after the Big Bang. ESA, 2024
  57. 57 The most massive black hole yet observed, TON 618, has 66 billion times the Sun’s mass. NASA
  58. 58 The lightest known black hole has only 3.8 times the Sun’s mass. It is paired up with a star. NASA
  59. 59 NASA’s black hole page lists the most distant black hole detected, at the centre of the galaxy QSO J0313-1806, as around 13 billion light-years away. NASA
  60. 60 A 2019 NASA article put the most distant black hole then known in a galaxy about 13.1 billion light-years from Earth, seen as it was about 690 million years after the Big Bang. NASA, 10 questions
  61. 61 Cygnus X-1, discovered in 1971, is a blue supergiant star and an invisible companion, a black hole, orbiting each other every 5.6 days. Britannica
  62. 62 In 2011 a NASA Chandra team gave the most precise mass yet for the Cygnus X-1 black hole: 14.8 times the Sun’s mass. NASA Chandra, 2011
  63. 63 ESO’s 2024 press release says Cygnus X-1 reaches 21 solar masses. ESO, 2024
  64. 64 ESA’s 2024 release puts Cygnus X-1 at around 20 times the Sun’s mass. ESA, 2024
  65. 65 Cygnus X-1 is about 6,070 light-years from Earth. NASA Chandra, 2011
  66. 66 Its event horizon spins round more than 800 times a second. NASA Chandra, 2011
  67. 67 Stephen Hawking lost a bet, first placed in 1974, that Cygnus X-1 did not contain a black hole. NASA Chandra, 2011
  68. 68 The supermassive black hole at the centre of the Milky Way is called Sagittarius A*, said “ay-star”. NASA
  69. 69 It has about 4 million times the mass of the Sun. NASA
  70. 70 That is relatively small compared with the black holes in some other galaxies. NASA, types
  71. 71 A NASA article puts Earth about 26,000 light-years from the Milky Way’s black hole. NASA, 10 questions
  72. 72 ESO’s 2022 press release says it is about 27,000 light-years from Earth. ESO, 2022
  73. 73 There is no danger of Earth being pulled in. NASA, 10 questions
  74. 74 It has probably already swallowed most or all of the stars that formed near it. NASA, 10 questions
  75. 75 From the early 1990s, Reinhard Genzel and Andrea Ghez each led a team that mapped the orbits of the brightest stars near the centre of our galaxy. Nobel Prize, 2020
  76. 76 Both teams found an extremely heavy, invisible object: around four million solar masses packed into a region no larger than our solar system. Nobel Prize, 2020
  77. 77 To see through the dust they used the world’s largest telescopes and new ways to correct for the blurring caused by Earth’s atmosphere. Nobel Prize, 2020
  78. 78 Genzel and Ghez shared half of the 2020 Nobel Prize in Physics “for the discovery of a supermassive compact object at the centre of our galaxy”. Nobel Prize, 2020
  79. 79 The Nobel committee said a supermassive black hole is the only explanation currently known. Nobel Prize, 2020
  80. 80 Andrea Ghez was born in New York City in 1965 and is a professor at the University of California, Los Angeles. Nobel Prize, 2020
  81. 81 Reinhard Genzel was born in 1952 in Bad Homburg vor der Höhe, Germany. Nobel Prize, 2020
  82. 82 NASA’s James Webb Space Telescope has seen bright flares and fainter flickers from Sagittarius A*. The flickers are so quick they must come from very close to the black hole. NASA
  83. 83 On 12 May 2022 astronomers unveiled the first image of Sagittarius A*, made by the Event Horizon Telescope. ESO, 2022
  84. 84 From Earth it looks about as big in the sky as a doughnut would on the Moon. ESO, 2022
  85. 85 The telescopes observed it on several nights in 2017, collecting data for hours at a time, like a long exposure on a camera. ESO, 2022
  86. 86 Gas takes days to weeks to circle M87*, but only minutes to circle Sagittarius A*, so its glow kept changing while the telescopes watched: “a bit like trying to take a clear picture of a puppy quickly chasing its tail”. ESO, 2022
  87. 87 The published picture is an average of the different images the team extracted. ESO, 2022
  88. 88 More than 300 researchers from 80 institutes worked on it, rigorously, for five years. ESO, 2022
  89. 89 The two pictured black holes look remarkably similar, even though ours is more than a thousand times smaller and less massive than M87*. ESO, 2022
  90. 90 The size of the ring agreed closely with the predictions of Einstein’s general relativity. ESO, 2022
