by Leon Mestel
© Oxford University Press 2004 All rights reserved
Cowling, Thomas George (1906-1990), mathematician and astrophysicist, was born on 17 June 1906 at 3 Harrogate Road, South Hackney, London, the second of the four sons of George Cowling, Post Office engineer, and Edith Eliza Cowling, née Nicholls. Cowling was educated at Sir George Monoux Grammar School, Walthamstow, from where in 1924 he went on an open scholarship in mathematics to Brasenose College, Oxford, where he won the university junior mathematical exhibition in 1925 and the scholarship in 1926. With a first-class degree, he took a diploma in education, followed by a three-year post-graduate scholarship at Brasenose. His research supervisor was Edward A. Milne, the first incumbent of the Rouse Ball chair of mathematics.
Cowling's acumen was quickly recognized. After gaining his DPhil he was invited to join Sydney Chapman at Imperial College as demonstrator. Their collaboration converted Chapman's 'skeleton' of a book into a standard text, The Mathematical Theory of non-Uniform Gases (1939), which treats kinetic theory by a systematic attack on the classical Boltzmann equation. Cowling was subsequently assistant lecturer in mathematics at Swansea (1933-7), and lecturer at Dundee (1937-8) and at Manchester (1938-45). In 1945-8 he was professor of mathematics at Bangor, moving then to his last post, as professor of applied mathematics at Leeds, becoming emeritus professor in 1970.
Over the three decades from 1928 to 1958 Cowling made outstanding contributions to knowledge about stellar structure and developments in cosmical electrodynamics, kinetic theory, and plasma physics. His achievements were recognized by many honours and awards: the Johnson memorial prize at Oxford (1935), election to the Royal Society (1947), the gold medal of the Royal Astronomical Society (1956) and its presidency (1965-7), honorary fellowship of Brasenose (1966), the Bruce medal of the Astronomical Society of the Pacific (1985), and the Hughes medal of the Royal Society (1990). Cowling married Doris Marjorie Moffatt on 24 August 1935. They had three children: Margaret Ann Morrison, Elizabeth Mary Offord (d. 1994), and Michael John Cowling, and six grandchildren.
A few years before Cowling began working on stellar structure, Sir Arthur Eddington had published his celebrated Internal Constitution of the Stars, in which he pictured a homogeneous star as a self-gravitating gaseous sphere in radiative equilibrium, with its luminosity fixed by its mass and chemical composition. From comparison of his 'standard model' with the masses and radii of observed 'main-sequence' stars, Eddington inferred that the as yet unknown stellar energy sources must be highly temperature-sensitive. Cowling's careful analysis strengthened Eddington's case--against the criticisms of Sir James Jeans--by showing that the models would nevertheless almost certainly be vibrationally stable. In parallel with Ludwig Biermann, he also showed that stars would normally have turbulent domains with very efficient convective energy transport.
The 'Cowling model', with its convective core and radiative envelope, is a physically more realistic replacement of Eddington's standard model, effectively removing the confusion surrounding Eddington's 'mass-luminosity' relation. Cowling's analysis of vibrational stability was done in the mid-1930s. Subsequent developments in nuclear physics by Hans Bethe and C.-F. von Weizsäcker showed the Cowling model to be a paradigm for stars somewhat more massive than the sun, for which energy release by fusion of hydrogen into helium takes place through a highly temperature-sensitive catalytic cycle involving carbon and nitrogen, so yielding a convective core.
In low-mass stars like the sun, hydrogen fusion occurs through a direct and less temperature-sensitive path. The energy-generating core is now radiative, but because of the low surface temperature there is a deep convective envelope, extending from just beneath the surface and driven by the latent heat associated with the ionization of hydrogen and helium. Cowling pointed out that the condition of radiative transport through the surface layers must still be satisfied. The profundity of this comment showed up many years later in studies of evolved, inhomogeneous red giant stars by Fred Hoyle and Martin Schwarzschild and of contracting pre-main sequence stars by Chushiro Hayashi. Earlier, Milne had argued that the luminosity of a star should be sensitive to conditions in the surface, rather than depending essentially on radiative transfer through the bulk of the star, as in Eddington's theory. Cowling's work vindicated Eddington's approach for main-sequence stars, while implicitly allowing for such sensitivity in stars with extensive sub-surface convective zones.
Cowling's other contributions to stellar structure include an important paper on binary stars and two pioneering papers on non-radial stellar oscillations, foreshadowing later work in 'helioseismology'. Simultaneously he published a series of studies on cosmical magnetism and on plasma physics, culminating in his superbly succinct monograph Magnetohydrodynamics (1959, 1976). His famous 'anti-dynamo theorem' for axisymmetric magnetic fields is implicit in virtually all subsequent studies of cosmical dynamos. His 'fossil' fields remain relevant to the observed strongly magnetic stars. A lively correspondence with Biermann brought out the requirements of a theory of both the structure and the origin of sunspots. While recognizing the seminal contributions of Hannes Alfvén (1908-1995) to magnetohydrodynamics, he applied his critical faculties to Alfvén's theory of the sunspot cycle, and defended the Chapman-Ferraro theory of geomagnetic storms against what he regarded as unjust attacks. Together with Biermann, Arnulf Schlüter, and Lyman Spitzer jun., he did much to clarify the concept of 'conductivity' for both fully and partially ionized gases.
Surprisingly, Cowling spent most of the years of the Second World War just teaching at Manchester University and doing service as an air-raid warden. Although initially involved by invitation in some defence-orientated work, he was later puzzled to find himself excluded. After the war he learned that he was considered a security risk on what now seem the flimsy grounds that he had 'undesirable associates', to wit the communist physicist Janossy and a pacifist church minister.
In his autobiographical essay 'Astronomer by accident' (1985) Cowling remarks that he and his brothers inherited the 'Puritan work ethic' from their nonconformist parents. He remained an active, non-fundamentalist Baptist, though in his later years he confessed to doubts, speaking of 'getting closer to mysticism rather than to a religion of set creeds'. His puritanism perhaps showed itself in his very high academic standards, which he applied as much to his own work as to that of others, and earned him the nickname Doubting Thomas. Everyone knew him by sight--his red hair and his great height made him conspicuous--and also looked up to him metaphorically, because of his combination of kindliness and intellectual power. Though he never had the opportunity to build up a school (his one outstanding PhD student was Eric Priest, later professor at St Andrews), many younger workers, including Nigel Weiss at Cambridge and Roger Tayler and Leon Mestel at Sussex, remembered with warmth his generous encouragement.
Cowling's research activity in his later decades was severely hampered by recurring bouts of ill health, beginning with a duodenal ulcer in 1954. He died in Newton Green Hospital, Leeds, on 16 June 1990, and his remains were cremated in Lawnswood cemetery, Leeds, on the 22nd. His wife survived him.
LEON MESTEL
Sources
T. G. Cowling, 'Astronomer by accident', Annual Review of Astronomy and Astrophysics, 23 (1985), 1
D. DeVorkin, 'Transcript of an interview with T. G. Cowling', Sources for the history of modern astrophysics (1978)
L. Mestel, Memoirs FRS, 37 (1991), 103-25
personal knowledge (2004)
private information (2004)
b. cert.
d. cert.
Archives
RS
Likenesses
W. Bird and others, photographs, c.1967, repro. in Mestel, Memoirs FRS
W. Bird and others, photographs, c.1967, repro. in Cowling, 'Astronomer by accident', 1
W. Bird, photograph, Royal Astronomical Society [see illus.]
Wealth at death
£139,256: probate, 16 Nov 1990, CGPLA Eng. & Wales
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