An appreciation of a giant of 20th-century astrophysics by Wyn Evans. View largeDownload slide View largeDownload slide Donald did not lecture in the modern way. I attended his graduate course on Galaxies in 1984–85, at the University of Cambridge. The lectures were chaotic and stimulating. Chaotic, because Donald would always work out formulae from first principles and without notes on the blackboard. This had the advantage that mistakes in chalk could be quickly erased, sign changes corrected. It also gave us students enormous insight into how to develop ideas into theory and how to catch errors as you made them (or at least shortly after making them). Stimulating, because his lectures were always intensely personal. If lecturing on, say, dark matter in dwarf galaxies, Donald would discuss whether he believed the data or whether he trusted the theoretical analysis. We were shown the building site – the place where the work was done, where a first-rate mind questioned, analysed and teased the data or theory. He showed us how to think, not what to learn for the exam. The exam was characteristically intractable. Donald did not supervise students in the modern way. I was his student for three years from 1985–88, and at no point was I actually working on a well-defined project. Rather, Donald was interested in … oh, pretty much everything astronomical, from general relativity to stellar dynamics to large-scale structure to stellar evolution to optical instruments. Donald's students were free to see him any time to talk about things he was interested in. Nominally, this weekly meeting took an hour, but in practice, if his imagination was fired up, the session might last all morning or afternoon. He posed problem after problem, and we would attempt to answer the problems together on the blackboard in his office. Usually, we students would emerge hours later, covered in chalk and sapped of all energy reserves. If unlucky, we had missed lunch. But Donald was always still perky and fresh for more problems after a session with a graduate student. Astronomy on the brain Donald never stopped thinking about astronomy. As a grad student, I wondered how he relaxed. I remember talking to him about movies. “The only movies I've seen in 20 years are N-body movies,” he said. Other things Donald seemed to have no interest in included departmental politics, gossip, fiction, fine food and drink. Those meetings in his office in the Observatory, room O2, were some of the most vivid encounters of my life. “The star's motion is like a slippery grape in a rotating fruit bowl,” he would cry. And off we would go, writing down the equations in chalk on the board, correcting the sign of the Coriolis term as we went, trying substitutions and transformations to make the equations tractable and solvable. There would always come a time when I wanted to stop, the board was densely covered in chalk and scrawl, some equations were partly rubbed out, some equations were probably not even correct, maybe it made sense to write it down on paper now. But Donald never wanted to stop, his energy was inexhaustible. At times, out of his battered brown briefcase would come enormous, ledger-sized books with various problems in different states of working, sometimes labelled with the name of the poser. One impressive volume was called “Roberto's Problem”, though I never actually learnt what that was. Another was “Alar's Problem” (the change in adiabatic invariant after separatrix crossing). Here, the eponymous Toomre had solved the problem, but had thoughtfully sealed his envelope containing the solution with lots of sticky tape to prevent Donald from peeking. Of course, Donald wanted to solve it himself. Donald knew a lot of mathematics, including some subjects no longer familiarly taught in undergraduate courses. But he never appreciated purely mathematical arguments. He always argued for the primacy of physical insight. I remember introducing him to Vladimir Arnold's Mathematical Methods of Classical Mechanics, a beautiful and splendid cathedral of differential geometry erected over Hamiltonian mechanics. “Not as good as Whittaker,” he said, referring to one of his favourite books, Whittaker's impenetrable late-Edwardian Analytical Dynamics. “Whittaker shows you the gaps.” For Donald, a good book was one that was incomplete in its argument and ragged in its exposition – because that stimulated further thinking and research. Of course, Donald loved Arthur Stanley Eddington's books more than anything else. His inscrutable and earnest face looked down from Donald's office wall. Any modern book on relativity or stellar structure or dynamics was “not a patch on Eddington”. Donald loved seminars, and he loved asking questions. Many is the time I have seen a speaker, arguments all polished and shining, thrown by an awkward question from Donald. He had a booming voice, and often an unusual perspective on a theoretical problem or on observational data, and so the question could