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22.12 Essays Dyson NS ESSAY NATURE|Vol 438|22/29 December 2005 The death of a star When Subrahmanyan Chandrasekhar asked in his twenties, ‘What happens to a massive star when it runs out of fuel?’ he had little idea that it would take a generation of astronomers to find the answer. Freeman Dyson Subrahmanyan Chandrasekhar, known to his friends and colleagues as Chandra, opened the door to our understanding of the death of stars. He was the first to calcu- BETTMANN/CORBIS late the possible final states of stars that have used up their supplies of energy. He did so in 1930, when he was a graduate student travelling by ship from his home in India to study at the University of Cambridge, UK. Even before he got to Cambridge, Chandrasekhar knew more about relativity and quantum mechanics than most of his teachers. He knew how to take account of both when building mathematical models of cold stars that had stopped shining. On board the ship, once he he had finished his calculations, he came to a startling conclu- sion: he found that there exists a critical mass, now known as the Chandrasekhar limit, beyond which no cold star made of Star performer: Subrahmanyan Chandrasekhar’s insight helped to revolutionize astronomy. ordinary matter can exist. He calculated this critical mass, and found that it is a few times 1930, his mentors had no inkling of the a magisterial book, summarizing his results the mass of the Sun, the exact value depend- revolution that his question was to bring and presenting the whole field in a new and ing on the chemical composition of the star. about. Chief among his mentors were clearer light. He worked in each of six fields When a star of less than the critical mass Arthur Eddington and Edward Milne, two in turn: in his third decade, he worked on has used up all its fuel, it will slowly radiate world-famous astronomers who thought the structure of dying stars; in his fourth on away its energy and cool down to reach a they knew everything worth knowing the transport of radiation through stellar state described by one of the models he about stars. Each of them had a private atmospheres; in his fifth on instabilities of had calculated. But once a star with greater theory of the Universe that was incompat- fluid motions; in his sixth on Einstein’s gen- than the critical mass has used up its ible with Chandra’s calculation. They eral theory of relativity; in his seventh on fuel, it cannot cool down gradually and die ignored his arguments and declared the theory of black holes; and in his eighth quietly. It must either change into some publicly that his conclusions were wrong. on a detailed historical study of Newton’s totally different form of matter, or end its But Chandra had a cool head. He pub- Principia Mathematica. life in a violent collapse and explosion. lished his work in reputable astronomical Everything that Chandra did was done When Chandra discovered the critical journals and waited for the next generation with elegance and style. He reached a deep mass, he had no idea what the ultimate of astronomers to recognize its importance. understanding of the mathematical and fate of a massive star should be. He opened He stayed in Cambridge for seven years and physical properties of black holes, those the door to understanding by raising the remained on friendly terms with Eddington objects of perfect symmetry that he saw question: what happens to a massive star and Milne. After their deaths many years as the crowning beauty of the Universe, a when it runs out of fuel and has no way to later, he wrote warm and sympathetic beauty to which Eddington and Milne and cool down? memorial lectures for each of them. even Einstein had been blind. His book The efforts of a whole generation of Once I went for a long walk with Chan- about black holes displays his unrivalled astronomers were needed to find the dra in the woods around Princeton and mathematical skill as well as his impressive answer to Chandra’s question, starting with listened to him talking about his friend- command of the English language. Fritz Zwicky’s observations of supernovae ships. His love and admiration for Edding- In his eighth decade, his first great in the 1930s and ending with the identifi- ton and Milne were genuine. He saw them discovery, the Chandrasekhar limit, was cation of stellar-mass black holes using clearly, on the one hand as misguided fools, recognized with the award of a belated X-ray telescopes in space in the 1960s. We and on the other hand as human beings of Nobel prize. His last book Truth and now know that stars with a mass greater rare quality, worthy of honour and respect. Beautyis a collection of meditations about than the Chandrasekhar limit mostly die In 1937 Chandra moved to the Univer- the place of beauty in science, including a in catastrophic explosions, which we call sity of Chicago, where he worked until his critical comparison of Newton with supernovae, leaving behind collapsed cores death in 1995. His output of research fol- Shakespeare and Beethoven, and ending which may be either neutron stars or black lowed a regular pattern. At the beginning with eloquent tributes to his old enemies holes. Chandra’s question led the way to the of each decade, he chose a fresh field of Eddington and Milne. ■ modern view of the Universe as a dynamic study. Then he wrote a series of papers Freeman Dyson is at the Institute for arena dominated by violent events. solving the outstanding problems in that Advanced Study, Princeton, When Chandra arrived in Cambridge in field. At the end of the decade he published New Jersey, USA. ESSAY 1086 © 2005Nature PublishingGroup.
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