Donald Lynden-Bell obituary | Science

Donald Lynden-Bell, who has died aged 82, was one of the leading theoretical astrophysicists of his generation. He proposed in 1969 that quasars are powered by supermassive black holes and that most large galaxies, including our own, would host a dead quasar in their nucleus. He was a brilliant dynamicist, inventing the concept of “violent relaxation” in stellar systems, but he was also interested in what could be learned from observational mapping of our galaxy and its surroundings.

Quasars, short for quasi-stellar radio sources, had been found during optical follow-up of radio surveys. In 1963 Maarten Schmidt had found that the quasar 3C273 had what was for the time a high red shift, indicating that it had a huge luminosity, and others soon followed. For a while, there was controversy about whether the red shifts were cosmological.

Donald’s black hole model could explain how such a huge luminosity could arise from a very small volume and it also placed quasars firmly in the nuclei of galaxies. Donald and Schmidt shared the first Kavli prize in astrophysics in 2008, a prize set up to honour fields not covered by the Nobel prize. The model and the idea that most large galaxies harbour a black hole have been amply confirmed by subsequent observations.

Son of Monica (nee Thring) and Lt-Col Lachlan Lynden-Bell, MC, Donald was born at Dover Castle, Kent, a military barracks at the time. At Marlborough college, Wiltshire, he encountered inspirational mathematics and physics teachers, one of whom also interested him in rock climbing. His great-grandfather had been a friend of the astronomer John Herschel and so his father inherited a telescope and introduced Donald to the wonders of astronomy.

Donald studied mathematics at Clare College, Cambridge, and in his second year he was tutored by Abdus Salam, who advised him to take up physics in his third year. He became a research student under the supervision of Leon Mestel. In his 2010 autobiographical essay for Annual Reviews of Astronomy and Astrophyscs, Donald claimed that the magnetohydrodynamic problems posed by Leon were “too intractable for a novice”, so instead, stimulated by Richard Woolley (the astronomer royal), he set about redeveloping stellar dynamics for his PhD thesis (1960).

He was to make profound contributions to this field, including his 1967 discovery of violent relaxation in stellar systems. If a globular star cluster evolved purely under the influence of gravitational interactions between pairs of stars, it would take longer than the age of the universe to relax down to the concentrated state we see today. Donald proved in his 1967 paper Statistical Mechanics of Violent Relaxation in Stellar Systems (published in Monthly Notices of the Royal Astronomical Society) that interactions with the global gravitational field made the relaxation process much faster.

Star Men, a 2015 documentary film about Lynden-Bell’s hike across the US southwest with Wal Sargent, Nick Woolf and Roger Griffin.

In 1960 Donald had taken a fellowship at Caltech and the Mt Wilson and Palomar observatories, in California, and there he offered his services to Allan Sandage, which resulted in the 1962 paper Evidence from the Motions of Old Stars that the Galaxy Collapsed, by Donald, Sandage and Olin Eggen (published in Astrophysical Journal). This opened up the field of galactic archaeology, where the motions of stars and their chemical abundances are used to trace the history of a galaxy’s formation and evolution.

Donald had met Ruth Truscott, a distinguished chemist, in Cambridge, and they were married in 1961, Ruth moving to Caltech where she continued to work on her PhD. She was later to become professor of chemistry at Belfast University and a fellow of the Royal Society.

During the Caltech period Donald embarked on a memorable hiking holiday across the US southwest with fellow postdocs Wal Sargent, Nick Woolf and Roger Griffin, which was delightfully recreated in the hit documentary film Star Men in 2015. I know that Donald enjoyed making this film and the subsequent interest in it.

He returned to Cambridge in 1962 to take up an assistant lectureship in mathematics. I remember attending his lectures on general relativity a year later. He was a forceful lecturer, with a booming voice and emphatic delivery, and he liked to provoke his audience, asking questions which seemed to have an obvious but incorrect answer.

In 1964 Donald decided to move to the Royal Greenwich Observatory at Herstmonceux, East Sussex, so as to have more time for research. Ruth took up a lectureship at the University of Sussex and she and Donald were bringing up their two young children, Marion and Edward. Donald also had a joint appointment with the nascent Astronomy Centre at the university, which also captured Roger Tayler and Mestel from Cambridge.

Sussex postgraduates attended seminars at Herstmonceux and I remember being commanded by Donald to accompany him on a lunchtime walk round the grounds of Herstmonceux Castle during which he recounted his ideas about black holes in the nuclei of galaxies. I don’t think I said much but I was pleasantly surprised to be acknowledged for “help and encouragement” at the end of his paper Galactic Nuclei as Collapsed Old Quasars (published in Nature, 1969).

In 1971 Donald decided to leave Hertsmonceux and applied for the vacant chair of astrophysics at Cambridge. Fred Hoyle, head of the university’s Institute of Theoretical Astronomy, which he had founded, was hoping to secure this position for Donald’s friend Sargent, but in a famous tactical error Wal failed to submit an application. Donald found himself director of the Cambridge Observatory but within a year, following Hoyle’s piqued resignation, he was having to negotiate the merger of the observatory and the institute, to form the new Institute of Astronomy. From 1972 to 1995 Donald and Martin Rees alternated five-year spells as director of the institute.

Although Donald was pre-eminently a theoretician, he was interested in the history of astronomy and in the nitty-gritty of astronomical observations. He was an admirer of the astrophysicist Arthur Eddington and would have relished following in his footsteps to the Royal Greenwich Observatory and the Cambridge Observatory. With Frank Kerr, Donald reviewed the fundamental constants describing the size and rotation speed of the Milky Way, and their new proposed values were adopted by the International Astronomical Union in 1985.

Donald joined a collaboration with six others which used a distance estimator based on the size and velocity spread of elliptical galaxies to map the peculiar velocities of galaxies, superposed on the Hubble expansion, across the sky and concluded in 1988 that there was a large hidden concentration of matter behind the galactic plane, in the direction of the Centaurus constellation, which they dubbed “the Great Attractor”. This was controversial because other surveys, particularly at infrared wavelengths, did not find this hidden object. In a temporary diversion from her distinguished career in chemistry, Ruth joined Donald and together they used globular clusters to map out streams of infalling matter around the Milky Way in 1995 and found a new member of the Sagittarius star stream.

Donald was a great supporter of the Royal Astronomical Society and was a regular attender of the monthly meetings, where he could be counted on to ask interesting questions. He was president of the Royal Astronomical Society from 1985 until 1987 and was their gold medallist in 1993. He received numerous other awards, including being made a CBE in 2000 and a fellow of the Royal Society in 1978.

He is survived by Ruth and their children.

Donald Lynden-Bell, astrophysicist, born 5 April 1935; died 6 February 2018

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