  91. 91 Britannica gives Sagittarius A* a mass equivalent to more than 4,000,000 Suns. Britannica
  92. 92 On 10 April 2019 the Event Horizon Telescope team revealed the first direct visual evidence of a supermassive black hole and its shadow. ESO, 2019
  93. 93 It is the black hole at the centre of Messier 87, a massive galaxy in the Virgo cluster: 55 million light-years away, with 6.5 billion times the mass of the Sun. ESO, 2019
  94. 94 The Event Horizon Telescope linked eight radio telescopes on the ground into one virtual telescope the size of Earth. ESO, 2019
  95. 95 Its telescopes sat on volcanoes in Hawaii and Mexico, mountains in Arizona and Spain’s Sierra Nevada, in Chile’s Atacama Desert and in Antarctica. ESO, 2019
  96. 96 The telescopes were synchronised by a technique called very-long-baseline interferometry, using Earth’s rotation, and observed at a wavelength of 1.3 mm. ESO, 2019
  97. 97 Its sharpness, 20 micro-arcseconds, is enough to read a newspaper in New York from a café in Paris. ESO, 2019
  98. 98 More than 200 researchers worked on the first picture. ESO, 2019
  99. 99 Petabytes of raw data were combined by specialised supercomputers at the Max Planck Institute for Radio Astronomy and MIT Haystack Observatory. ESO, 2019
  100. 100 ALMA in Chile, with 66 high-precision antennas, was the most sensitive telescope in the network. ESO, 2019
  101. 101 The result was published in six papers in a special issue of The Astrophysical Journal Letters. ESO, 2019
  102. 102 Britannica says the M87 black hole, with six and a half billion Suns’ worth of mass, is only 38 billion km (24 billion miles) across. Britannica
  103. 103 The ring is brighter on one side because the black hole is spinning: light from material turning towards Earth is boosted. Britannica
  104. 104 The picture was made from data collected in 2017 and released in 2019. Britannica
  105. 105 Since 2017 the network has added the Greenland Telescope, the NOEMA array and the 12-metre telescope on Kitt Peak. ESO, 2022
  106. 106 On 14 September 2015 gravitational waves were observed for the very first time. They came from a collision between two black holes. Nobel Prize, 2017
  107. 107 Both LIGO detectors, at Livingston, Louisiana, and Hanford, Washington, picked up the signal at 09:51 UTC. LIGO, 2016
  108. 108 The two black holes were about 29 and 36 times the mass of the Sun, and the collision happened 1.3 billion years ago. LIGO, 2016
  109. 109 About three Suns’ worth of mass was turned into gravitational waves in a fraction of a second, with a peak power about 50 times that of the whole visible universe. LIGO, 2016
  110. 110 Livingston recorded it 7 milliseconds before Hanford, which showed that the source lay in the southern sky. LIGO, 2016
  111. 111 In the final fraction of a second the two black holes collided at nearly half the speed of light. LIGO, 2016
  112. 112 Gravitational waves are ripples in space-time. They are made whenever a mass speeds up, like an ice-skater spinning or a pair of black holes circling each other. Nobel Prize, 2017
  113. 113 LIGO measured a change thousands of times smaller than the nucleus of an atom as the wave passed Earth. Nobel Prize, 2017
  114. 114 Einstein predicted gravitational waves about a hundred years earlier, but was convinced they could never be measured. Nobel Prize, 2017
  115. 115 LIGO is a joint project of more than a thousand researchers from more than twenty countries. Nobel Prize, 2017
  116. 116 Rainer Weiss, Barry Barish and Kip Thorne won the 2017 Nobel Prize in Physics “for decisive contributions to the LIGO detector and the observation of gravitational waves”. Nobel Prize, 2017
  117. 117 The waves took 1.3 billion years to reach the LIGO detectors in the USA. Nobel Prize, 2017
  118. 118 Russell Hulse and Joseph Taylor’s pulsar, found in 1974, showed indirectly that gravitational waves exist, and won them the 1993 Nobel Prize in Physics. LIGO, 2016
  119. 119 Collisions of neutron stars and black holes make bigger black holes quickly, and send out gravitational waves. NASA, 10 questions
  120. 120 Before Gaia BH3, black holes of 30-odd Suns had only been detected in distant galaxies, through gravitational waves picked up by the LIGO–Virgo–KAGRA collaboration. ESA, 2024
  121. 121 In 1783 the clergyman and astronomer John Michell described ‘dark stars’ whose light could not reach us, in a letter to Henry Cavendish that the Royal Society published. Royal Society