come fast and unexpectedly to the speaker from left-field. Donald did not restrict himself to subjects in which he had done research. His mastery over astronomy was so great that he could devise a fiendish question in any area. But, he never asked to bamboozle the speaker, he asked because he was playfully and inexhaustibly curious. In astronomy, Donald often had the greatest respect for individuals who did things he couldn't do himself, such as instrument building (Roger Angel), or pioneering N-body simulations (Sverre Aarseth and Simon White) or stellar evolution codes (Peter Eggleton). Golden period Donald completed his PhD under the supervision of the late Leon Mestel in 1960. His thesis laid the foundations for Donald's golden period, an incredibly productive decade-and-a-half in which glorious paper followed glorious paper. The finest achievements include solo papers in 1962, which derive exact solutions for steady-state elliptical galaxies and which systematically study the potentials that can support triaxial stellar systems; two fundamental papers with Peter Goldreich in 1965, which led to the discovery of swing amplification, the mechanism that drives spiral structure in galaxies; an important paper with Jim Pringle in 1974, which lays the foundations for the evolution of accretion discs around the “nebular variables” or T Tauri stars and predicts the signature of excess infrared emission. In 1968, puzzled by work sent to him from Leningrad by Vadim Antonov, Donald rediscovered the gravothermal catastrophe and comprehended its implication for the evolution of the globular clusters. This awakened his interest in negative heat capacities, a research subject he subsequently pursued with his wife, Ruth Lynden-Bell (professor of chemistry at Queen's University, Belfast). His theoretical papers, often staunchly mathematical, were relieved with humour and analogies, such as Donald's Demon who impishly reverses the velocities of stars. “It's important that observers read them too,” he often said. And then in 1969 came the Nature paper – Donald's single most important contribution – which hypothesized the existence of supermassive black holes, or dead quasars, at the very centres of galaxies. Years later, in 2008, this heart-stopper of a paper would win Donald the inaugural Kavli Prize, shared with Maarten Schmidt. This splendid flowering of theoretical astrophysics gave way to increasing interest in the observations in the 1980s and 1990s. Donald joined forces with six other warriors to form the Seven Samurai. Devising new distance estimators, the Samurai analysed the local flows of galaxies and discovered the Great Attractor, an enormously massive agglomeration into which most of the nearby galaxies are falling. In the Milky Way, Donald was the first to recognize the great future of tidal streams, both as measures of the gravity field and as evidence of the hierarchical accretion that builds up the galaxies. This was bittersweet, as merging and accretion had gradually displaced the older picture of collapse that Donald had himself devised with Olin Eggen and Allan Sandage in a highly cited and influential paper in 1962. They used evidence from chemistry and kinematics of local halo stars to suggest that our galaxy had collapsed from a single large gas cloud. Though this idea has not survived, the lineal descendants of Donald's paper are all around us today in the wide-field spectroscopic and multi-band photometric surveys of the galaxy. Donald loved the Royal Astronomical Society and its even older sibling, the RAS Dining Club. He regarded the Presidency of the RAS as a high honour and was proud of his service from 1985–87. He viewed the RAS as playing a hugely important role in fostering interest in, and support for, astronomy for two centuries. Star Men Donald remained mentally vigorous almost to the very end. His interests in recent years had become focused on fundamental questions as to the origin of Mach's Principle and the localization of energy in general relativity. He was still regularly attending morning coffee and seminars at the Institute of Astronomy right up to his final illness, still bounding up to grad students and asking, “What are you working on?”, before explaining excitedly to them what he was working on. He was still taking the pen, ever ready to scribble equations on the whiteboards over coffee. He was still smilingly curious. His collaboration with Alison Rose led to the graceful Star Men movie about his recreated journey to Rainbow Bridge, Utah, accompanied by Wal Sargent, Nick Woolf and Roger Griffin, together with meditations on science, life, hiking and ageing. But to me, Donald never aged. He was immutable. He always looked the same as he did when I was his grad student in the late 1980s. Some people you really think will be there for ever… © 2018 Royal Astronomical Society
Astronomy & Geophysics – Oxford University Press
Published: Jun 1, 2018
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