  122. 122 He worked out that a star as dense as the Sun but more than 500 times as wide would hold in its own light. Royal Society
  123. 123 He suggested that such a hidden body might still be found from the movements of bright bodies circling it. Royal Society
  124. 124 His idea was published more than 130 years before Einstein’s theory of gravity. Royal Society
  125. 125 Michell never used the words ‘black hole’, and nor did Einstein. Royal Society
  126. 126 Michell (1724–1793) was a British geologist and astronomer, counted among the fathers of seismology, the science of earthquakes. Britannica
  127. 127 In 1916 Karl Schwarzschild predicted that there could be collapsed stars that give out no radiation. Britannica
  128. 128 Schwarzschild found the first exact solution of Einstein’s equations of gravity, and laid the foundation of black hole theory. Britannica
  129. 129 Schwarzschild was born in Frankfurt am Main in 1873 and died in Potsdam on 11 May 1916, after falling fatally ill while serving in the German army in the First World War. Britannica
  130. 130 Einstein himself did not believe that black holes really exist. Nobel Prize, 2020
  131. 131 In January 1965, ten years after Einstein’s death, Roger Penrose proved that black holes really can form, and described them in detail. Nobel Prize, 2020
  132. 132 Penrose’s article is still regarded as the most important contribution to general relativity since Einstein. It won him half of the 2020 Nobel Prize in Physics. Nobel Prize, 2020
  133. 133 The Royal Society says the term ‘black hole’ was coined by the American physicist John Archibald Wheeler in 1967. Royal Society
  134. 134 Britannica says Wheeler popularised the term black hole. Britannica
  135. 135 Wheeler (1911–2008) studied with Niels Bohr in Copenhagen and joined Princeton University in 1938. Britannica
  136. 136 With Charles Misner and Kip Thorne he wrote Gravitation (1973), a major textbook on Einstein’s relativity. Britannica
  137. 137 The physicist Stephen Hawking proposed that black holes also slowly shrink, because they lose tiny amounts of energy, now called Hawking radiation. NASA, 10 questions
  138. 138 For most black holes we know about, shrinking noticeably would take much longer than the entire age of the universe. NASA, 10 questions
  139. 139 Hawking also proposed that many tiny primordial black holes, perhaps no heavier than an asteroid, might have formed in the Big Bang. Britannica
  140. 140 Even bigger black holes, about 10 billion Suns each, are inferred at the centres of the nearby galaxies NGC 3842 and NGC 4889. Britannica
  141. 141 Black holes are not cosmic vacuum cleaners. They don’t suck in other matter: from far enough away, their gravity acts just like that of anything else with the same mass. NASA
  142. 142 Black holes are not wormholes. They don’t give shortcuts between places in space, or portals to other dimensions or universes. NASA
  143. 143 A black hole cannot eat a whole galaxy. Even a supermassive one’s reach is nowhere near big enough. NASA, 10 questions
  144. 144 If the Sun were swapped for a black hole of the same mass, the planets would stay in their orbits, though the solar system would get a lot colder. NASA
  145. 145 The universe won’t all be swallowed: each black hole’s zone of strong gravity is small compared with a galaxy, and the universe is big and expanding. NASA, 10 questions
  146. 146 Black holes are not rare. Most galaxies the size of the Milky Way have a giant one at their centre. NASA
  147. 147 The exploding stars that leave black holes behind spread elements needed for life, such as carbon, nitrogen and oxygen. In a sense, NASA says, we owe our existence to them. NASA, 10 questions
  148. 148 Which came first, the galaxy or its black hole, is still one of the great puzzles of the universe. NASA, 10 questions
  149. 149 Matter that falls in adds to the black hole’s mass. Its gravity doesn’t disappear from the universe. NASA, 10 questions
  150. 150 A quasar is a supermassive black hole with gas pouring in so fast that it gives out a thousand times more energy than its whole galaxy. NASA, 10 questions
  151. 151 Telescopes don’t see black holes themselves. They see what happens around them: hot gas, orbiting stars and bent light. NASA